Arch Capital (ACGL) 10-K 27 Feb 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
In Billions, except Share data, unless otherwise specified	Dec. 31, 2014	Feb. 20, 2015	Jun. 30, 2014
Document and Entity Information [Abstract]
Entity Registrant Name	ARCH CAPITAL GROUP LTD.
Entity Central Index Key	947484
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Document Type	10-K
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Amendment Flag	FALSE
Entity Common Stock, Shares Outstanding		126,226,689
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$7.05

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Thousands, unless otherwise specified
Investments:
Fixed maturities available for sale, at fair value (amortized cost: $10,701,557 and $9,564,634)	$10,750,770		$9,571,776
Short-term investments available for sale, at fair value (amortized cost: $801,758 and $1,477,584)	797,226		1,478,367
Investment of funds received under securities lending, at fair value (amortized cost: $40,473 and $97,943)	44,301		100,584
Equity securities available for sale, at fair value (cost: $562,534 and $433,275)	658,182		496,824
Other investments available for sale, at fair value (cost: $264,747 and $488,687)	296,224		498,310
Investments accounted for using the fair value option	2,435,532		1,221,534
Investments accounted for using the equity method	349,014		244,339
Total investments	15,331,249		13,611,734
Cash	485,702		434,057
Accrued investment income	74,316		66,848
Investment in joint venture (cost: $100,000)	90,426		104,856
Fixed maturities and short-term investments pledged under securities lending, at fair value	50,802		105,081
Premiums receivable	948,695		753,924
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	1,812,845		1,804,330
Contractholder receivables	1,309,192		1,064,246
Prepaid reinsurance premiums	377,078		328,343
Deferred acquisition costs, net	414,525		342,314
Receivable for securities sold	78,170		50,555
Goodwill and intangible assets	109,539		27,319
Other assets	927,004		872,487
Total assets	22,009,543		19,566,094
Liabilities
Reserve for losses and loss adjustment expenses	9,036,448		8,824,696
Unearned premiums	2,231,578		1,896,365
Reinsurance balances payable	219,312		196,167
Contractholder payables	1,309,192		1,064,246
Deposit accounting liabilities	327,384		421,297
Senior notes	800,000		800,000
Revolving credit agreement borrowings	100,000		100,000
Securities lending payable	50,529		107,999
Payable for securities purchased	128,413		51,318
Other liabilities	688,041		456,510
Total liabilities	14,890,897		13,918,598
Commitments and Contingencies
Redeemable noncontrolling interests	219,512	[1]	0	[1]
Shareholders’ Equity
Non-cumulative preferred shares	325,000		325,000
Common shares ($0.0033 par, shares issued: 171,672,408 and 169,560,591)	572		565
Additional paid-in capital	383,073		299,517
Retained earnings	6,854,571		6,042,154
Accumulated other comprehensive income, net of deferred income tax	128,856		74,964
Common shares held in treasury, at cost (shares: 44,304,474 and 35,885,707)	-1,562,019		-1,094,704
Total shareholders' equity available to Arch	6,130,053		5,647,496
Non-redeemable noncontrolling interests	769,081	[1]	0	[1]
Total shareholders' equity	6,899,134		5,647,496
Total liabilities, noncontrolling interests and shareholders' equity	$22,009,543		$19,566,094
[1]	See Note 4.

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parentheticals) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, except Share data, unless otherwise specified
Fixed maturities available for sale, at amortized cost	$10,701,557	$9,564,634
Short-term investments available for sale, at amortized cost	801,758	1,477,584
Investment of funds received under securities lending agreements, at cost	40,473	97,943
Equity securities, at cost	562,534	433,275
Other investments, at cost	264,747	488,687
Investment in joint venture, at cost	$100,000	$100,000
Common shares, par value per share	$0.00	$0.00
Common shares issued (shares)	171,672,408	169,560,591
Common shares held in treasury (shares)	44,304,474	35,885,707

Consolidated_Statements_of_Inc

Consolidated Statements of Income (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Revenues
Net premiums written	$3,891,938		$3,351,367		$3,052,235
Change in unearned premiums	-298,190		-205,415		-117,095
Net premiums earned	3,593,748		3,145,952		2,935,140
Net investment income	302,585		267,219		294,895
Net realized gains	102,917		74,018		194,228
Other-than-temporary impairment losses	-30,470		-3,961		-12,175
Less investment impairments recognized in other comprehensive income, before taxes	320		175		787
Net impairment losses recognized in earnings	-30,150		-3,786		-11,388
Other underwriting income	10,142		7,639		8,090
Equity in net income of investment funds accounted for using the equity method	19,883		35,701		73,510
Other income (loss)	-10,252		-586		-12,094
Total revenues	3,988,873		3,526,157		3,482,381
Expenses
Losses and loss adjustment expenses	1,919,250		1,679,424		1,861,277
Acquisition expenses	657,262		564,103		508,884
Other operating expenses	606,224		500,730		465,353
Interest expense	45,634		27,060		28,525
Net foreign exchange (gains) losses	-83,744		12,335		28,955
Total expenses	3,144,626		2,783,652		2,892,994
Income before income taxes	844,247		742,505		589,387
Current tax expense	30,550		32,696		9,004
Deferred tax (benefit) expense	-7,563		78		-13,014
Income tax expense (benefit)	22,987		32,774		-4,010
Net income	821,260		709,731		593,397
Amounts attributable to noncontrolling interests	13,095	[1]	0	[1]	0	[1]
Net income available to Arch	834,355		709,731		593,397
Preferred dividends	-21,938		-21,938		-25,079
Loss on repurchase of preferred shares	0		0		-10,612
Net income available to Arch common shareholders	$812,417		$687,793		$557,706
Net income per common share
Basic (per share)	$6.21		$5.24		$4.15
Diluted (per share)	$6.02		$5.07		$4.03
Weighted average common shares and common share equivalents outstanding
Basic (shares)	130,817,610		131,355,392		134,446,158
Diluted (shares)	134,922,322		135,777,183		138,258,847
[1]	See Note 4.

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Income (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Comprehensive Income
Net income	$821,260		$709,731		$593,397
Unrealized appreciation (decline) in value of available-for-sale investments:
Unrealized holding gains (losses) arising during period	146,330		-176,403		273,931
Portion of other-than-temporary impairment losses recognized in other comprehensive income, net of deferred income tax	-320		-175		-787
Reclassification of net realized gains, net of income taxes, included in net income	-65,104		-32,686		-157,824
Foreign currency translation adjustments	-27,014		-2,789		17,774
Comprehensive income	875,152		497,678		726,491
Amounts attributable to noncontrolling interests	13,095	[1]	0	[1]	0	[1]
Comprehensive income available to Arch	$888,247		$497,678		$726,491
[1]	See Note 4.

Consolidated_Statements_of_Cha

Consolidated Statements of Changes in Shareholders' Equity (USD $)		Total	Non-cumulative preferred shares	Common shares	Additional paid-in capital	Retained earnings	Accumulated other comprehensive income	Unrealized Appreciation on Available-For-Sale Investments	Foreign Currency Translation Adjustments	Common shares held in treasury, at cost
In Thousands
Balance at beginning of year at Dec. 31, 2011			$325,000	$549	$161,419	$4,796,655	$153,923	$174,636	($20,713)	($845,472)
Shares issued - Series C			325,000
Shares repurchased			-325,000		10,612	-10,612
Common shares issued, net				12	6,823
Issue costs on Series C preferred shares					-9,398
Exercise of stock options					14,218
Amortization of share-based compensation					42,303
Other					1,801
Net income		593,397				593,397
Amounts attributable to noncontrolling interests	[1]	0
Dividends declared on preferred shares		-25,079				-25,079
Unrealized holding gains (losses) arising during period, net of reclassification adjustment								116,107
Portion of other-than-temporary impairment losses recognized in other comprehensive income, net of deferred income tax								-787
Foreign currency translation adjustments		17,774							17,774
Shares repurchased for treasury										-180,367
Non-redeemable noncontrolling interests	[1]	0
Total shareholders' equity		5,168,878
Balance at end of year at Dec. 31, 2012		5,168,878	325,000	561	227,778	5,354,361	287,017	289,956	-2,939	-1,025,839
Shares issued - Series C			0
Shares repurchased			0		0	0
Common shares issued, net				4	8,237
Issue costs on Series C preferred shares					0
Exercise of stock options					10,561
Amortization of share-based compensation					49,237
Other					3,704
Net income		709,731				709,731
Amounts attributable to noncontrolling interests	[1]	0
Dividends declared on preferred shares		-21,938				-21,938
Unrealized holding gains (losses) arising during period, net of reclassification adjustment								-209,089
Portion of other-than-temporary impairment losses recognized in other comprehensive income, net of deferred income tax								-175
Foreign currency translation adjustments		-2,789							-2,789
Shares repurchased for treasury										-68,865
Non-redeemable noncontrolling interests	[1]	0
Total shareholders' equity		5,647,496
Balance at end of year at Dec. 31, 2013		5,647,496	325,000	565	299,517	6,042,154	74,964	80,692	-5,728	-1,094,704
Shares issued - Series C			0
Shares repurchased			0		0	0
Common shares issued, net				7	9,590
Issue costs on Series C preferred shares					0
Exercise of stock options					18,662
Amortization of share-based compensation					54,789
Other					515
Net income		821,260				821,260
Amounts attributable to noncontrolling interests	[1]	13,095
Dividends declared on preferred shares		-21,938				-21,938
Unrealized holding gains (losses) arising during period, net of reclassification adjustment								81,226
Portion of other-than-temporary impairment losses recognized in other comprehensive income, net of deferred income tax								-320
Foreign currency translation adjustments		-27,014							-27,014
Shares repurchased for treasury										-467,315
Non-redeemable noncontrolling interests	[1]	769,081
Total shareholders' equity		6,899,134
Balance at end of year at Dec. 31, 2014		$6,130,053	$325,000	$572	$383,073	$6,854,571	$128,856	$161,598	($32,742)	($1,562,019)
[1]	See Note 4.

Unaudited_Consolidated_Stateme

Unaudited Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Activities
Net income	$821,260	$709,731	$593,397
Adjustments to reconcile net income to net cash provided by operating activities:
Net realized losses	-127,511	-78,084	-199,547
Net impairment losses recognized in earnings	30,150	3,786	11,388
Equity in net income or loss of investment funds accounted for using the equity method and other income or loss	13,340	52,824	-43,633
Share-based compensation	54,789	49,237	42,303
Changes in:
Reserve for losses and loss adjustment expenses, net of unpaid losses and loss adjustment expenses recoverable	156,211	-29,393	395,901
Unearned premiums, net of prepaid reinsurance premiums	298,190	205,415	117,095
Premiums receivable	-217,035	-60,224	-120,380
Deferred acquisition costs, net	-81,584	-75,948	-34,371
Reinsurance balances payable	26,699	6,830	42,740
Other liabilities	101,757	-29,989	41,049
Other items, net	-39,136	96,683	75,661
Net Cash Provided By Operating Activities	1,037,130	850,868	921,603
Investing Activities
Purchases of fixed maturity investments	-28,745,279	-18,174,988	-17,568,592
Purchases of equity securities	-520,817	-535,857	-268,999
Purchases of other investments	-2,257,481	-1,326,729	-1,000,049
Proceeds from the sale of fixed maturity investments	26,823,192	17,196,614	16,366,306
Proceeds from the sale of equity securities	411,362	462,787	313,617
Proceeds from sales, redemptions and maturities of other investments	1,643,000	1,162,707	443,630
Proceeds from redemptions and maturities of fixed maturity investments	762,995	731,708	1,115,594
Net sales (purchases) of short-term investments	577,126	-750,613	185,919
Change in investment of securities lending collateral	57,470	-55,643	6,190
Purchase of business, net of cash acquired	-237,106	0	28,948
Purchases of furniture, equipment and other assets	-19,883	-17,499	-18,532
Net Cash Used For Investing Activities	-1,505,421	-1,307,513	-395,968
Financing Activities
Proceeds from issuance of Series C preferred shares, net	0	0	315,763
Repurchase of Series A and B preferred shares	0	0	-325,000
Purchases of common shares under share repurchase program	-454,137	-57,796	-172,056
Proceeds from common shares issued, net	6,827	3,051	7,033
Proceeds from borrowings	0	494,228	0
Repayments of borrowings	0	0	-310,868
Change in securities lending collateral	-57,470	55,643	-6,190
Third party investment in non-redeemable noncontrolling interests	796,903	0	0
Third party investment in redeemable noncontrolling interests	219,233	0	0
Dividends paid to redeemable noncontrolling interests	-14,448	0	0
Other	64,973	50,830	6,664
Preferred dividends paid	-21,938	-21,938	-28,381
Net Cash Used For Financing Activities	539,943	524,018	-513,035
Effects of exchange rate changes on foreign currency cash	-20,007	-4,357	6,742
Increase in cash	51,645	63,016	19,342
Cash beginning of year	434,057	371,041	351,699
Cash end of period	485,702	434,057	371,041
Income taxes paid (recovered)	20,923	15,288	-1,801
Interest paid	$46,429	$23,733	$29,133

General

General	12 Months Ended
	Dec. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
General	General
	Arch Capital Group Ltd. (“ACGL”) is a Bermuda public limited liability company which provides insurance and reinsurance on a worldwide basis through its wholly owned subsidiaries. ACGL was formed in September 2000 and became the sole shareholder of Arch Capital Group (U.S.) Inc. (“Arch-U.S.”) pursuant to an internal reorganization transaction completed in November 2000. In October 2001, the Company launched an underwriting initiative to meet current and future demand in the global insurance and reinsurance markets that included the recruitment of new management teams and an equity capital infusion of $763.2 million.
	On January 30, 2014, the Company acquired CMG Mortgage Insurance Company and its affiliated mortgage insurance companies (together, “CMG Entities”) and the mortgage insurance platform and related assets from PMI Mortgage Insurance Co., in Rehabilitation (“PMI”) (see Note 2).
	On March 20, 2014, the Company acquired approximately 11% of Watford Holdings Ltd.’s common equity and a warrant to purchase additional common equity for $100 million. Watford Holdings Ltd. is the parent of Watford Re Ltd., a newly-formed multi-line Bermuda reinsurance company (together with Watford Holdings Ltd., “Watford Re”). Watford Re is considered a variable interest entity (“VIE”) and the Company concluded that it is the primary beneficiary of Watford Re. As such, the results of Watford Re are included in the Company’s consolidated financial statements (see Note 4).
	As used herein, the “Company” means ACGL and its subsidiaries. Similarly, “Common Shares” means the common shares, par value $0.0033, of ACGL. The Company has reclassified the presentation of certain prior year information to conform to the current presentation. Such reclassifications had no effect on the Company’s net income, shareholders’ equity or cash flows. Tabular amounts are in U.S. Dollars in thousands, except share amounts, unless otherwise noted.

Business_Acquired

Business Acquired	12 Months Ended
	Dec. 31, 2014
Business Combinations [Abstract]
Business Acquired	Business Acquired
	On January 30, 2014, the Company’s U.S.-based subsidiaries completed the acquisition of the CMG Entities through a stock purchase agreement (“SPA”) from its previous owners, PMI, which has been in rehabilitation under the receivership of the Arizona Department of Insurance since 2011, and CMFG Life Insurance Company (“CUNA Mutual”). In addition, the Company entered into a distribution agreement with CUNA Mutual and a reinsurance agreement with an affiliate of CUNA Mutual. CMG Mortgage Insurance Company has been renamed “Arch Mortgage Insurance Company” (“Arch MI U.S.”). As part of the transaction, Arch MI U.S. obtained approval as an eligible mortgage insurer from Federal National Mortgage Association and Federal Home Loan Mortgage Corporation (each a government sponsored enterprise or “GSE”), subject to maintaining certain ongoing requirements.
	In addition, through an asset purchase agreement (“APA”) with PMI, the Company acquired the mortgage insurance operating platform of PMI, 100% of the capital stock of PMI Mortgage Assurance Co., a mortgage insurance company licensed in all 50 states (renamed Arch Mortgage Guaranty Company), and entered into a quota share reinsurance agreement pursuant to which Arch Reinsurance Ltd. (“Arch Re Bermuda”) agreed to provide 100% quota share indemnity reinsurance to PMI for all certificates of insurance that were issued by PMI between and including January 1, 2009 and December 31, 2011 that were not in default as of an agreed upon effective date. Other than this quota share, no PMI legacy exposures were assumed in the transaction. As part of the transaction, the Company entered into a services agreement with PMI to provide certain necessary operational services to administer the run-off of PMI’s legacy business at the direction of PMI. Arch MI U.S. also entered into a quota share reinsurance agreement whereby it will cede to PMI 20% of all new primary flow mortgage insurance business post-closing (both credit union and non-credit union business) on the first $25 billion in original loan amounts, on a funds-withheld basis.
	The completion of the SPA and APA transactions enabled the Company to enter the U.S. mortgage insurance marketplace and serve all lenders nationwide. The arrangements with CUNA Mutual also provided a seamless transition and enabled the Company to provide uninterrupted access and services to the credit union marketplace.
	At closing, the Company paid aggregate consideration of $160.6 million (80% of the actual closing date book value of the CMG Entities) under the SPA and $84.6 million under the APA. Additionally, the SPA contains provisions for contingent consideration payments, subject to an overall maximum payment of 150% of closing book value of the pre-closing portfolio of the CMG Entities as re-calculated over an earn-out period and payable at the third, fifth and sixth anniversaries after closing (subject to a one time extension period of one to three years at the sellers’ discretion). The maximum amount of contingent consideration payments is $136.9 million. To the extent that the adjusted book value of the CMG Entities drops below the cumulative amount paid by the Company, no additional payments would be due. To determine the fair value of the contingent consideration liability, the Company estimated future payments using a weighted average cost of capital approach at a rate of return of 15% which reflects the industry-weighted average rate of return on debt and equity as required by market participants. The fair value of the contingent consideration liability was $41.8 million at closing. The contingent consideration liability, which is included in ‘other liabilities’ in the consolidated balance sheets, is remeasured at fair value at each balance sheet date ($61.8 million at December 31, 2014) until the contingency is resolved with changes in fair value recognized in ‘net realized gains (losses).’
	The following table summarizes the fair value of net assets acquired and allocation of purchase price, measured as of the acquisition date:
		Total		Useful Life
	Purchase price
	Cash paid	$	245,157
	Contingent consideration liability	41,762
	Total purchase price (a)	$	286,919
	Assets acquired
	Cash	$	9,579
	Investments, at fair value	312,093
	Intangible asset -- acquired insurance contracts	46,473		5 years
	Intangible asset -- operating platform	29,900		5 years
	Intangible asset -- favorable lease contract	1,056		5 years
	Intangible asset -- insurance licenses	16,858		Indefinite
	Other assets acquired	21,691
	Total assets acquired	437,650
	Liabilities acquired
	Reserves for losses and loss adjustment expenses	$	121,572
	Unearned premiums	26,261
	Intangible liability -- unfavorable service contract	9,533		9 years
	Other liabilities acquired	7,217
	Total liabilities acquired	164,583
	Net assets acquired (b)	$	273,067
	Goodwill (a)-(b)	$	13,852
	From the acquisition date to December 31, 2014, the Company recorded amortization expense of $20.1 million related to net intangible assets acquired. The Company recognized goodwill of $13.9 million that is primarily attributed to PMI’s assembled workforce, access to the mortgage insurance market and additional synergies to be realized in the future. Under U.S. tax principles, which differentiate between taxable and non-taxable business combinations, the Company estimates that $48.0 million of goodwill is expected to be deductible for tax purposes.
	The Company includes the operations of Arch MI U.S. in its mortgage segment (see Note 5).

Significant_Accounting_Policie

Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Significant accounting policies	Significant Accounting Policies
	(a) Basis of Presentation
	The consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of ACGL and its subsidiaries, including Arch Re Bermuda, Arch Reinsurance Company (“Arch Re U.S.”), Arch-U.S., Arch Insurance Company, Arch Specialty Insurance Company, Arch Excess & Surplus Insurance Company, Arch Indemnity Insurance Company, Arch Insurance Canada Ltd. (“Arch Insurance Canada”), Arch Risk Transfer Services Ltd., Arch Reinsurance Europe Underwriting Limited (“Arch Re Europe”), Arch MI U.S., Arch Mortgage Guaranty Company, Arch Mortgage Insurance Limited (“Arch MI Europe”), Arch Insurance Company (Europe) Limited (“Arch Insurance Company Europe”), Lloyd’s of London syndicate 2012 and related companies (“Arch Syndicate 2012”) and Watford Re. All significant intercompany transactions and balances have been eliminated in consolidation. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates and assumptions.
	(b) Premium Revenues and Related Expenses
	Insurance. Insurance premiums written are generally recorded at the policy inception and are primarily earned on a pro rata basis over the terms of the policies for all products, usually 12 months. Premiums written include estimates in the Company’s programs, specialty lines, lenders products business and for participation in involuntary pools. Such premium estimates are derived from multiple sources which include the historical experience of the underlying business, similar business and available industry information. Unearned premium reserves represent the portion of premiums written that relates to the unexpired terms of in-force insurance policies.
	Reinsurance. Reinsurance premiums written include amounts reported by brokers and ceding companies, supplemented by the Company’s own estimates of premiums where reports have not been received. The determination of premium estimates requires a review of the Company’s experience with the ceding companies, familiarity with each market, the timing of the reported information, an analysis and understanding of the characteristics of each line of business, and management’s judgment of the impact of various factors, including premium or loss trends, on the volume of business written and ceded to the Company. On an ongoing basis, the Company’s underwriters review the amounts reported by these third parties for reasonableness based on their experience and knowledge of the subject class of business, taking into account the Company’s historical experience with the brokers or ceding companies. In addition, reinsurance contracts under which the Company assumes business generally contain specific provisions which allow the Company to perform audits of the ceding company to ensure compliance with the terms and conditions of the contract, including accurate and timely reporting of information. Based on a review of all available information, management establishes premium estimates where reports have not been received. Premium estimates are updated when new information is received and differences between such estimates and actual amounts are recorded in the period in which estimates are changed or the actual amounts are determined.
	Reinsurance premiums written are recorded based on the type of contracts the Company writes. Premiums on the Company’s excess of loss and pro rata reinsurance contracts are estimated when the business is underwritten. For excess of loss contracts, premiums are recorded as written based on the terms of the contract. Estimates of premiums written under pro rata contracts are recorded in the period in which the underlying risks are expected to incept and are based on information provided by the brokers and the ceding companies. For multi-year reinsurance treaties which are payable in annual installments, generally, only the initial annual installment is included as premiums written at policy inception due to the ability of the reinsured to commute or cancel coverage during the term of the policy. The remaining annual installments are included as premiums written at each successive anniversary date within the multi-year term.
	Reinstatement premiums for the Company’s insurance and reinsurance operations are recognized at the time a loss event occurs, where coverage limits for the remaining life of the contract are reinstated under pre-defined contract terms. Reinstatement premiums, if obligatory, are fully earned when recognized. The accrual of reinstatement premiums is based on an estimate of losses and loss adjustment expenses, which reflects management’s judgment.
	Premium estimates are reviewed by management at least quarterly. Such review includes a comparison of actual reported premiums to expected ultimate premiums along with a review of the aging and collection of premium estimates. Based on management’s review, the appropriateness of the premium estimates is evaluated, and any adjustment to these estimates is recorded in the period in which it becomes known. Adjustments to premium estimates could be material and such adjustments could directly and significantly impact earnings favorably or unfavorably in the period they are determined because the estimated premium may be fully or substantially earned. A significant portion of amounts included as premiums receivable, which represent estimated premiums written, net of commissions, are not currently due based on the terms of the underlying contracts.
	Reinsurance premiums written, irrespective of the class of business, are generally earned on a pro rata basis over the terms of the underlying policies or reinsurance contracts. Contracts and policies written on a “losses occurring” basis cover claims that may occur during the term of the contract or policy, which is typically 12 months. Accordingly, the premium is earned evenly over the term. Contracts which are written on a “risks attaching” basis cover claims which attach to the underlying insurance policies written during the terms of such contracts. Premiums earned on such contracts usually extend beyond the original term of the reinsurance contract, typically resulting in recognition of premiums earned over a 24-month period. Certain of the Company’s reinsurance contracts include provisions that adjust premiums or acquisition expenses based upon the experience under the contracts. Premiums written and earned, as well as related acquisition expenses, are recorded based upon the projected experience under such contracts.
	The Company also writes certain reinsurance business that is intended to provide insurers with risk management solutions that complement traditional reinsurance. Under these contracts, the Company assumes a measured amount of insurance risk in exchange for an anticipated margin, which is typically lower than on traditional reinsurance contracts. The terms and conditions of these contracts may include additional or return premiums based on loss experience, loss corridors, sublimits and caps. Examples of such business include aggregate stop-loss coverages, financial quota share coverages and multi-year retrospectively rated excess of loss coverages. If these contracts are deemed to transfer risk, they are accounted for as reinsurance.
	Mortgage. Mortgage guaranty insurance policies are contracts that are generally non-cancelable by the insurer, are renewable at a fixed price, and provide for payment of premiums on a monthly, annual or single basis. Upon renewal, the Company is not able to re-underwrite or re-price its policies. Consistent with industry accounting practices, premiums written on a monthly basis are earned as coverage is provided. Premiums written on an annual basis are amortized on a monthly pro rata basis over the year of coverage. Primary mortgage insurance premiums written on policies covering more than one year are referred to as single premiums. A portion of the revenue from single premiums is recognized in premiums earned in the current period, and the remaining portion is deferred as unearned premiums and earned over the estimated expiration of risk of the policy. If single premium policies related to insured loans are canceled due to repayment by the borrower and the policy is a non-refundable product, the remaining unearned premium related to each canceled policy is recognized as earned premium upon notification of the cancellation.
	Unearned premiums represent the portion of premiums written that is applicable to the estimated unexpired risk of insured loans. A portion of premium payments may be refundable if the insured cancels coverage, which generally occurs when the loan is repaid, the loan amortizes to a sufficiently low amount to trigger a lender permitted or legally required cancellation, or the value of the property has increased sufficiently in accordance with the terms of the contract. Premium refunds reduce premiums earned in the consolidated statements of income. Generally, only unearned premiums are refundable. However, when the Company pays a claim on a delinquent loan, all servicer paid premiums received on the delinquent loan covering any period after the default date will be refunded, in accordance with the terms of the contract.
	Acquisition Costs. Acquisition expenses and other expenses related to the Company’s underwriting operations that vary with, and are directly related to, the successful acquisition or renewal of business are deferred and amortized based on the type of contract. For property and casualty insurance and reinsurance contracts, deferred acquisition costs are amortized over the period in which the related premiums are earned. Consistent with mortgage insurance industry accounting practice, amortization of acquisition costs related to the mortgage insurance contracts for each underwriting year’s book of business is recorded in proportion to estimated gross profits. Estimated gross profits are comprised of earned premiums and losses and loss adjustment expenses. For each underwriting year, the Company estimates the rate of amortization to reflect actual experience and any changes to persistency or loss development.
	Acquisition expenses, net of ceding commissions received from reinsurers, consist principally of commissions and premium taxes paid to obtain the Company’s business. Other operating expenses also include expenses that vary with, and are directly related to, the successful acquisition of business. Deferred acquisition costs are carried at their estimated realizable value and take into account anticipated losses and loss adjustment expenses, based on historical and current experience, and anticipated investment income.
	A premium deficiency occurs if the sum of anticipated losses and loss adjustment expenses, unamortized acquisition costs and maintenance costs exceed unearned premiums (including expected future premiums) and anticipated investment income. A premium deficiency reserve (“PDR”) is recorded by charging any unamortized acquisition costs to expense to the extent required in order to eliminate the deficiency. If the premium deficiency exceeds unamortized acquisition costs then a liability is accrued for the excess deficiency.
	To assess the need for a PDR on mortgage exposures, the Company develops loss projections based on modeled loan defaults related to its current policies in force. This projection is based on recent trends in default experience, severity and rates of defaulted loans moving to claim, as well as recent trends in the rate at which loans are prepaid. Evaluating the expected profitability of the Company’s existing mortgage insurance business and the need for a PDR for its mortgage business involves significant reliance upon assumptions and estimates with regard to the likelihood, magnitude and timing of potential losses and premium revenues.
	No premium deficiency charges were recorded by the Company during 2014, 2013 or 2012.
	(c) Deposit Accounting
	Certain assumed reinsurance contracts, which pursuant to Financial Accounting Standards Board (“FASB”) guidance regarding the accounting and reporting for reinsurance and short-duration and long-duration contracts, that are deemed not to transfer insurance risk, are accounted for using the deposit method of accounting as prescribed in the FASB guidance. However, it is possible that the Company could incur financial losses on such contracts. Management exercises significant judgment in the assumptions used in determining whether assumed contracts should be accounted for as reinsurance contracts or deposit contracts. Under the FASB guidance, for those contracts that contain only significant underwriting risk, the estimated profit margin is deferred and amortized over the contract period and such amount is included in the Company’s underwriting results. When the estimated profit margin is explicit, the margin is reflected as other underwriting income and any adverse financial results on such contracts are reflected as incurred losses. When the estimated profit margin is implicit, the margin is reflected as an offset to paid losses and any adverse financial results on such contracts are reflected as incurred losses. Additional judgments are required when applying the accounting guidance with respect to the revenue recognition criteria for contracts deemed to transfer only significant underwriting risk. For those contracts that contain only significant timing risk, an accretion rate is established at inception of the contract based on actuarial estimates whereby the deposit accounting liability is increased to the estimated amount payable over the contract term. The accretion on the deposit is based on the expected rate of return required to fund the expected future payment obligations. Periodically the Company reassesses the estimated ultimate liability and the related expected rate of return. The accretion of the deposit accounting liability as well as changes to the estimated ultimate liability and the accretion rate are reflected as part of interest expense in the Company’s results of operations.
	Deposit accounting liabilities totaled $327.4 million and $421.3 million, respectively, at December 31, 2014 and 2013. Under some of these contracts, the ceding company retains the related assets on a funds-held basis. In those instances, the Company records the asset as a deposit accounting asset. Deposit accounting assets of $284.4 million and $370.5 million, respectively, at December 31, 2014 and 2013 are included in “Other assets” on the Company’s balance sheet. Interest income produced by those assets are recorded as part of net investment income in the Company's results of operations.
	(d) Retroactive Accounting
	Retroactive reinsurance reimburses a ceding company for liabilities incurred as a result of past insurable events covered by the underlying policies reinsured. In certain instances, reinsurance contracts cover losses both on a prospective basis and on a retroactive basis and, accordingly, the Company bifurcates the prospective and retrospective elements of these reinsurance contracts and accounts for each element separately. Underwriting income generated in connection with retroactive reinsurance contracts is deferred and amortized into income over the settlement period while losses are charged to income immediately. Subsequent changes in estimated or actual cash flows under such retroactive reinsurance contracts are accounted for by adjusting the previously deferred amount to the balance that would have existed had the revised estimate been available at the inception of the reinsurance transaction, with a corresponding charge or credit to income.
	(e) Reinsurance Ceded
	In the normal course of business, the Company purchases reinsurance to increase capacity and to limit the impact of individual losses and events on its underwriting results by reinsuring certain levels of risk with other insurance enterprises or reinsurers. The Company uses pro rata, excess of loss and facultative reinsurance contracts. Reinsurance ceding commissions are recognized as a reduction to acquisition costs on a pro rata basis over the period of risk while the portion of such commissions that will be earned in the future is deferred. The accompanying consolidated statement of income reflects premiums and losses and loss adjustment expenses and acquisition costs, net of reinsurance ceded. See Note 7 for information on the Company's reinsurance usage. Ceded unearned premiums are reported as prepaid reinsurance premiums and estimated amounts of reinsurance recoverable on unpaid losses are reported as unpaid losses and loss adjustment expenses recoverable. Reinsurance premiums ceded and unpaid losses and loss adjustment expenses recoverable are estimated in a manner consistent with that of the original policies issued and the terms of the reinsurance contracts. If the reinsurers are unable to satisfy their obligations under the agreements, the Company’s insurance or reinsurance subsidiaries would be liable for such defaulted amounts.
	(f) Cash
	Cash includes cash equivalents, which are investments with original maturities of three months or less that are not managed by external or internal investment advisors.
	(g) Investments
	The Company currently classifies substantially all of its fixed maturity investments, equity securities and short-term investments as “available for sale” and, accordingly, they are carried at estimated fair value (also known as fair value) with the changes in fair value recorded as an unrealized gain or loss component of accumulated other comprehensive income in shareholders’ equity. The fair value of fixed maturity securities and equity securities is generally determined from quotations received from nationally recognized pricing services, or when such prices are not available, by reference to broker or underwriter bid indications. Short-term investments comprise securities due to mature within one year of the date of issue. Short-term investments include certain cash equivalents which are part of investment portfolios under the management of external and internal investment managers.
	The Company participates in a securities lending program as a mechanism for generating additional interest income on its fixed income portfolio. Under the security lending agreements, certain of its fixed income portfolio securities are loaned to third parties, primarily major brokerage firms, for short periods of time through a lending agent. Such securities have been reclassified as “Fixed maturities and short-term investments pledged under securities lending agreements, at fair value.” The Company maintains legal control over the securities it lends, retains the earnings and cash flows associated with the loaned securities and receives a fee from the borrower for the temporary use of the securities. Collateral received, primarily in the form of cash, is required at a rate of 102% of the fair value of the loaned securities including accrued investment income and is monitored and maintained by the lending agent. Such collateral is reinvested and is reflected as “Investment of funds received under securities lending agreements, at fair value.”
	The Company’s investment portfolio includes certain funds that, due to their ownership structure, are accounted for by the Company using the equity method. In applying the equity method, these investments are initially recorded at cost and are subsequently adjusted based on the Company’s proportionate share of the net income or loss of the funds (which include changes in the fair value of the underlying securities in the funds). Such investments are generally recorded on a one to three month lag based on the availability of reports from the investment funds. Changes in the carrying value of such investments are recorded in net income as “Equity in net income (loss) of investment funds accounted for using the equity method.” As such, fluctuations in the carrying value of the investment funds accounted for using the equity method may increase the volatility of the Company’s reported results of operations.
	Other investments include funds and separately managed accounts with holdings in Asian and emerging markets, fixed maturities, term loans and other investment strategies. The fair value for certain of the Company’s other investments are determined using net asset values (“NAVs”) as advised by external fund managers. The NAV is based on the fund manager’s valuation of the underlying holdings in accordance with the fund’s governing documents. Certain of the funds are accounted for as available for sale equity securities, regardless of the nature of the investments held within the fund.
	The Company elected to carry certain fixed maturity securities, equity securities and other investments at fair value under the fair value option afforded by accounting guidance regarding the fair value option for financial assets and liabilities. Changes in fair value of investments accounted for using the fair value option are included in “Net realized gains (losses).” The primary reasons for electing the fair value option were to address simplification and cost-benefit considerations.
	The Company invests in limited partner equity interests issued by third party variable interest entities (“VIE”). Such amounts are included in investments accounted for using the equity method, other investments available for sale and investments accounted for using the fair value option. A VIE refers to entities that have characteristics such as (i) insufficient equity at risk to allow the entity to finance its activities without additional financial support or (ii) instances where the equity investors, as a group, do not have the characteristic of a controlling financial interest. If the Company is determined to be the primary beneficiary, it is required to consolidate the VIE. The primary beneficiary is defined as the variable interest holder that is determined to have the controlling financial interest as a result of having both (i) the power to direct the activities of a VIE that most significantly impact the economic performance of the VIE and (ii) the obligation to absorb losses or right to receive benefits from the VIE that could potentially be significant to the VIE. At inception of the VIE as well as on an ongoing basis, the Company determines whether it is the primary beneficiary based on an analysis of the Company’s level of involvement in the VIE, the contractual terms, and the overall structure of the VIE. The Company's maximum exposure to loss with respect to these investments is limited to the investment carrying amounts reported in the Company's consolidated balance sheet and any unfunded commitment.
	The Company performs quarterly reviews of its investments to determine whether declines in fair value below the cost basis are considered other-than-temporary in accordance with applicable accounting guidance regarding the recognition and presentation of other-than-temporary impairments (“OTTI”). The process of determining whether a security is other-than-temporarily impaired requires judgment and involves analyzing many factors. These factors include (i) an analysis of the liquidity, business prospects and overall financial condition of the issuer, (ii) the time period in which there was a significant decline in value, (iii) the significance of the decline and (iv) the analysis of specific credit events. The Company evaluates the unrealized losses of its equity securities by issuer and forecasts a reasonable period of time by which the fair value of the securities would increase and the Company would recover its cost. If the Company is unable to forecast a reasonable period of time in which to recover the cost of its equity securities, a net impairment loss in earnings equivalent to the entire unrealized loss is recognized.
	When there are credit-related losses associated with debt securities for which the Company does not have an intent to sell and it is more likely than not that it will not be required to sell the security before recovery of its cost basis, the amount of the OTTI related to a credit loss is recognized in earnings and the amount of the OTTI related to other factors (e.g., interest rates, market conditions, etc.) is recorded as a component of other comprehensive income (loss). The amount of the credit loss of an impaired debt security is the difference between the amortized cost and the greater of (i) the present value of expected future cash flows and (ii) the fair value of the security. In instances where no credit loss exists but it is more likely than not that the Company will have to sell the debt security prior to the anticipated recovery, the decline in fair value below amortized cost is recognized as an OTTI in earnings. In periods after the recognition of an OTTI on debt securities, the Company accounts for such securities as if they had been purchased on the measurement date of the OTTI at an amortized cost basis equal to the previous amortized cost basis less the OTTI recognized in earnings. For debt securities for which OTTI were recognized in earnings, the difference between the new amortized cost basis and the cash flows expected to be collected will be accreted or amortized into net investment income. See Note 8, “Investment Information—Other-Than-Temporary Impairments” for additional information.
	Net investment income includes interest and dividend income together with amortization of market premiums and discounts and is net of investment management and custody fees. Anticipated prepayments and expected maturities are used in applying the interest method for certain investments such as mortgage and other asset-backed securities. When actual prepayments differ significantly from anticipated prepayments, the effective yield is recalculated to reflect actual payments to date and anticipated future payments. The net investment in such securities is adjusted to the amount that would have existed had the new effective yield been applied since the acquisition of the security. Such adjustments, if any, are included in net investment income when determined. Equity in net income (loss) of investment funds accounted for using the equity method includes changes in the fair value of certain alternative investments accounted for under the equity method.
	Investment gains or losses realized on the sale of investments, except for certain fund investments, are determined on a first-in, first-out basis and are reflected in net income. Investments gains or losses realized on the sale of certain fund investments are determined on an average cost basis. Unrealized appreciation or decline in the value of securities, which are carried at fair value, is excluded from net income and recorded as a separate component of accumulated other comprehensive income, net of applicable deferred income tax.
	(h) Derivative Instruments
	The Company recognizes all derivative instruments, including embedded derivative instruments, at fair value in the Consolidated Balance Sheets. The Company employs the use of derivative instruments within its operations to mitigate risks arising from assets and liabilities held in foreign currencies as well as part of its overall investment strategy. For such instruments, changes in assets and liabilities measured at fair value are recorded as “Net realized gains” in the consolidated statements of income. In addition, the Company’s derivative instruments include amounts related to underwriting activities where an insurance or reinsurance contract meets the accounting definition of a derivative instrument. For such contracts, changes in fair value are reflected in “Other underwriting income” in the consolidated statements of income as the underlying contract originates from the Company’s underwriting operations. For the periods ended 2014, 2013, and 2012, the Company did not designate any derivative instruments as hedges under the relevant accounting guidance. See Note 11 for information on the Company’s derivative instruments.
	(i) Reserves for Losses and Loss Adjustment Expenses
	Insurance and Reinsurance. The reserve for losses and loss adjustment expenses consists of estimates of unpaid reported losses and loss adjustment expenses and estimates for losses incurred but not reported. The reserve for unpaid reported losses and loss adjustment expenses, established by management based on reports from ceding companies and claims from insureds, excludes estimates of amounts related to losses under high deductible policies, and represents the estimated ultimate cost of events or conditions that have been reported to or specifically identified by the Company. Such reserves are supplemented by management’s estimates of reserves for losses incurred for which reports or claims have not been received. The Company’s reserves are based on a combination of reserving methods, incorporating both Company and industry loss development patterns. The Company selects the initial expected loss and loss adjustment expense ratios based on information derived by its underwriters and actuaries during the initial pricing of the business, supplemented by industry data where appropriate. Such ratios consider, among other things, rate changes and changes in terms and conditions that have been observed in the market. These estimates are reviewed regularly and, as experience develops and new information becomes known, the reserves are adjusted as necessary. Such adjustments, if any, are reflected in income in the period in which they are determined. As actual loss information has been reported, the Company has developed its own loss experience and its reserving methods include other actuarial techniques. Over time, such techniques have been given further weight in its reserving process based on the continuing maturation of the Company’s reserves. Inherent in the estimates of ultimate losses and loss adjustment expenses are expected trends in claims severity and frequency and other factors which may vary significantly as claims are settled. Accordingly, ultimate losses and loss adjustment expenses may differ materially from the amounts recorded in the accompanying consolidated financial statements. Losses and loss adjustment expenses are recorded on an undiscounted basis, except for excess workers’ compensation and employers’ liability business written by the Company’s insurance operations.
	Mortgage. The reserves for mortgage guaranty insurance losses and loss adjustment expenses are the estimated claim settlement costs on notices of default that have been received by the Company, as well as loan defaults that have been incurred but have not been reported by the lenders. Consistent with industry accounting practice, the Company does not establish loss reserves for future claims on insured loans that are not currently in default (defined as two consecutive missed payments). The Company establishes loss reserves on a case-by-case basis when insured loans are identified as currently in default using estimated claim rates and average claim sizes for each report year, net of any salvage recoverable. The Company also reserves for defaults that have occurred but have not yet been reported to the Company prior to the close of an accounting period. To determine this reserve, the Company estimates the number of defaults not yet reported using historical information regarding late reported delinquencies and applies estimated claim rates and claim sizes for the estimated defaults not yet reported.
	The establishment of reserves across the Company’s segments is an inherently uncertain process, are necessarily based on estimates, and the ultimate net cost may vary from such estimates. The methods for making such estimates and for establishing the resulting liability are reviewed and updated using the most current information available. Any resulting adjustments, which may be material, are reflected in current operations.
	(j) Contractholder Receivables and Payables
	Certain insurance policies written by the Company’s insurance operations feature large deductibles, primarily in its construction and national accounts lines of business. Under such contracts, the Company is obligated to pay the claimant for the full amount of the claim. The Company is subsequently reimbursed by the policyholder for the deductible amount. These amounts are included on a gross basis in the consolidated balance sheet in contractholder payables and contractholder receivables, respectively. In the event that the Company is unable to collect from the policyholder, the Company would record an increase to losses and loss adjustment expenses related to such policy.
	(k) Foreign Exchange
	Assets and liabilities of foreign operations whose functional currency is not the U.S. Dollar are translated at the prevailing exchange rates at each balance sheet date. Revenues and expenses of such foreign operations are translated at average exchange rates during the year. The net effect of the translation adjustments for foreign operations is included in accumulated other comprehensive income, net of applicable deferred income tax. Monetary assets and liabilities, such as premiums receivable and the reserve for losses and loss adjustment expenses, denominated in foreign currencies are revalued at the exchange rate in effect at the balance sheet date with the resulting foreign exchange gains and losses included in net income. Accounts that are classified as non-monetary, such as deferred acquisition costs and the unearned premium reserves, are not revalued. In the case of foreign currency denominated fixed maturity securities which are classified as “available for sale,” the change in exchange rates between the local currency in which the investments are denominated and the Company’s functional currency at each balance sheet date is included in unrealized appreciation or decline in value of securities, a component of accumulated other comprehensive income, net of applicable deferred income tax.
	(l) Income Taxes
	Deferred income taxes reflect the expected future tax consequences of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and amounts used for income tax purposes. A valuation allowance is recorded if it is more likely than not that some or all of a deferred tax asset may not be realized. The Company considers future taxable income and feasible tax planning strategies in assessing the need for a valuation allowance. In the event the Company determines that it will not be able to realize all or part of its deferred income tax assets in the future, an adjustment to the deferred income tax assets would be charged to income in the period in which such determination is made. In addition, if the Company subsequently assesses that the valuation allowance is no longer needed, a benefit would be recorded to income in the period in which such determination is made. See Note 14 for more information.
	The Company recognizes a tax benefit where it concludes that it is more likely than not that the tax benefit will be sustained on audit by the taxing authority based solely on the technical merits of the associated tax position. If the recognition threshold is met, the Company recognizes a tax benefit measured at the largest amount of the tax benefit that, in the Company’s judgment, is greater than 50% likely to be realized. The Company records interest and penalties related to unrecognized tax benefits in the provision for income taxes.
	(m) Share-Based Payment Arrangements
	The Company applies a fair value based measurement method in accounting for its share-based payment arrangements with eligible employees and directors. Compensation expense is estimated based on the fair value of the award at the grant date and is recognized in net income over the requisite service period with a corresponding increase in shareholders’ equity. No value is attributed to awards that employees forfeit because they fail to satisfy vesting conditions. As such, the number of shares granted is reduced by assumed forfeitures and adjusted based on actual forfeitures until vesting. The Company’s share-based payment arrangements generally vest over a three year period with one-third vesting on the first, second and third anniversaries of the grant date. The share-based compensation expense associated with such awards that have graded vesting features and vest based on service conditions only is calculated on a straight-line basis over the requisite service period for the entire award. For awards granted to retirement-eligible employees where no service is required for the employee to retain the award, the grant date fair value is immediately recognized as compensation expense at the grant date because the employee is able to retain the award without continuing to provide service. For employees near retirement eligibility, attribution of compensation cost is over the period from the grant date to the retirement eligibility date. In November 2012, the Company issued off-cycle share-based awards, which will cliff vest on the fifth anniversary of the grant date. The expense for such grant is being amortized on a straight-line basis over the five-year requisite service period. The off-cycle awards have similar terms as the annual award agreements, however with respect to retirement eligible employees service is required for the employee to retain the award. Retirement-eligible employees retiring prior to the fifth anniversary of the grant date will vest in a pro-rated portion of the award commensurate with the service performed. Such employees will receive the vested portion at the end of the five-year cliff period. These charges had no impact on the Company’s cash flows or total shareholders’ equity. See Note 20 for information relating to the Company’s share-based payment awards.
	(n) Guaranty Fund and Other Related Assessments
	Liabilities for guaranty fund and other related assessments in the Company’s insurance and reinsurance operations are accrued when the Company receives notice that an amount is payable, or earlier if a reasonable estimate of the assessment can be made.
	(o) Treasury Shares
	Treasury shares are common shares purchased by the Company and not subsequently canceled. These shares are recorded at cost and result in a reduction of the Company’s shareholders’ equity in its Consolidated Balance Sheets.
	(p) Goodwill and Intangible Assets
	Goodwill represents the excess of the purchase price of an acquisition over the fair value of the net assets acquired and is assigned to the applicable reporting unit at acquisition. Goodwill is not amortized and is evaluated for impairment on an annual basis. Impairment tests may be performed more frequently if the facts and circumstances indicate a possible impairment. In performing impairment tests, the Company may first assess qualitative factors to determine whether it is more likely than not (that is, more than a 50% probability) that the fair value of a reporting unit exceeds its carrying amount as a basis for determining whether it is necessary to perform the two-step goodwill impairment test described in the accounting guidance.
	Indefinite-lived intangible assets, such as insurance licenses, are not amortized and are evaluated for impairment similar to goodwill. Finite-lived intangible assets and liabilities include the value of acquired insurance and reinsurance contracts, which are estimated based on the present value of future expected cash flows and amortized in ‘acquisition expenses’ in proportion to the estimated profits expected to be realized. Other finite-lived intangible assets or liabilities, including favorable or unfavorable contracts, are amortized in ‘other operating expenses’ over their useful lives. Finite-lived intangible assets and liabilities are periodically reviewed for indicators of impairment. An impairment is recognized when the carrying amount is not recoverable from its undiscounted cash flows and is measured as the difference between the carrying amount and fair value.
	If goodwill or intangible assets are impaired, such assets are written down to their fair values with the related expense recorded in the Company’s results of operations. See Note 18 for information relating to the Company’s goodwill and intangible assets.
	(q) Recent Accounting Pronouncements
	A new accounting standard issued in the 2014 second quarter will change the manner in which most companies recognize revenue. The standard requires that revenue reflect the transfer of goods or services to customers based on the consideration or payment the company expects to be entitled to in exchange for those goods or services; however, the standard does not change the accounting for insurance contracts or financial instruments. The new standard also requires enhanced disclosures about revenue. This accounting guidance is effective in the 2017 first quarter and may be applied on a full retrospective or modified retrospective approach. The Company is currently assessing the impact the implementation of this standard will have on its consolidated financial statements.
	A new accounting standard was issued in the 2015 first quarter providing targeted improvements to consolidation guidance for limited partnerships and other similarly structured entities. The new standard addresses instances where a reporting entity consolidates another entity when the reporting entity is simply acting on the behalf of others, amongst other related issues. While the standard is targeted, the application is relevant for all companies that are required to assess whether or not to consolidate certain entities. The standard is effective in the 2016 first quarter and early adoption is permitted. The Company is currently assessing the impact the implementation of this standard will have on its consolidated financial statements.

Variable_Interest_Entity_and_N

Variable Interest Entity and Noncontrolling Interests	12 Months Ended
	Dec. 31, 2014
Variable Interest Entity and Noncontrolling Interest Disclosure [Abstract]
Variable Interest Entity and Noncontrolling Interests	Variable Interest Entity and Noncontrolling Interests
	Variable interest entity
	A VIE refers to an entity that has characteristics such as (i) insufficient equity at risk to allow the entity to finance its activities without additional financial support or (ii) instances where the equity investors, as a group, do not have characteristics of a controlling financial interest. The primary beneficiary of a VIE is defined as the variable interest holder that is determined to have the controlling financial interest as a result of having both (i) the power to direct the activities of a VIE that most significantly impact the economic performance of the VIE and (ii) the obligation to absorb losses or right to receive benefits from the VIE that could potentially be significant to the VIE. If a company is determined to be the primary beneficiary, it is required to consolidate the VIE in its financial statements.
	In March 2014, Watford Re raised approximately $1.1 billion of capital consisting of $907.3 million in common equity ($895.6 million net of issuance costs) and $226.6 million in preference equity ($219.2 million net of issuance costs and discount). The Company invested $100.0 million and acquired approximately 11% of Watford Holdings Ltd.’s common equity and a warrant to purchase additional common equity. Arch Underwriters Ltd. (“AUL”), a subsidiary of the Company, acts as Watford Re’s reinsurance underwriting manager, and Highbridge Principal Strategies, LLC (“Highbridge”), a subsidiary of JPMorgan Chase & Co., manages Watford Re’s investment assets, each under separate long term services agreements. In connection with the capital raise at Watford Re, warrants to purchase a total of 1.7 million common shares were issued to the Company and Highbridge. The warrants are only exercisable if Watford Re has consummated an initial public offering of its common shares or otherwise effected a listing of its common shares on a U.S. national securities exchange and certain targeted returns are achieved for existing common shareholders. The warrants expire on March 25, 2020. John Rathgeber, previously Vice Chairman of Arch Worldwide Reinsurance Group, was named CEO of Watford Re. In addition, Marc Grandisson, Chairman and CEO of Arch Worldwide Reinsurance and Mortgage Groups, and Nicolas Papadopoulo, CEO Reinsurance Group, were appointed to the board of directors of Watford Re.
	The Company concluded that Watford Re is a VIE due to both the reinsurance management services agreement with AUL and the investment management agreement with Highbridge. Both of these agreements provide for services for an extended period of time with limited termination rights by Watford Re. In addition, these agreements allow for both AUL and Highbridge to participate in the favorable results of Watford Re in the form of performance fees. To determine if the Company is the primary beneficiary of Watford Re, the Company concluded that the most significant activity of Watford Re pertains to the insurance activities arising from the reinsurance management services agreement, as these activities will ultimately generate the investable assets required by Highbridge to execute the investment strategy. In addition, the Company factored into its analysis qualitative aspects of its relationship with Watford Re that are indicative of power to direct the insurance activities. These aspects coupled with the Company’s board seats and a former executive of the Company serving as Watford Re’s CEO resulted in the Company concluding that it is the primary beneficiary of Watford Re. As such, the results of Watford Re are included in the Company’s consolidated financial statements.
	The Company concluded that Watford Re represents a separate operating segment and provides the income statement and total investable assets, total assets and total liabilities of Watford Re within Note 5. At December 31, 2014, Watford Re’s liabilities included unearned premiums of $180.0 million and reserves for losses and loss adjustment expenses of $66.3 million, some of which is related to transactions with the Company. During 2014, Watford Re generated $39.3 million of cash provided by operating activities and $1.16 billion of cash provided by financing activities, partially offset by $1.18 billion of cash used for investing activities.
	Because Watford Re is an independent company, the assets of Watford Re can be used only to settle obligations of Watford Re and Watford Re is solely responsible for its own liabilities and commitments. The Company’s financial exposure to Watford Re is limited to its investment in Watford Re’s common shares and counterparty credit risk (mitigated by collateral) arising from the reinsurance transactions.
	Non-redeemable noncontrolling interests
	The Company accounts for the portion of Watford Re’s common equity attributable to third party investors in the shareholders’ equity section of its consolidated balance sheets. The noncontrolling ownership in Watford Re’s common shares was approximately 89% at December 31, 2014. The following table sets forth activity in the non-redeemable noncontrolling interests:
		Year Ended December 31, 2014
	Balance, beginning of year	$	—
	Sale of shares to noncontrolling interests	796,904
	Amounts attributable to noncontrolling interests	(27,823		)
	Balance, end of year	$	769,081
	Redeemable noncontrolling interests
	The Company accounts for redeemable noncontrolling interests in the mezzanine section of its consolidated balance sheets in accordance with applicable accounting guidance. Such redeemable noncontrolling interests represent the 9,065,200 cumulative redeemable preference shares (“Watford Preference Shares”) issued in late March 2014 with a par value of $0.01 per share and a liquidation preference of $25.00 per share. The Watford Preference Shares were issued at a discounted amount of $24.50 per share. Holders of the Watford Preference Shares will be entitled to receive, if declared by Watford Re’s board, quarterly cash dividends on the last day of March, June, September, and December. Dividends will accrue from the closing date to June 30, 2019 at a fixed rate of 8.5% per annum. From June 30, 2019 and subsequent, dividends will accrue based on a floating rate equal to the 3 month U.S. dollar LIBOR (with a 1% floor) plus a margin based on the difference between the fixed rate and the 5 year mid swap rate to the floating rate as set out on the Bloomberg Screen IRSB 18. The Watford Preference Shares may be redeemed by Watford Re on or after June 30, 2019 or at the option of the preferred shareholders at any time on or after June 30, 2034. Because the redemption features are not solely within the control of Watford Re, the Company accounts for the redeemable noncontrolling interests in the Watford Preference Shares in the mezzanine section of its consolidated balance sheets. Third party investors own 100% of the Watford Preference Shares at December 31, 2014. Preferred dividends on the Watford Preference Shares, including the accretion of the discount and issuance costs, totaled $14.7 million for 2014.
	The portion of Watford Re’s income or loss attributable to third party investors is recorded in the consolidated statements of income in ‘amounts attributable to noncontrolling interests.’ For 2014, amounts attributable to noncontrolling interests of $13.1 million reflected the net impact of amounts attributable to non-redeemable noncontrolling interests of $27.8 million and amounts attributable to redeemable noncontrolling interests of $14.7 million, as described above.

Segment_Information

Segment Information	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Segment Information	Segment Information
	During the 2014 first quarter, to reflect activity during the period as described below, the Company changed its segment structure and added two new segments (mortgage and ‘other’). The Company now classifies its businesses into three underwriting segments — insurance, reinsurance and mortgage — and two other operating segments — ‘other’ and corporate (non-underwriting). The Company determined its reportable segments using the management approach described in accounting guidance regarding disclosures about segments of an enterprise and related information. The accounting policies of the segments are the same as those used for the preparation of the Company’s consolidated financial statements. Intersegment business is allocated to the segment accountable for the underwriting results.
	The Company’s insurance, reinsurance and mortgage segments each have managers who are responsible for the overall profitability of their respective segments and who are directly accountable to the Company’s chief operating decision makers, the Chairman, President and Chief Executive Officer of ACGL and the Chief Financial Officer of ACGL. The chief operating decision makers do not assess performance, measure return on equity or make resource allocation decisions on a line of business basis. Management measures segment performance for its three underwriting segments based on underwriting income or loss. The Company does not manage its assets by underwriting segment, with the exception of goodwill and intangible assets, and, accordingly, investment income is not allocated to each underwriting segment.
	The insurance segment consists of the Company’s insurance underwriting units which offer specialty product lines on a worldwide basis. Product lines include: construction and national accounts; excess and surplus casualty; lenders products; professional lines; programs; property, energy, marine and aviation; travel, accident and health; and other (consisting of alternative markets, excess workers' compensation and surety business).
	The reinsurance segment consists of the Company’s reinsurance underwriting units which offer specialty product lines on a worldwide basis. Product lines include: casualty; marine and aviation; other specialty; property catastrophe; property excluding property catastrophe (losses on a single risk, both excess of loss and pro rata); and other (consisting of life reinsurance, casualty clash and other).
	The mortgage segment consists of the Company’s mortgage insurance and reinsurance business and includes the results of Arch MI U.S. Arch MI U.S. was approved as an eligible mortgage insurer by the GSEs (see Note 2) and provides mortgage insurance to banks and other lenders nationwide, including existing credit union customers. The mortgage segment also provides reinsurance on a global basis, direct mortgage insurance in Europe and various risk-sharing products to government agencies and mortgage lenders.
	The ‘other’ segment includes the results of Watford Re (see Note 4). Watford Re has its own management and board of directors that is responsible for the overall profitability of the ‘other’ segment. For the ‘other’ segment, performance is measured based on net income or loss.
	The corporate (non-underwriting) segment results include net investment income, other income (loss), other expenses incurred by the Company, interest expense, net realized gains or losses, net impairment losses included in earnings, equity in net income (loss) of investment funds accounted for using the equity method, net foreign exchange gains or losses, income taxes and items related to the Company’s non-cumulative preferred shares. Such amounts exclude the results of the ‘other’ segment.
	The following tables summarize the Company’s underwriting income or loss by segment, together with a reconciliation of underwriting income to net income available to Arch common shareholders, summary information regarding net premiums written and earned by major line of business and net premiums written by location:
		Year Ended December 31, 2014
		Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
	Gross premiums written (1)	$	3,008,669		$	1,527,245		$	227,356		$	4,760,394		$	288,627		$	4,840,616
	Premiums ceded	(862,015		)	(261,254		)	(22,519		)	(1,142,912		)	(14,171		)	(948,678		)
	Net premiums written	2,146,654			1,265,991			204,837			3,617,482			274,456			3,891,938
	Change in unearned premiums	(129,284		)	13,337			(11,264		)	(127,211		)	(170,979		)	(298,190		)
	Net premiums earned	2,017,370			1,279,328			193,573			3,490,271			103,477			3,593,748
	Other underwriting income	2,135			3,167			4,840			10,142			—			10,142
	Losses and loss adjustment expenses	(1,260,953		)	(532,450		)	(55,674		)	(1,849,077		)	(70,173		)	(1,919,250		)
	Acquisition expenses, net	(316,308		)	(261,438		)	(49,400		)	(627,146		)	(30,116		)	(657,262		)
	Other operating expenses	(335,157		)	(147,964		)	(66,891		)	(550,012		)	(6,268		)	(556,280		)
	Underwriting income (loss)	$	107,087		$	340,643		$	26,448		474,178			(3,080		)	471,098
	Net investment income										284,336			18,249			302,585
	Net realized gains (losses)										133,384			(30,467		)	102,917
	Net impairment losses recognized in earnings										(30,150		)	—			(30,150		)
	Equity in net income of investment funds accounted for using the equity method										19,883			—			19,883
	Other income (loss)										(10,252		)	—			(10,252		)
	Other expenses										(47,615		)	(2,329		)	(49,944		)
	Interest expense										(45,634		)	—			(45,634		)
	Net foreign exchange gains (losses)										82,658			1,086			83,744
	Income before income taxes										860,788			(16,541		)	844,247
	Income tax expense										(22,987		)	—			(22,987		)
	Net income										837,801			(16,541		)	821,260
	Dividends attributable to redeemable noncontrolling interests										—			(14,728		)	(14,728		)
	Amounts attributable to noncontrolling interests										—			27,823			27,823
	Net income available to Arch										837,801			(3,446		)	834,355
	Preferred dividends										(21,938		)	—			(21,938		)
	Net income available to Arch common shareholders										$	815,863		$	(3,446	)	$	812,417
	Underwriting Ratios
	Loss ratio	62.5		%	41.6		%	28.8		%	53		%	67.8		%	53.4		%
	Acquisition expense ratio	15.7		%	20.4		%	25.5		%	18		%	29.1		%	18.3		%
	Other operating expense ratio	16.6		%	11.6		%	34.6		%	15.8		%	6.1		%	15.5		%
	Combined ratio	94.8		%	73.6		%	88.9		%	86.8		%	103		%	87.2		%
	Goodwill and intangible assets	$	28,331		$	3,333		$	77,875		$	109,539		$	—		$	109,539
	Total investable assets										$	14,609,969		$	1,163,240		$	15,773,209
	Total assets										20,527,027			1,482,516			22,009,543
	Total liabilities										14,492,237			398,660			14,890,897
	-1	Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.
		Year Ended December 31, 2013
		Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
	Gross premiums written (1)	$	2,712,509		$	1,399,621		$	89,570		$	4,196,623		$	—		$	4,196,623
	Premiums ceded	(763,713		)	(86,620		)	—			(845,256		)	—			(845,256		)
	Net premiums written	1,948,796			1,313,001			89,570			3,351,367			—			3,351,367
	Change in unearned premiums	(72,782		)	(94,329		)	(38,304		)	(205,415		)	—			(205,415		)
	Net premiums earned	1,876,014			1,218,672			51,266			3,145,952			—			3,145,952
	Other underwriting income	2,122			5,258			259			7,639			—			7,639
	Losses and loss adjustment expenses	(1,188,445		)	(486,236		)	(4,743		)	(1,679,424		)	—			(1,679,424		)
	Acquisition expenses, net	(311,904		)	(234,373		)	(17,826		)	(564,103		)	—			(564,103		)
	Other operating expenses	(315,387		)	(134,563		)	(8,377		)	(458,327		)	—			(458,327		)
	Underwriting income (loss)	$	62,400		$	368,758		$	20,579		451,737			—			451,737
	Net investment income										267,219			—			267,219
	Net realized gains										74,018			—			74,018
	Net impairment losses recognized in earnings										(3,786		)	—			(3,786		)
	Equity in net income (loss) of investment funds accounted for using the equity method										35,701			—			35,701
	Other income (loss)										(586		)	—			(586		)
Other expenses										(42,403		)	—			(42,403		)
Interest expense										(27,060		)	—			(27,060		)
Net foreign exchange gains (losses)										(12,335		)	—			(12,335		)
Income before income taxes										742,505			—			742,505
Income tax expense										(32,774		)	—			(32,774		)
Net income										709,731			—			709,731
Dividends attributable to redeemable noncontrolling interests										—			—			—
Amounts attributable to noncontrolling interests										—			—			—
Net income available to Arch										709,731			—			709,731
Preferred dividends										(21,938		)	—			(21,938		)
Net income available to Arch common shareholders										$	687,793		$	—		$	687,793
Underwriting Ratios
Loss ratio	63.3		%	39.9		%	9.3		%	53.4		%	—			53.4		%
Acquisition expense ratio	16.6		%	19.2		%	34.8		%	17.9		%	—			17.9		%
Other operating expense ratio	16.8		%	11		%	16.3		%	14.6		%	—			14.6		%
Combined ratio	96.7		%	70.1		%	60.4		%	85.9		%	—			85.9		%
Goodwill and intangible assets	$	20,419		$	6,900		$	—		$	27,319		$	—		$	27,319
Total investable assets										$	14,049,525		$	—		$	14,049,525
Total assets										19,566,094			—			19,566,094
Total liabilities										13,918,598			—			13,918,598
-1	Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.
	Year Ended December 31, 2012
	Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
Gross premiums written (1)	$	2,593,959		$	1,213,889		$	68,111		$	3,869,161		$	—		$	3,869,161
Premiums ceded	(768,625		)	(55,099		)	—			(816,926		)	—			(816,926		)
Net premiums written	1,825,334			1,158,790			68,111			3,052,235			—			3,052,235
Change in unearned premiums	(24,991		)	(40,663		)	(51,441		)	(117,095		)	—			(117,095		)
Net premiums earned	1,800,343			1,118,127			16,670			2,935,140			—			2,935,140
Other underwriting income	2,335			5,755			—			8,090			—			8,090
Losses and loss adjustment expenses	(1,283,841		)	(574,821		)	(2,615		)	(1,861,277		)	—			(1,861,277		)
Acquisition expenses, net	(298,983		)	(204,903		)	(4,998		)	(508,884		)	—			(508,884		)
Other operating expenses	(307,489		)	(118,245		)	(4,301		)	(430,035		)	—			(430,035		)
Underwriting income (loss)	$	(87,635	)	$	225,913		$	4,756		143,034			—			143,034
Net investment income										294,895			—			294,895
Net realized gains										194,228			—			194,228
Net impairment losses recognized in earnings										(11,388		)	—			(11,388		)
Equity in net income (loss) of investment funds accounted for using the equity method										73,510			—			73,510
Other income (loss)										(12,094		)	—			(12,094		)
Other expenses										(35,318		)	—			(35,318		)
Interest expense										(28,525		)	—			(28,525		)
Net foreign exchange gains (losses)										(28,955		)	—			(28,955		)
Income before income taxes										589,387			—			589,387
Income tax benefit										4,010			—			4,010
Net income										593,397			—			593,397
Dividends attributable to redeemable noncontrolling interests										—			—			—
Amounts attributable to noncontrolling interests										—			—			—
Net income available to Arch										593,397			—			593,397
Preferred dividends										(25,079		)	—			(25,079		)
Loss on repurchase of preferred shares										(10,612		)	—			(10,612		)
Net income available to Arch common shareholders										$	557,706		$	—		$	557,706
Underwriting Ratios
Loss ratio	71.3		%	51.4		%	15.7		%	63.4		%	—		%	63.4		%
Acquisition expense ratio	16.6		%	18.3		%	30		%	17.3		%	—		%	17.3		%
Other operating expense ratio	17.1		%	10.6		%	25.8		%	14.7		%	—		%	14.7		%
Combined ratio	105		%	80.3		%	71.5		%	95.4		%	—		%	95.4		%
Goodwill and intangible assets	$	21,357		$	16,902		$	—		$	38,259		$	—		$	38,259
Total investable assets										$	13,045,134		$	—		$	13,045,134
Total assets										17,816,762			—			17,816,762
Total liabilities										12,647,884			—			12,647,884
-1	Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.
The following tables provide summary information regarding net premiums written and earned by major line of business and net premiums written by location:
INSURANCE SEGMENT	Year Ended December 31,
	2014			2013			2012
Net premiums written (1)
Programs	$	480,580		$	419,673		$	340,130
Professional lines (2)	476,604			476,193			548,423
Construction and national accounts	286,994			271,110			211,130
Property, energy, marine and aviation	244,640			280,551			294,690
Excess and surplus casualty (3)	212,519			149,286			112,307
Travel, accident and health	145,732			104,903			106,871
Lenders products	100,407			101,576			99,724
Other (4)	199,178			145,504			112,059
Total	$	2,146,654		$	1,948,796		$	1,825,334
Net premiums earned (1)
Programs	$	460,392		$	386,840		$	318,740
Professional lines (2)	456,508			491,791			536,971
Construction and national accounts	277,811			250,729			209,217
Property, energy, marine and aviation	244,974			304,294			313,081
Excess and surplus casualty (3)	182,024			118,395			113,597
Travel, accident and health	127,691			97,135			98,686
Lenders products	94,438			99,847			103,478
Other (4)	173,532			126,983			106,573
Total	$	2,017,370		$	1,876,014		$	1,800,343
Net premiums written by client location (1)
United States	$	1,726,181		$	1,526,156		$	1,314,577
Europe	240,136			226,254			271,278
Asia and Pacific	79,564			95,970			120,492
Other	100,773			100,416			118,987
Total	$	2,146,654		$	1,948,796		$	1,825,334
Net premiums written by underwriting location (1)
United States	$	1,688,887		$	1,478,930		$	1,254,623
Europe	394,430			389,763			472,132
Other	63,337			80,103			98,579
Total	$	2,146,654		$	1,948,796		$	1,825,334
-1	Insurance segment results include premiums written and earned assumed through intersegment transactions and exclude premiums written and earned ceded through intersegment transactions.
-2	Includes professional liability, executive assurance and healthcare business.
-3	Includes casualty and contract binding business.
-4	Includes alternative markets, excess workers' compensation and surety business.
REINSURANCE SEGMENT	Year Ended December 31,
	2014			2013			2012
Net premiums written (1)
Other specialty (2)	$	405,126		$	417,865		$	308,104
Property excluding property catastrophe (3)	343,043			292,536			265,783
Casualty (4)	317,996			306,304			205,925
Property catastrophe	137,471			220,749			283,677
Marine and aviation	50,444			64,380			84,649
Other (5)	11,911			11,167			10,652
Total	$	1,265,991		$	1,313,001		$	1,158,790
Net premiums earned (1)
Other specialty (2)	$	424,725		$	387,630		$	309,101
Property excluding property catastrophe (3)	303,496			274,719			254,338
Casualty (4)	327,518			241,774			188,963
Property catastrophe	150,761			232,423			280,185
Marine and aviation	61,118			70,105			76,145
Other (5)	11,710			12,021			9,395
Total	$	1,279,328		$	1,218,672		$	1,118,127
Net premiums written (1)
Pro rata	$	663,135		$	692,024		$	530,763
Excess of loss	602,856			620,977			628,027
Total	$	1,265,991		$	1,313,001		$	1,158,790
Net premiums earned (1)
Pro rata	$	686,201		$	608,586		$	499,094
Excess of loss	593,127			610,086			619,033
Total	$	1,279,328		$	1,218,672		$	1,118,127
Net premiums written by client location (1)
United States	$	589,255		$	706,388		$	568,043
Europe	355,735			327,059			341,653
Asia and Pacific	142,626			94,252			97,879
Bermuda	77,620			87,047			72,864
Other	100,755			98,255			78,351
Total	$	1,265,991		$	1,313,001		$	1,158,790
Net premiums written by underwriting location (1)
United States	$	492,891		$	507,183		$	379,239
Bermuda	394,351			459,467			527,909
Europe	343,823			309,129			225,470
Other	34,926			37,222			26,172
Total	$	1,265,991		$	1,313,001		$	1,158,790
-1	Reinsurance segment results include premiums written and earned assumed through intersegment transactions and exclude premiums written and earned ceded through intersegment transactions.
-2	Includes non-excess motor, surety, accident and health, workers’ compensation catastrophe, agriculture, trade credit and other.
-3	Includes facultative business.
-4	Includes executive assurance, professional liability, workers’ compensation, excess motor, healthcare and other.
-5	Includes life, casualty clash and other.
	Year Ended December 31,
MORTGAGE SEGMENT	2014			2013			2012
Net premiums written by client location
United States	$	184,333		$	63,692		$	61,571
Other	20,504			25,878			6,540
Total	$	204,837		$	89,570		$	68,111
Net premiums written by underwriting location
United States	$	98,809		$	—		$	—
Other	106,028			89,570			68,111
Total	$	204,837		$	89,570		$	68,111
OTHER SEGMENT	Year Ended December 31,
	2014			2013			2012
Net premiums written (1)
Casualty (2)	$	179,054		$	—		$	—
Other specialty (3)	66,524			—			—
Property catastrophe	9,280			—			—
Marine and aviation	1,251			—			—
Property excluding property catastrophe	564			—			—
Other (4)	17,783			—			—
Total	$	274,456		$	—		$	—
Net premiums earned (1)
Casualty (2)	$	72,395		$	—		$	—
Other specialty (3)	24,157			—			—
Property catastrophe	5,115			—			—
Marine and aviation	789			—			—
Property excluding property catastrophe	182			—			—
Other (4)	839			—			—
Total	$	103,477		$	—		$	—
Net premiums written by client location (1)
United States	$	186,795		$	—		$	—
Europe	54,131			—			—
Bermuda	27,977			—			—
Other	5,553			—			—
Total	$	274,456		$	—		$	—
-1	Other segment results include premiums written and earned assumed through intersegment transactions and exclude premiums written and earned ceded through intersegment transactions.
-2	Includes professional liability, excess motor, programs and other.
-3	Includes non-excess motor and other.
-4	Includes mortgage and other.

Reserve_for_Losses_and_Loss_Ad

Reserve for Losses and Loss Adjustment Expenses	12 Months Ended
	Dec. 31, 2014
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Reserve for losses and loss adjustment expenses	Reserve for Losses and Loss Adjustment Expenses
	The following table represents an analysis of losses and loss adjustment expenses and a reconciliation of the beginning and ending reserve for losses and loss adjustment expenses:
		Year Ended December 31,
		2014			2013			2012
	Reserve for losses and loss adjustment expenses at beginning of year	$	8,824,696		$	8,933,292		$	8,456,210
	Unpaid losses and loss adjustment expenses recoverable	1,748,250			1,829,070			1,818,047
	Net reserve for losses and loss adjustment expenses at beginning of year	7,076,446			7,104,222			6,638,163
	Net incurred losses and loss adjustment expenses relating to losses occurring in:
	Current year	2,246,152			1,943,466			2,082,805
	Prior years	(326,902		)	(264,042		)	(221,528		)
	Total net incurred losses and loss adjustment expenses	1,919,250			1,679,424			1,861,277
	Net losses and loss adjustment expense reserves of acquired business (1)	120,671			—			31,977
	Foreign exchange (gains) losses	(160,486		)	1,617			38,184
	Net paid losses and loss adjustment expenses relating to losses occurring in:
	Current year	(347,270		)	(288,114		)	(295,984		)
	Prior years	(1,350,466		)	(1,420,703		)	(1,169,395		)
	Total net paid losses and loss adjustment expenses	(1,697,736		)	(1,708,817		)	(1,465,379		)
	Net reserve for losses and loss adjustment expenses at end of year	7,258,145			7,076,446			7,104,222
	Unpaid losses and loss adjustment expenses recoverable	1,778,303			1,748,250			1,829,070
	Reserve for losses and loss adjustment expenses at end of year	$	9,036,448		$	8,824,696		$	8,933,292
	-1	2014 amount relates to the Company’s acquisition of Arch MI U.S. while the 2012 amount relates to the Company’s acquisition of the trade credit and surety operations of Ariel Reinsurance Company.
	2014 Prior Year Reserve Development
	During 2014, the Company recorded estimated net favorable development on prior year loss reserves of $326.9 million, which consisted of $267.3 million from the reinsurance segment, $58.7 million from the insurance segment and $0.9 million from the mortgage segment.
	The reinsurance segment’s net favorable development of $267.3 million, or 20.9 points of net earned premium, consisted of $146.7 million from short-tailed lines and $120.6 million of net favorable development from medium-tailed and long-tailed lines. Favorable development in short-tailed lines included $107.6 million from property catastrophe and property other than property catastrophe reserves, primarily from the 2011 to 2013 underwriting years (i.e., losses attributable to contracts having an inception or renewal date within the given twelve-month period). Contained within this release was favorable development of $23.3 million from the 2005 to 2013 named catastrophic events. The net reduction of loss estimates for the reinsurance segment’s short-tailed lines primarily resulted from varying levels of reported and paid claims activity than previously anticipated which led to decreases in certain loss ratio selections during 2014. Net favorable development of $120.6 million in medium-tailed and long-tailed lines included reductions in casualty reserves of $101.6 million, primarily from the 2003 to 2006 underwriting years and the 2009 and 2010 underwriting years, and marine and aviation reserves of $14.7 million, primarily from the 2008 to 2012 underwriting years. The balance of net favorable development was spread across various lines and underwriting years.
	The insurance segment’s net favorable development of $58.7 million, or 2.9 points of net earned premium, consisted of $73.5 million of net favorable development from short-tailed lines, partially offset by $14.8 million of net adverse development from medium-tailed and long-tailed lines. Favorable development in short-tailed lines predominantly consisted of $60.5 million of net favorable development in property lines, primarily from the 2008 to 2013 accident years (i.e., the year in which a loss occurred), with the balance emanating from lenders products, primarily from the 2012 accident year, and travel and accident, primarily from the 2012 and 2013 accident years. Contained within this short tail release was favorable development of $7.9 million from 2005 to 2013 named catastrophic events. Net adverse development in medium-tailed and long-tailed lines of $14.8 million included a net increase of $41.3 million in construction reserves, primarily from the 2009 to 2013 accident years. The adverse development in construction was largely driven by general liability claims involving New York labor law matters. In addition, the insurance segment experienced $26.2 million of net adverse development in program business, primarily from the 2011 and 2012 accident years. Such amounts were partially offset by $39.0 million of net favorable development on professional lines, primarily from the 2003 to 2011 accident years, and $7.8 million of net favorable development on surety business, primarily from the 2011 to 2013 accident years.
	2013 Prior Year Reserve Development
	During 2013, the Company recorded estimated net favorable development on prior year loss reserves of $264.0 million, which consisted of $217.9 million from the reinsurance segment, $45.1 million from the insurance segment and $1.0 million from the mortgage segment.
	The reinsurance segment’s net favorable development of $217.9 million, or 17.9 points of net earned premium, consisted of $111.1 million from short-tailed lines and $106.8 million of net favorable development from medium-tailed and long-tailed lines. Favorable development in short-tailed lines included $110.1 million from property catastrophe and property other than property catastrophe reserves, primarily from the 2009 to 2012 underwriting years. Contained within this release was favorable development of $28.6 million from the 2005 to 2012 named catastrophic events. The net reduction of loss estimates for the reinsurance segment’s short-tailed lines primarily resulted from varying levels of reported and paid claims activity than previously anticipated which led to decreases in certain loss ratio selections during 2013. Net favorable development of $106.8 million in medium-tailed and long-tailed lines included reductions in casualty reserves of $98.8 million, primarily from the 2003 to 2009 underwriting years, and marine and aviation reserves of $8.1 million, primarily from the 2007 and 2009 underwriting years. The balance of net favorable development was spread across various lines and underwriting years.
	The insurance segment’s net favorable development of $45.1 million, or 2.4 points of net earned premium, consisted of $67.0 million of net favorable development from short-tailed lines, partially offset by $21.9 million of net adverse development from medium-tailed and long-tailed lines. Favorable development in short-tailed lines predominantly consisted of $62.4 million of net favorable development in property lines from the 2008 to 2011 accident years, with the balance emanating from lenders products, primarily from the 2011 and 2012 accident years, and travel and accident, primarily from the 2009 and 2010 accident years. Contained within this short tail release was favorable development of $15.9 million from 2005 to 2012 named catastrophic events. Net adverse development in medium-tailed and long-tailed lines of $21.9 million included a net increase of $28.0 million in specialty casualty reserves. Such development primarily stemmed from losses in two captive programs written by the insurance segment's Canadian operation, which was partially offset by favorable development in non-captive business, and from three large excess casualty claims in the insurance segment's U.S. operation, all of the above in the more recent accident years. In addition, the insurance segment experienced $26.2 million of net adverse development in program business, with nearly 90% of this development emanating from a single canceled program whose last inception year was 2011. Such amounts were partially offset by net favorable development spread across various lines and accident years.
	2012 Prior Year Reserve Development
	During 2012, the Company recorded estimated net favorable development on prior year loss reserves of $221.5 million, which consisted of $190.3 million from the reinsurance segment and $31.2 million from the insurance segment.
	The reinsurance segment’s net favorable development of $190.3 million, or 17.0 points of net earned premium, consisted of $117.6 million from short-tailed lines and $72.7 million of net favorable development from medium-tailed and long-tailed lines. Favorable development in short-tailed lines included $92.1 million from property catastrophe and property other than property catastrophe reserves, primarily from the 2008 to 2011 underwriting years. Contained within this release was favorable development of $16.8 million from the 2005 to 2011 named catastrophic events. The net reduction of loss estimates for the reinsurance segment’s short-tailed lines primarily resulted from varying levels of reported and paid claims activity than previously anticipated which led to decreases in certain loss ratio selections during 2012. Net favorable development of $72.7 million in medium-tailed and long-tailed lines included reductions in casualty reserves of $55.9 million, primarily from the 2003 to 2009 underwriting years, and marine and aviation reserves of $16.8 million, primarily from the 2008 to 2010 underwriting years. The balance of net favorable development was spread across various lines and underwriting years.
	The insurance segment’s net favorable development of $31.2 million, or 1.7 points of net earned premium, consisted of $79.3 million of net favorable development from short-tailed lines, partially offset by $48.1 million of net adverse development from medium-tailed and long-tailed lines. Favorable development in short-tailed lines predominantly consisted of $69.0 million of net favorable development in property lines from the 2007 to 2011 accident years, with the balance emanating from travel and accident and lender’s products, both primarily from the 2009 to 2011 accident years. Contained within this short tail release was favorable development of $19.1 million from 2005 to 2011 named catastrophic events. Net adverse development in medium-tailed and long-tailed lines of $48.1 million included a net increase of $38.9 million from the insurance segment’s U.K. underwriting operations for the 2007 to 2010 accident years, primarily due to an increase in Australian executive assurance reserves. In addition, U.S. primary specialty casualty reserves reflected a net increase of $23.0 million, primarily from the 2003 to 2005 accident years, and $19.8 million from U.S. professional liability in the two most recent accident years. Such amounts were partially offset by net favorable development spread across various lines and accident years.

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2014
Reinsurance Disclosures [Abstract]
Reinsurance	Reinsurance
	In the normal course of business, the Company’s insurance subsidiaries cede a portion of their premium through pro rata and excess of loss reinsurance agreements on a treaty or facultative basis. The Company’s reinsurance subsidiaries participate in “common account” retrocessional arrangements for certain pro rata treaties. Such arrangements reduce the effect of individual or aggregate losses to all companies participating on such treaties, including the reinsurers, such as the Company’s reinsurance subsidiaries, and the ceding company. In addition, the Company’s reinsurance subsidiaries may purchase retrocessional coverage as part of their risk management program. Reinsurance recoverables are recorded as assets, predicated on the reinsurers’ ability to meet their obligations under the reinsurance agreements. If the reinsurers are unable to satisfy their obligations under the agreements, the Company’s insurance or reinsurance subsidiaries would be liable for such defaulted amounts.
	The effects of reinsurance on the Company’s written and earned premiums and losses and loss adjustment expenses with unaffiliated reinsurers were as follows:
		Year Ended December 31,
		2014			2013			2012
	Premiums Written
	Direct	$	3,100,946		$	2,754,582		$	2,673,864
	Assumed	1,739,670			1,442,041			1,195,297
	Ceded	(948,678		)	(845,256		)	(816,926		)
	Net	$	3,891,938		$	3,351,367		$	3,052,235
	Premiums Earned
	Direct	$	2,914,755		$	2,666,104		$	2,572,078
	Assumed	1,574,049			1,296,048			1,165,371
	Ceded	(895,056		)	(816,200		)	(802,309		)
	Net	$	3,593,748		$	3,145,952		$	2,935,140
	Losses and Loss Adjustment Expenses
	Direct	$	1,763,492		$	1,603,369		$	1,725,707
	Assumed	621,346			450,618			578,081
	Ceded	(465,588		)	(374,563		)	(442,511		)
	Net	$	1,919,250		$	1,679,424		$	1,861,277
	The Company monitors the financial condition of its reinsurers and attempts to place coverages only with substantial, financially sound carriers. At December 31, 2014, approximately 83.0% of the Company’s reinsurance recoverables on paid and unpaid losses (not including prepaid reinsurance premiums) of $1.81 billion were due from carriers which had an A.M. Best rating of “A-” or better and the largest reinsurance recoverables from any one carrier was approximately 3.7% of the Company’s total shareholders’ equity. At December 31, 2013, approximately 86.5% of the Company’s reinsurance recoverables on paid and unpaid losses (not including prepaid reinsurance premiums) of $1.80 billion were due from carriers which had an A.M. Best rating of “A-” or better and the largest reinsurance recoverables from any one carrier was approximately 4.2% of the Company’s total shareholders’ equity. Although the Company has not experienced any material credit losses to date, an inability of its reinsurers or retrocessionaires to meet their obligations to it over the relevant exposure periods for any reason could have a material adverse effect on its financial condition and results of operations.

Investment_Information

Investment Information	12 Months Ended
	Dec. 31, 2014
Disclosure Investment Information [Abstract]
Investment Information	Investment Information
	At December 31, 2014, total investable assets of $15.77 billion included $14.61 billion managed by the Company and $1.16 billion attributable to Watford Re.
	Available For Sale Securities
	The following table summarizes the fair value and cost or amortized cost of the Company’s securities classified as available for sale:
		Estimated			Gross			Gross			Cost or			OTTI
		Fair			Unrealized			Unrealized			Amortized			Unrealized
		Value			Gains			Losses			Cost			Losses (2)
	31-Dec-14
	Fixed maturities and fixed maturities pledged
	under securities lending agreements (1):
	Corporate bonds	$	3,108,513		$	37,928		$	(38,974	)	$	3,109,559		$	(317	)
	Mortgage backed securities	943,343			18,843			(3,842		)	928,342			(3,307		)
	Municipal bonds	1,494,122			31,227			(1,044		)	1,463,939			—
	Commercial mortgage backed securities	1,114,528			14,594			(3,822		)	1,103,756			—
	U.S. government and government agencies	1,447,972			8,345			(1,760		)	1,441,387			—
	Non-U.S. government securities	1,015,153			21,311			(37,203		)	1,031,045			—
	Asset backed securities	1,677,941			8,425			(6,089		)	1,675,605			(22		)
	Total	10,801,572			140,673			(92,734		)	10,753,633			(3,646		)
	Equity securities	658,182			109,012			(13,364		)	562,534			—
	Other investments	296,224			31,839			(362		)	264,747			—
	Short-term investments	797,226			738			(5,270		)	801,758			—
	Total	$	12,553,204		$	282,262		$	(111,730	)	$	12,382,672		$	(3,646	)
	31-Dec-13
	Fixed maturities and fixed maturities pledged
	under securities lending agreements (1):
	Corporate bonds	$	2,267,263		$	35,289		$	(35,537	)	$	2,267,511		$	(16	)
	Mortgage backed securities	1,133,095			16,270			(22,209		)	1,139,034			(9,269		)
	Municipal bonds	1,481,738			29,378			(9,730		)	1,462,090			(17		)
	Commercial mortgage backed securities	1,074,497			13,972			(15,224		)	1,075,749			(199		)
	U.S. government and government agencies	1,301,809			3,779			(11,242		)	1,309,272			(19		)
	Non-U.S. government securities	1,085,861			14,729			(19,363		)	1,090,495			—
	Asset backed securities	1,332,594			20,033			(13,795		)	1,326,356			(3,422		)
	Total	9,676,857			133,450			(127,100		)	9,670,507			(12,942		)
	Equity securities	496,824			69,487			(5,938		)	433,275			—
	Other investments	498,310			28,082			(18,459		)	488,687			—
	Short-term investments	1,478,367			1,654			(871		)	1,477,584			—
	Total	$	12,150,358		$	232,673		$	(152,368	)	$	12,070,053		$	(12,942	)
	-1	In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged. For purposes of this table, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged. See “—Securities Lending Agreements.”
	-2	Represents the total other-than-temporary impairments (“OTTI”) recognized in accumulated other comprehensive income (“AOCI”). It does not include the change in fair value subsequent to the impairment measurement date. At December 31, 2014, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $0.9 million, compared to a net unrealized gain of $6.0 million at December 31, 2013.
	The following table summarizes, for all available for sale securities in an unrealized loss position, the fair value and gross unrealized loss by length of time the security has been in a continual unrealized loss position:
		Less than 12 Months						12 Months or More						Total
		Estimated			Gross			Estimated			Gross			Estimated			Gross
		Fair			Unrealized			Fair			Unrealized			Fair			Unrealized
		Value			Losses			Value			Losses			Value			Losses
	31-Dec-14
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	1,309,637		$	(32,903	)	$	148,963		$	(6,071	)	$	1,458,600		$	(38,974	)
	Mortgage backed securities	293,624			(1,476		)	59,107			(2,366		)	352,731			(3,842		)
	Municipal bonds	210,614			(588		)	13,643			(456		)	224,257			(1,044		)
	Commercial mortgage backed securities	232,147			(770		)	125,894			(3,052		)	358,041			(3,822		)
	U.S. government and government agencies	618,381			(1,626		)	3,438			(134		)	621,819			(1,760		)
	Non-U.S. government securities	510,766			(31,172		)	46,910			(6,031		)	557,676			(37,203		)
	Asset backed securities	612,950			(2,486		)	243,452			(3,603		)	856,402			(6,089		)
	Total	3,788,119			(71,021		)	641,407			(21,713		)	4,429,526			(92,734		)
	Equity securities	181,002			(13,364		)	—			—			181,002			(13,364		)
	Other investments	59,638			(362		)	—			—			59,638			(362		)
	Short-term investments	79,271			(5,270		)	—			—			79,271			(5,270		)
	Total	$	4,108,030		$	(90,017	)	$	641,407		$	(21,713	)	$	4,749,437		$	(111,730	)
	31-Dec-13
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	1,183,625		$	(32,837	)	$	46,673		$	(2,700	)	$	1,230,298		$	(35,537	)
	Mortgage backed securities	778,693			(20,253		)	43,634			(1,956		)	822,327			(22,209		)
	Municipal bonds	589,009			(9,422		)	6,092			(308		)	595,101			(9,730		)
	Commercial mortgage backed securities	677,617			(15,110		)	1,612			(114		)	679,229			(15,224		)
U.S. government and government agencies	1,144,809			(11,242		)	—			—			1,144,809			(11,242		)
Non-U.S. government securities	821,506			(15,776		)	24,334			(3,587		)	845,840			(19,363		)
Asset backed securities	692,362			(10,431		)	88,629			(3,364		)	780,991			(13,795		)
Total	5,887,621			(115,071		)	210,974			(12,029		)	6,098,595			(127,100		)
Equity securities	76,563			(5,938		)	—			—			76,563			(5,938		)
Other investments	165,891			(15,775		)	47,316			(2,684		)	213,207			(18,459		)
Short-term investments	28,170			(871		)	—			—			28,170			(871		)
Total	$	6,158,245		$	(137,655	)	$	258,290		$	(14,713	)	$	6,416,535		$	(152,368	)
-1	In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged. For purposes of this table, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged. See “—Securities Lending Agreements.”
At December 31, 2014, on a lot level basis, approximately 1,900 security lots out of a total of approximately 4,790 security lots were in an unrealized loss position and the largest single unrealized loss from a single lot in the Company’s fixed maturity portfolio was $2.9 million. At December 31, 2013, on a lot level basis, approximately 2,080 security lots out of a total of approximately 4,400 security lots were in an unrealized loss position and the largest single unrealized loss from a single lot in the Company’s fixed maturity portfolio was $3.5 million.
The contractual maturities of the Company’s fixed maturities and fixed maturities pledged under securities lending agreements are shown in the following table. Expected maturities, which are management’s best estimates, will differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties.
	December 31, 2014						December 31, 2013
Maturity	Estimated Fair Value			Amortized Cost			Estimated Fair Value			Amortized Cost
Due in one year or less	$	235,930		$	233,794		$	235,330		$	232,652
Due after one year through five years	4,074,562			4,077,408			3,738,500			3,718,920
Due after five years through 10 years	2,475,726			2,461,356			1,966,536			1,979,510
Due after 10 years	279,542			273,372			196,305			198,286
	7,065,760			7,045,930			6,136,671			6,129,368
Mortgage backed securities	943,343			928,342			1,133,095			1,139,034
Commercial mortgage backed securities	1,114,528			1,103,756			1,074,497			1,075,749
Asset backed securities	1,677,941			1,675,605			1,332,594			1,326,356
Total	$	10,801,572		$	10,753,633		$	9,676,857		$	9,670,507
Securities Lending Agreements
At December 31, 2014, the fair value and amortized cost of fixed maturities and short-term investments pledged under securities lending agreements were $50.8 million and $52.1 million, respectively, compared to $105.1 million and $105.9 million at December 31, 2013, respectively. At December 31, 2014, the portfolio of collateral backing the Company’s securities lending program included approximately $5.8 million of sub-prime securities, compared to $6.3 million at December 31, 2013.
Other Investments
The following table summarizes the Company’s other investments, including available for sale and fair value option components:
	December 31,
	2014			2013
Available for sale securities:
Asian and emerging markets	$	236,586		$	331,984
Investment grade fixed income	59,638			159,115
Other	—			7,211
Total available for sale	296,224			498,310
Fair value option:
Term loan investments (par value: $1,076,011 and $360,204)	1,073,649			399,591
Mezzanine debt funds	121,341			102,172
Credit related funds	114,436			63,271
Investment grade fixed income	69,108			75,062
Asian and emerging markets	25,800			14,054
Other (1)	147,573			119,130
Total fair value option	1,551,907			773,280
Total	$	1,848,131		$	1,271,590
-1	Includes fund investments with strategies in mortgage servicing rights, transportation and infrastructure assets and other.
Certain of the Company’s other investments are in investment funds for which the Company has the option to redeem at agreed upon values as described in each investment fund’s subscription agreement. Depending on the terms of the various subscription agreements, investments in investment funds may be redeemed daily, monthly, quarterly or on other terms. Two common redemption restrictions which may impact the Company’s ability to redeem these investment funds are gates and lockups. A gate is a suspension of redemptions which may be implemented by the general partner or investment manager of the fund in order to defer, in whole or in part, the redemption request in the event the aggregate amount of redemption requests exceeds a predetermined percentage of the investment fund's net assets which may otherwise hinder the general partner or investment manager's ability to liquidate holdings in an orderly fashion in order to generate the cash necessary to fund extraordinarily large redemption payouts. A lockup period is the initial amount of time an investor is contractually required to hold the security before having the ability to redeem. If the investment funds are eligible to be redeemed, the time to redeem such fund can take weeks or months following the notification.
Fair Value Option
The following table summarizes the Company’s assets and liabilities which are accounted for using the fair value option:
	December 31,
	2014			2013
Fixed maturities	$	632,024		$	448,254
Other investments	1,551,907			773,280
Short term investments	251,601			—
Investments accounted for using the fair value option	$	2,435,532		$	1,221,534
Net Investment Income
The components of net investment income were derived from the following sources:
	Year Ended December 31,
	2014			2013			2012
Fixed maturities	$	265,219		$	249,833		$	281,140
Term loan investments	39,940			20,608			15,283
Equity securities (dividends)	13,005			8,919			7,963
Short-term investments	1,888			1,259			1,980
Other (1)	28,869			19,710			14,196
Gross investment income	348,921			300,329			320,562
Investment expenses	(46,336		)	(33,110		)	(25,667		)
Net investment income	$	302,585		$	267,219		$	294,895
-1	Includes dividends on investment funds and other items.
Net Realized Gains (Losses)
Net realized gains (losses) were as follows, excluding the other-than-temporary impairment provisions:
	Year Ended December 31,
	2014			2013			2012
Available for sale securities:
Gross gains on investment sales	$	248,031		$	239,421		$	247,291
Gross losses on investment sales	(145,808		)	(203,077		)	(71,722		)
Change in fair value of assets and liabilities accounted for using the fair value option:
Fixed maturities	(20,743		)	54			13,195
Other investments	2,672			17,503			7,383
Equity securities	—			704			(73		)
Short term investments	(395		)	—			—
Derivative instruments (1)	43,934			20,912			(1,326		)
Other (2)	(24,774		)	(1,499		)	(520		)
Net realized gains	$	102,917		$	74,018		$	194,228
-1	See Note 11 for information on the Company’s derivative instruments.
-2	Includes accretion of contingent consideration liability amounts related to the acquisition of the CMG Entities (see Note 2 for further details).
Other-Than-Temporary Impairments
The Company performs quarterly reviews of its available for sale investments in order to determine whether declines in fair value below the amortized cost basis were considered other-than-temporary in accordance with applicable guidance. The following table details the net impairment losses recognized in earnings by asset class:
	Year Ended December 31,
	2014			2013			2012
Fixed maturities:
Corporate bonds	$	(2,062	)	$	(88	)	$	(1,512	)
Mortgage backed securities	(1,525		)	(295		)	(2,491		)
Asset backed securities	(40		)	(128		)	(127		)
Non-U.S. government securities	—			—			(261		)
Commercial mortgage backed securities	(7		)	—			(211		)
U.S. government and government agencies	—			—			(10		)
Total	(3,634		)	(511		)	(4,612		)
Other investments	(25,410		)	—			—
Equity securities	(1,106		)	(3,275		)	(6,689		)
Investment of funds received under securities lending agreements	—			—			(87		)
Net impairment losses recognized in earnings	$	(30,150	)	$	(3,786	)	$	(11,388	)
A description of the methodology and significant inputs used to measure the amount of net impairment losses recognized in earnings in 2014 is as follows:
•	Other investments — the Company utilized information received from fund managers and positive and negative evidence, including the business prospects, recent events, industry and market data and other factors. Net impairment losses for 2014 primarily related to a reduction in the carrying value of a fund investment which was in an unrealized loss position and where the Company determined that it did not intend to hold such investment for a reasonable period of time by which the fair value of the fund would increase and the Company would recover its cost. The Company redeemed its holding in such fund in early October 2014;
•	Corporate bonds – the Company reviewed the business prospects, credit ratings, estimated loss given default factors, foreign currency impacts and information received from asset managers and rating agencies for certain corporate bonds. The amortized cost basis of the corporate bonds were adjusted down, if required, to the expected recovery value calculated in the OTTI review process;
•	Mortgage backed securities – the Company utilized underlying data provided by asset managers, cash flow projections and additional information from credit agencies in order to determine an expected recovery value for each security. The analysis includes expected cash flow projections under base case and stress case scenarios which modify the expected default expectations and loss severities and slow down prepayment assumptions. The significant inputs in the models include the expected default rates, delinquency rates and foreclosure costs. Net impairment losses for 2014 primarily resulted from a revised analysis of prepayment assumptions on one interest only mortgage backed security and small adjustments in other holdings. The amortized cost basis of the mortgage backed securities were adjusted down, if required, to the expected recovery value calculated in the OTTI review process;
•	Equity securities – the Company utilized information received from asset managers on common stocks, including the business prospects, recent events, industry and market data and other factors. For certain equities which were in an unrealized loss position and where the Company determined that it did not have the intent or ability to hold such securities for a reasonable period of time by which the fair value of the securities would increase and the Company would recover its cost, the cost basis of such securities was adjusted down accordingly.
The Company believes that the $3.6 million of OTTI included in accumulated other comprehensive income at December 31, 2014 on the securities which were considered by the Company to be impaired was due to market and sector-related factors (i.e., not credit losses). At December 31, 2014, the Company did not intend to sell these securities, or any other securities which were in an unrealized loss position, and determined that it is more likely than not that the Company will not be required to sell such securities before recovery of their cost basis.
The following table provides a roll forward of the amount related to credit losses recognized in earnings for which a portion of an OTTI was recognized in accumulated other comprehensive income:
	Year Ended December 31,
	2014			2013			2012
Balance at start of year	$	60,062		$	62,001		$	66,545
Credit loss impairments recognized on securities not previously impaired	691			423			1,962
Credit loss impairments recognized on securities previously impaired	162			88			2,735
Reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security	—			—			—
Reductions for securities sold during the period	(40,719		)	(2,450		)	(9,241		)
Balance at end of year	$	20,196		$	60,062		$	62,001
Restricted Assets
The Company is required to maintain assets on deposit, which primarily consist of fixed maturities, with various regulatory authorities to support its insurance and reinsurance operations. The Company’s insurance and reinsurance subsidiaries maintain assets in trust accounts as collateral for insurance and reinsurance transactions with affiliated companies and also have investments in segregated portfolios primarily to provide collateral or guarantees for letters of credit to third parties. See Note 16, “Commitments and Contingencies—Letter of Credit and Revolving Credit Facilities,” for further details. The following table details the value of the Company’s restricted assets:
	December 31,
	2014			2013
Assets used for collateral or guarantees:
Affiliated transactions	$	4,138,527		$	4,060,533
Third party agreements	970,120			856,890
Deposits with U.S. regulatory authorities	337,981			302,809
Trust funds	72,461			75,264
Deposits with non-U.S. regulatory authorities	—			6,546
Total restricted assets	$	5,519,089		$	5,302,042
In addition, because Watford Re is an independent company, the assets of Watford Re can be used only to settle obligations of Watford Re and Watford Re is solely responsible for its own liabilities and commitments. See Note 5, “Segment Information,” for information on Watford Re’s assets and liabilities.
Investment Funds Accounted For Using the Equity Method
The Company recorded equity in net income related to investment funds accounted for using the equity method of $19.9 million for 2014, compared to $35.7 million for 2013 and $73.5 million for 2012. Due to the ownership structure of these investment funds (e.g., limited partnership), the Company uses the equity method where it does not have a controlling interest and is not the primary beneficiary. In applying the equity method, these investments are initially recorded at cost and are subsequently adjusted based on the Company’s proportionate share of the net income or loss of the funds (which include changes in the fair value of the underlying securities in the funds). Such investments are generally recorded on a one month lag with some investments reported on a three month lag based on the availability of reports from the investment funds. Changes in the carrying value of such investments are recorded in net income as “Equity in net income (loss) of investment funds accounted for using the equity method.” As such, fluctuations in the carrying value of the investment funds accounted for using the equity method may increase the volatility of the Company’s reported results of operations. Investment funds accounted for using the equity method totaled $349.0 million at December 31, 2014, compared to $244.3 million at December 31, 2013.

Fair_Value

Fair Value	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value	Fair Value
	Accounting guidance regarding fair value measurements addresses how companies should measure fair value when they are required to use a fair value measure for recognition or disclosure purposes under GAAP and provides a common definition of fair value to be used throughout GAAP. It defines fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly fashion between market participants at the measurement date. In addition, it establishes a three-level valuation hierarchy for the disclosure of fair value measurements. The valuation hierarchy is based upon the transparency of inputs to the valuation of an asset or liability as of the measurement date. The level in the hierarchy within which a given fair value measurement falls is determined based on the lowest level input that is significant to the measurement (Level 1 being the highest priority and Level 3 being the lowest priority).
	The levels in the hierarchy are defined as follows:
	Level 1:	Inputs to the valuation methodology are observable inputs that reflect quoted prices (unadjusted) for identical assets or liabilities in active markets
	Level 2:	Inputs to the valuation methodology include quoted prices for similar assets and liabilities in active markets, and inputs that are observable for the asset or liability, either directly or indirectly, for substantially the full term of the financial instrument
	Level 3:	Inputs to the valuation methodology are unobservable and significant to the fair value measurement
	Fair Value Measurements on a Recurring Basis
	Following is a description of the valuation methodologies used for securities measured at fair value, as well as the general classification of such securities pursuant to the valuation hierarchy.
	The Company determines the existence of an active market based on its judgment as to whether transactions for the financial instrument occur in such market with sufficient frequency and volume to provide reliable pricing information. The independent pricing sources obtain market quotations and actual transaction prices for securities that have quoted prices in active markets. The Company uses quoted values and other data provided by nationally recognized independent pricing sources as inputs into its process for determining fair values of its fixed maturity investments. To validate the techniques or models used by pricing sources, the Company's review process includes, but is not limited to: (i) quantitative analysis (e.g., comparing the quarterly return for each managed portfolio to its target benchmark, with significant differences identified and investigated); (ii) a review of the average number of prices obtained in the pricing process and the range of resulting fair values; (iii) initial and ongoing evaluation of methodologies used by outside parties to calculate fair value; (iv) comparing the fair value estimates to its knowledge of the current market; (v) a comparison of the pricing services' fair values to other pricing services' fair values for the same investments; and (vi) periodic back-testing, which includes randomly selecting purchased or sold securities and comparing the executed prices to the fair value estimates from the pricing service. For a majority of investments, the Company obtained multiple quotes. A price source hierarchy was maintained in order to determine which price source would be used (i.e., a price obtained from a pricing service with more seniority in the hierarchy will be used over a less senior one in all cases). The hierarchy prioritizes pricing services based on availability and reliability and assigns the highest priority to index providers. Based on the above review, the Company will challenge any prices for a security or portfolio which are considered not to be representative of fair value. At December 31, 2014, the Company adjusted certain prices (primarily on structured securities) obtained from the pricing services and substituted alternate prices (primarily broker-dealer quotes) for such securities. Such adjustments did not have a material impact on the overall fair value of the Company’s investment portfolio at December 31, 2014.
	The independent pricing sources obtain market quotations and actual transaction prices for securities that have quoted prices in active markets. Each source has its own proprietary method for determining the fair value of securities that are not actively traded. In general, these methods involve the use of “matrix pricing” in which the independent pricing source uses observable market inputs including, but not limited to, investment yields, credit risks and spreads, benchmarking of like securities, broker-dealer quotes, reported trades and sector groupings to determine a reasonable fair value. In addition, pricing vendors use model processes, such as an Option Adjusted Spread model, to develop prepayment and interest rate scenarios. The Option Adjusted Spread model is commonly used to estimate fair value for securities such as mortgage backed and asset backed securities. In certain circumstances, when fair values are unavailable from these independent pricing sources, quotes are obtained directly from broker-dealers who are active in the corresponding markets. Such quotes are subject to the validation procedures noted above. Of the $15.1 billion of financial assets and liabilities measured at fair value at December 31, 2014, approximately $260.8 million, or 1.7%, were priced using non-binding broker-dealer quotes. Of the $13.4 billion of financial assets and liabilities measured at fair value at December 31, 2013, approximately $601.9 million, or 4.5%, were priced using non-binding broker-dealer quotes.
	The Company reviews its securities measured at fair value and discusses the proper classification of such investments with investment advisors and others. A discussion of the general classification of the Company’s financial instruments follows:
	Fixed maturities. The Company determined that all U.S. Treasuries would be classified as Level 1 securities due to observed levels of trading activity, the high number of strongly correlated pricing quotes received on U.S. Treasuries and other factors. Where the Company believes that quoted market prices are not available or that the market is not active, fair values are estimated by using quoted prices of securities with similar characteristics, pricing models or matrix pricing and are generally classified as Level 2 securities. The Company determined that Level 2 securities included corporate bonds, mortgage backed securities, municipal bonds, asset backed securities and non-U.S. government securities. The Company determined that certain corporate bonds and asset backed securities for which there is a low level of transparency around inputs to the valuation process should be classified within Level 3 of the valuation hierarchy.
	Equity securities. The Company determined that exchange-traded equity securities would be included in Level 1 as their fair values are based on quoted market prices in active markets. Other equity securities are included in Level 2 of the valuation hierarchy.
	Other investments. The fair values for certain of the Company's other investments are determined using net asset values (“NAV”) as advised by external fund managers. The NAV is based on the fund manager's valuation of the underlying holdings in accordance with the fund's governing documents. Periodically, the Company performs a number of monitoring procedures in order to assess the quality of the NAVs, including regular review and discussion of each fund's performance, regular evaluation of fund performance against applicable benchmarks and the backtesting of the NAVs against audited and interim financial statements. Other investments with liquidity terms allowing the Company to substantially redeem its holdings in a short time frame at the applicable NAV are reflected in Level 2. Other investments with redemption restrictions that prevent the Company from redeeming in the near term are classified in Level 3 of the valuation hierarchy.
	Short-term investments. The Company determined that certain of its short-term investments held in highly liquid money market-type funds would be included in Level 1 as their fair values are based on quoted market prices in active markets. Other short-term investments are classified in Level 2 of the valuation hierarchy.
	In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged under securities lending agreements. For purposes of the following tables, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged under securities lending agreements, at fair value.
	Contingent consideration liability. The contingent consideration liability (included in ‘other liabilities’ in the consolidated balance sheets) resulted from the acquisition of the CMG Entities and is remeasured at fair value at each balance sheet date. Changes in fair value are recognized in ‘net realized gains (losses).’ To determine the fair value of the contingent consideration liability, the Company estimates future payments using an income approach based on modeled inputs which include a weighted average cost of capital. The Company determined that the contingent consideration liability would be included in Level 3 of the valuation hierarchy.
	The Company reviews the classification of its investments each quarter. No transfers were made between Level 1 and Level 2 in 2012, 2013 or 2014. In the 2014 fourth quarter, the Company determined that certain asset backed securities would be included in Level 3 due to a review of the pricing inputs used on such securities. This resulted in a transfer of $57.5 million from Level 2 into Level 3 in the 2014 fourth quarter.
	The following table presents the Company’s financial assets and liabilities measured at fair value by level at December 31, 2014:
					Fair Value Measurement Using:
		Estimated			Quoted Prices in Active Markets for Identical Assets			Significant Other Observable Inputs			Significant Unobservable Inputs
		Fair Value			(Level 1)			(Level 2)			(Level 3)
	Assets measured at fair value:
	Available for sale securities:
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	3,108,513		$	—		$	3,108,513		$	—
	Mortgage backed securities	943,343			—			943,343			—
	Municipal bonds	1,494,122			—			1,494,122			—
	Commercial mortgage backed securities	1,114,528			—			1,114,528			—
	U.S. government and government agencies	1,447,972			1,447,972			—			—
	Non-U.S. government securities	1,015,153			—			1,015,153			—
	Asset backed securities	1,677,941			—			1,620,441			57,500
	Total	10,801,572			1,447,972			9,296,100			57,500
	Equity securities	658,182			658,182			—			—
	Other investments	296,224			—			192,687			103,537
	Short-term investments	797,226			759,621			37,605			—
	Fair value option:
	Investments accounted for using the fair value option:
	Corporate bonds	497,101			—			497,101			—
	Non-U.S. government bonds	88,411			—			88,411			—
	Mortgage backed securities	22,190			—			22,190			—
	Asset backed securities	24,322			—			24,322			—
	Other investments	1,551,907			—			1,085,557			466,350
	Short-term investments	251,601			250,580			1,021			—
	Total	2,435,532			250,580			1,718,602			466,350
	Total assets measured at fair value	$	14,988,736		$	3,116,355		$	11,244,994		$	627,387
	Liabilities measured at fair value:
	Contingent consideration liability	$	(61,845	)	$	—		$	—		$	(61,845	)
	Total liabilities measured at fair value	$	(61,845	)	$	—		$	—		$	(61,845	)
	-1	In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged. For purposes of this table, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged.
	The following table presents the Company’s financial assets and liabilities measured at fair value by level at December 31, 2013:
					Fair Value Measurement Using:
		Estimated			Quoted Prices in Active Markets for Identical Assets			Significant Other Observable Inputs			Significant Unobservable Inputs
		Fair Value			(Level 1)			(Level 2)			(Level 3)
	Assets measured at fair value:
	Available for sale securities:
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	2,267,263		$	—		$	2,265,218		$	2,045
	Mortgage backed securities	1,133,095			—			1,133,095			—
	Municipal bonds	1,481,738			—			1,481,738			—
	Commercial mortgage backed securities	1,074,497			—			1,074,497			—
	U.S. government and government agencies	1,301,809			1,301,809			—			—
	Non-U.S. government securities	1,085,861			—			1,085,861			—
	Asset backed securities	1,332,594			—			1,332,594			—
Total	9,676,857			1,301,809			8,373,003			2,045
Equity securities	496,824			496,738			86			—
Other investments	498,310			—			327,890			170,420
Short-term investments	1,478,367			1,427,744			50,623			—
Fair value option:
Investments accounted for using the fair value option:
Corporate bonds	334,065			—			334,065			—
Non-U.S. government bonds	73,156			—			73,156			—
Mortgage backed securities	41,033			—			41,033			—
Other investments	773,280			—			395,755			377,525
Total	1,221,534			—			844,009			377,525
Total assets measured at fair value	$	13,371,892		$	3,226,291		$	9,595,611		$	549,990
-1	In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged. For purposes of this table, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged.
The following table presents a reconciliation of the beginning and ending balances for all investments measured at fair value on a recurring basis using Level 3 inputs for 2014, 2013 and 2012:
	Fair Value Measurements Using:
	Significant Unobservable Inputs (Level 3)
				Assets												Liabilities
	Available for sale									Fair value option
	Asset Backed Securities			Corporate Bonds			Other Investments			Other Investments			Total			Contingent
																Consideration
																Liability
Year Ended December 31, 2014
Balance at beginning of year	$	—		$	2,045		$	170,420		$	377,525		$	549,990		$	—
Total gains or (losses) (realized/unrealized)
Included in earnings (1)	—			—			(39		)	16,818			16,779			(20,083		)
Included in other comprehensive income	—			—			(680		)	932			252			—
Purchases, issuances and settlements
Purchases	—			—			—			185,224			185,224			—
Issuances	—			—			—			—			—			(41,762		)
Sales	—			(2,045		)	(66,164		)	—			(68,209		)	—
Settlements	—			—			—			(114,149		)	(114,149		)	—
Transfers in and/or out of Level 3	57,500			—			—			—			57,500			—
Balance at end of year	$	57,500		$	—		$	103,537		$	466,350		$	627,387		$	(61,845	)
Year Ended December 31, 2013
Balance at beginning of year	$	—		$	98,404		$	184,202		$	195,350		$	477,956		$	—
Total gains or (losses) (realized/unrealized)
Included in earnings (1)	—			4,679			8,915			11,743			25,337			—
Included in other comprehensive income	—			(3,051		)	473			—			(2,578		)	—
Purchases, issuances and settlements
Purchases	—			—			25,000			275,067			300,067			—
Issuances	—			—			—			—			—			—
Sales	—			(96,655		)	—			(20,156		)	(116,811		)	—
Settlements	—			(1,332		)	(48,170		)	(84,479		)	(133,981		)	—
Transfers in and/or out of Level 3	—			—			—			—			—			—
Balance at end of year	$	—		$	2,045		$	170,420		$	377,525		$	549,990		$	—
-1	Gains or losses on corporate bonds were included in net realized gains (losses) while gains or losses on other investments were included in net realized gains (losses) or net investment income. Amounts related to the contingent consideration liability were included in net realized gains (losses).
The amount of total gains for 2014 included in earnings attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2014 was $11.5 million. The amount of total gains for 2013 included in earnings attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2013 was $22.0 million.
Fair Value Measurements on a Non-Recurring Basis
The Company measures the fair value of certain assets on a non-recurring basis, generally quarterly, annually, or when events or changes in circumstances indicate that the carrying amount of the assets may not be recoverable. These assets include investment funds accounted for using the equity method, investment in joint venture, certain other investments, goodwill and intangible assets, and long-lived assets. The Company uses a variety of techniques to measure the fair value of these assets when appropriate, as described below:
Investments accounted for using the equity method and Investment in joint venture. When the Company determines that the carrying value of these assets may not be recoverable, the Company records the assets at fair value with the loss recognized in income. In such cases, the Company measures the fair value of these assets using the techniques discussed above in “—Fair Value Measurements on a Recurring Basis.”
Goodwill and Intangible Assets. The Company tests goodwill and intangible assets for impairment whenever events or changes in circumstances indicate the carrying amount may not be recoverable, but at least annually for goodwill. When the Company determines goodwill and intangible assets may be impaired, the Company uses techniques including discounted expected future cash flows, to measure fair value.
Long-Lived Assets. The Company tests its long-lived assets for impairment whenever events or changes in circumstances indicate the carrying amount of a long-lived asset may not be recoverable.

Investment_in_Joint_Venture

Investment in Joint Venture	12 Months Ended
	Dec. 31, 2014
Equity Method Investments and Joint Ventures [Abstract]
Investment in Joint Venture	Investment in Joint Venture
	In May 2008, the Company provided $100.0 million of funding to Gulf Reinsurance Limited (“Gulf Re”), a newly formed reinsurer based in the Dubai International Financial Centre, pursuant to the joint venture agreement with Gulf Investment Corporation GSC (“GIC”). Under the agreement, Arch Re Bermuda and GIC each own 50% of Gulf Re, which commenced underwriting activities in June 2008. Gulf Re provides property and casualty reinsurance primarily in the member states of the Gulf Cooperation Council, which include Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the United Arab Emirates. The initial capital of the joint venture consisted of $200.0 million. The Company accounts for its investment in Gulf Re, shown as “Investment in joint venture,” using the equity method and records its equity in the operating results of Gulf Re in “Other income (loss).”
	For 2014, the Company recorded $14.1 million of losses from Gulf Re, compared to $0.4 million of losses for 2013 and $1.1 million of losses for 2012. Gulf Re’s losses for 2014 primarily reflected the impact of a number of large technical risk claims. The Company entered into a number of strategic initiatives related to Gulf Re in the 2014 fourth quarter, including an agreement to acquire complete ownership of Gulf Re, which is pending approval by the Dubai Financial Services Authority. To further support Gulf Re’s business in advance of the January 1 renewal season, the Company entered into a 90% whole account quota share retrocession arrangement of Gulf Re’s net liabilities and a portfolio transfer of all of Gulf Re’s existing business on a funds withheld basis, effective as of October 1, 2014. The portfolio transfer included approximately $50 million of net unearned premium reserves and $65 million of net reserves for losses and loss adjustment expenses.

Derivative_Instruments

Derivative Instruments	12 Months Ended
	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments	Derivative Instruments
	The Company’s investment strategy allows for the use of derivative securities. The Company’s derivative instruments are recorded on its consolidated balance sheets at fair value. The fair values of those derivatives are based on quoted market prices. All realized and unrealized contract gains and losses are reflected in the Company’s results of operations. The Company utilizes exchange traded U.S. Treasury note, Eurodollar and other futures contracts and commodity futures to manage portfolio duration or replicate investment positions in its portfolios. Certain of the Company’s corporate bonds are managed in a global bond portfolio which incorporates the use of foreign currency forward contracts which are intended to provide an economic hedge against foreign currency movements on the portfolio’s non-U.S. Dollar denominated holdings. The Company routinely utilizes other foreign currency forward contracts, currency options, index futures contracts and other derivatives as part of its total return objective.
	In addition, the Company purchases to-be-announced mortgage backed securities (“TBAs”) as part of its investment strategy. TBAs represent commitments to purchase a future issuance of agency mortgage backed securities. For the period between purchase of a TBA and issuance of the underlying security, the Company’s position is accounted for as a derivative. The Company purchases TBAs in both long and short positions to enhance investment performance and as part of its overall investment strategy. The Company did not hold any derivatives which were designated as hedging instruments at December 31, 2014 or 2013.
	The following table summarizes information on the fair values and notional values of the Company’s derivative instruments. The fair value of TBAs is included in “fixed maturities available for sale, at fair value.”
		Asset			Liability Derivatives			Net
		Derivatives						Derivatives
		Fair Value			Fair Value			Fair Value			Notional
											Value (1)
	31-Dec-14
	Futures contracts	$	2,156		$	(1,907	)	$	249		$	2,549,027
	Foreign currency forward contracts	10,511			(1,145		)	9,366			397,106
	TBAs	10,592			—			10,592			10,056
	Other	3,209			(2,345		)	864			735,684
	Total	$	26,468		$	(5,397	)	$	21,071
	31-Dec-13
	Futures contracts	$	461		$	(110	)	$	351		$	475,967
	Foreign currency forward contracts	5,023			(3,090		)	1,933			330,746
	TBAs	33,455			(21,731		)	11,724			56,160
	Other	920			(1,541		)	(621		)	347,916
	Total	$	39,859		$	(26,472	)	$	13,387
	-1	Represents the absolute notional value of all outstanding contracts, consisting of long and short positions.
	The Company’s derivative instruments are generally traded under master netting agreements, which establish terms that apply to all derivative transactions with a counterparty. In the event of a bankruptcy or other stipulated event of default, such agreements provide that the non-defaulting party may elect to terminate all outstanding derivative transactions, in which case all individual derivative positions (loss or gain) with a counterparty are closed out and netted and replaced with a single amount, usually referred to as the termination amount, which is expressed in a single currency. The resulting single net amount, where positive, is payable to the party “in-the-money” regardless of whether or not it is the defaulting party, unless the parties have agreed that only the non-defaulting party is entitled to receive a termination payment where the net amount is positive and is in its favor. Effectively, contractual close-out netting reduces derivatives credit exposure from gross to net exposure. At December 31, 2014, $25.3 million and $5.4 million, respectively, of asset derivatives and liability derivatives were subject to a master netting agreement or similar arrangement, compared to $28.0 million and $14.6 million, respectively, at December 31, 2013. The remaining derivatives included in the table above were not subject to a master netting agreement.
	The following table summarizes net realized gains (losses) recorded on the Company’s derivative instruments in the consolidated statements of income:
	Derivatives not designated	Year Ended December 31,
	as hedging instruments	2014			2013			2012
	Futures contracts	$	30,444		$	10,742		$	(3,307	)
	Foreign currency forward contracts	13,430			9,762			(214		)
	TBAs	(342		)	(1,623		)	4,413
	Other	402			2,031			(2,218		)
	Total	$	43,934		$	20,912		$	(1,326	)

Other_Comprehensive_Income

Other Comprehensive Income	12 Months Ended
	Dec. 31, 2014
Comprehensive Income Note Disclosure [Abstract]
Other Comprehensive Income (Loss)	Other Comprehensive Income (Loss)
	The following table presents the changes in each component of accumulated other comprehensive income (“AOCI”), net of tax:
		Unrealized Appreciation on Available-For-Sale Investments				Foreign Currency Translation Adjustments				Total
	Year Ended December 31, 2014
	Beginning balance	$	80,692			$	(5,728	)		$	74,964
	Other comprehensive income (loss) before reclassifications	146,010				(27,014		)		118,996
	Amounts reclassified from accumulated other comprehensive income	(65,104		)		—				(65,104		)
	Net current period other comprehensive income (loss)	80,906				(27,014		)		53,892
	Ending balance	$	161,598			$	(32,742	)		$	128,856
	Year Ended December 31, 2013
	Beginning balance	$	289,956			$	(2,939	)		$	287,017
	Other comprehensive income (loss) before reclassifications	(176,578		)		(2,789		)		(179,367		)
	Amounts reclassified from accumulated other comprehensive income	(32,686		)		—				(32,686		)
	Net current period other comprehensive income (loss)	(209,264		)		(2,789		)		(212,053		)
	Ending balance	$	80,692			$	(5,728	)		$	74,964
	Year Ended December 31, 2012
	Beginning balance	$	174,636			$	(20,713	)		$	153,923
	Other comprehensive income (loss) before reclassifications	273,144				17,774				290,918
	Amounts reclassified from accumulated other comprehensive income	(157,824		)		—				(157,824		)
	Net current period other comprehensive income (loss)	115,320				17,774				133,094
	Ending balance	$	289,956			$	(2,939	)		$	287,017
	The following table presents details about amounts reclassified from accumulated other comprehensive income:
			Consolidated Statement of Income		Amounts Reclassed from AOCI
			Line Item That Includes		Year Ended December 31,
	Details About AOCI Components		Reclassification		2014				2013				2012
	Unrealized appreciation on available-for-sale investments
			Net realized gains		$	102,223			$	39,308			$	180,144
			Other-than-temporary impairment losses		(30,470		)		(3,961		)		(12,175		)
			Total before tax		71,753				35,347				167,969
			Income tax expense		(6,649		)		(2,661		)		(10,145		)
			Net of tax		$	65,104			$	32,686			$	157,824
	Following are the related tax effects allocated to each component of other comprehensive income (loss):
		Before				Tax				Net
		Tax				Expense				of Tax
		Amount				(Benefit)				Amount
	Year Ended December 31, 2014
	Unrealized appreciation (decline) in value of available-for-sale investments:
	Unrealized holding losses arising during period	$	161,685			$	15,355			$	146,330
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	(320		)		—				(320		)
	Less reclassification of net realized gains included in net income	71,753				6,649				65,104
	Foreign currency translation adjustments	(27,014		)		—				(27,014		)
	Other comprehensive income (loss)	$	62,598			$	8,706			$	53,892
	Year Ended December 31, 2013
	Unrealized appreciation (decline) in value of available-for-sale investments:
	Unrealized holding gains arising during period	$	(201,386	)		$	(24,983	)		$	(176,403	)
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	(175		)		—				(175		)
	Less reclassification of net realized gains included in net income	35,347				2,661				32,686
	Foreign currency translation adjustments	(2,789		)		—				(2,789		)
	Other comprehensive income (loss)	$	(239,697	)		$	(27,644	)		$	(212,053	)
	Year Ended December 31, 2012
	Unrealized appreciation (decline) in value of available-for-sale investments:
	Unrealized holding gains arising during period	$	278,917			$	4,986			$	273,931
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	(787		)		—				(787		)
	Less reclassification of net realized gains included in net income	167,969				10,145				157,824
	Foreign currency translation adjustments	18,748				974				17,774
	Other comprehensive income (loss)	$	128,909			$	(4,185	)		$	133,094

Earnings_Per_Common_Share

Earnings Per Common Share	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Earnings Per Common Share	Earnings Per Common Share
	The calculation of basic earnings per common share excludes dilutive securities and is computed by dividing income available to Arch common shareholders by the weighted average number of Common Shares, including vested restricted shares, outstanding for the periods. The following table sets forth the computation of basic and diluted earnings per common share:
		Year Ended December 31,
		2014			2013			2012
	Numerator:
	Net income	$	821,260		$	709,731		$	593,397
	Amounts attributable to noncontrolling interests	13,095			—			—
	Net income available to Arch	834,355			709,731			593,397
	Preferred dividends	(21,938		)	(21,938		)	(25,079		)
	Loss on repurchase of preferred shares	—			—			(10,612		)
	Net income available to Arch common shareholders	$	812,417		$	687,793		$	557,706
	Denominator:
	Weighted average common shares outstanding – basic	130,817,610			131,355,392			134,446,158
	Effect of dilutive common share equivalents:
	Nonvested restricted shares	1,128,540			1,090,100			857,174
	Stock options (1)	2,976,172			3,331,691			2,955,515
	Weighted average common shares and common share	134,922,322			135,777,183			138,258,847
	equivalents outstanding – diluted
	Earnings per common share:
	Basic	$	6.21		$	5.24		$	4.15
	Diluted	$	6.02		$	5.07		$	4.03
	-1	Certain stock options were not included in the computation of diluted earnings per share where the exercise price of the stock options exceeded the average market price and would have been anti-dilutive or where, when applying the treasury stock method to in-the-money options, the sum of the proceeds, including unrecognized compensation, exceeded the average market price and would have been anti-dilutive. For 2014, 2013 and 2012, the number of stock options excluded were 1,435,955, 892,439 and 839,414, respectively.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes
	ACGL is incorporated under the laws of Bermuda and, under current Bermuda law, is not obligated to pay any taxes in Bermuda based upon income or capital gains. The Company has received a written undertaking from the Minister of Finance in Bermuda under the Exempted Undertakings Tax Protection Act 1966 that, in the event that any legislation is enacted in Bermuda imposing any tax computed on profits, income, gain or appreciation on any capital asset, or any tax in the nature of estate duty or inheritance tax, such tax will not be applicable to ACGL or any of its operations until March 31, 2035. This undertaking does not, however, prevent the imposition of taxes on any person ordinarily resident in Bermuda or any company in respect of its ownership of real property or leasehold interests in Bermuda.
	ACGL and its non-U.S. subsidiaries will be subject to U.S. federal income tax only to the extent that they derive U.S. source income that is subject to U.S. withholding tax or income that is effectively connected with the conduct of a trade or business within the U.S. and is not exempt from U.S. tax under an applicable income tax treaty with the U.S. ACGL and its non-U.S. subsidiaries will be subject to a withholding tax on dividends from U.S. investments and interest from certain U.S. payors (subject to reduction by any applicable income tax treaty). ACGL and its non-U.S. subsidiaries intend to conduct their operations in a manner that will not cause them to be treated as engaged in a trade or business in the United States and, therefore, will not be required to pay U.S. federal income taxes (other than U.S. excise taxes on insurance and reinsurance premium and withholding taxes on dividends and certain other U.S. source investment income). However, because there is uncertainty as to the activities which constitute being engaged in a trade or business within the United States, there can be no assurances that the U.S. Internal Revenue Service will not contend successfully that ACGL or its non-U.S. subsidiaries are engaged in a trade or business in the United States. If ACGL or any of its non-U.S. subsidiaries were subject to U.S. income tax, ACGL’s shareholders’ equity and earnings could be materially adversely affected. ACGL has subsidiaries and branches that operate in various jurisdictions around the world that are subject to tax in the jurisdictions in which they operate. The significant jurisdictions in which ACGL’s subsidiaries and branches are subject to tax include the United States, United Kingdom, Ireland, Canada, Switzerland and Denmark.
	The components of income taxes attributable to operations were as follows:
		Year Ended December 31,
		2014			2013			2012
	Current expense (benefit):
	United States	$	29,749		$	27,537		$	8,267
	Non-U.S.	801			5,159			737
		30,550			32,696			9,004
	Deferred expense (benefit):
	United States	(8,168		)	1,937			(9,779		)
	Non-U.S.	605			(1,859		)	(3,235		)
		(7,563		)	78			(13,014		)
	Income tax expense (benefit)	$	22,987		$	32,774		$	(4,010	)
	The Company’s income or loss before income taxes was earned in the following jurisdictions:
		Year Ended December 31,
		2014			2013			2012
	Income (Loss) Before Income Taxes:
	Bermuda	$	794,926		$	717,661		$	609,710
	United States	53,055			87,032			9,600
	Other	(3,734		)	(62,188		)	(29,923		)
	Total	$	844,247		$	742,505		$	589,387
	The expected tax provision computed on pre-tax income or loss at the weighted average tax rate has been calculated as the sum of the pre-tax income in each jurisdiction multiplied by that jurisdiction’s applicable statutory tax rate. The statutory tax rates by jurisdiction were as follows: Bermuda (0.0%), United States (35.0%), United Kingdom (21.5%), Ireland (12.5%), Denmark (24.5%), Canada (24.0%), Gibraltar (10.0%) and the Netherlands (23.9%).
	A reconciliation of the difference between the provision for income taxes and the expected tax provision at the weighted average tax rate follows:
		Year Ended December 31,
		2014			2013			2012
	Expected income tax expense (benefit) computed on pre-tax income	$	18,178		$	17,506		$	(3,426	)
	at weighted average income tax rate
	Addition (reduction) in income tax expense (benefit) resulting from:
	Tax-exempt investment income	(8,048		)	(8,255		)	(9,257		)
	Meals and entertainment	711			599			688
	State taxes, net of U.S. federal tax benefit	1,281			431			270
	Foreign branch taxes	464			703			544
	Prior year adjustment	320			2,810			(1,581		)
	Foreign exchange gains & losses	746			(1,254		)	(436		)
	Changes in applicable tax rate	(51		)	2,007			1,193
	Dividend withholding taxes	3,276			4,619			2,511
	Change in valuation allowance	(736		)	11,795			4,281
	Contingent consideration	6,763			—			—
	Other	83			1,813			1,203
	Income tax expense (benefit)	$	22,987		$	32,774		$	(4,010	)
	Deferred income tax assets and liabilities reflect temporary differences based on enacted tax rates between the carrying amounts of assets and liabilities for financial reporting and income tax purposes. Significant components of the Company’s deferred income tax assets and liabilities were as follows:
		December 31,
		2014			2013
	Deferred income tax assets:
	Net operating loss	$	12,017		$	15,294
	AMT credit carryforward	5,309			14,456
	Discounting of net loss reserves	63,880			53,293
	Net unearned premium reserve	34,087			30,698
	Compensation liabilities	27,765			26,481
	Foreign tax credit carryforward	4,628			4,343
	Interest expense	5,208			6,049
	Goodwill	885			—
	Tax and loss bonds	21,247			—
	Other, net	13,408			13,347
	Deferred tax assets before valuation allowance	188,434			163,961
	Valuation allowance	(12,624		)	(15,548		)
	Deferred tax assets net of valuation allowance	175,810			148,413
	Deferred income tax liabilities:
	Depreciation and amortization	(7,579		)	(3,810		)
	Deferred acquisition costs, net	(2,744		)	(3,369		)
	Deposit accounting liability	(4,170		)	(4,581		)
	Contingency reserve	(18,480		)	—
Net unrealized appreciation of investments	(11,695		)	(6,354		)
Other, net	(150		)	(1,738		)
Total deferred tax liabilities	(44,818		)	(19,852		)
Net deferred income tax assets	$	130,992		$	128,561
The Company provides a valuation allowance to reduce certain deferred tax assets to an amount which management expects to more likely than not be realized. As of December 31, 2014, the Company’s valuation allowance was $12.6 million, compared to $15.5 million at December 31, 2013. The valuation allowance in both periods was primarily attributable to (1) a full valuation allowance on the Company’s Canadian operations; (2) unutilized foreign tax credits; and (3) certain other deferred tax assets relating to carryforwards that have a limited use.
At December 31, 2014, the Company has net operating loss carryforwards in its U.K. operating subsidiaries of approximately $20.7 million. Additionally, the Company’s U.K. operations have a foreign tax credit carryforward of $4.6 million at December 31, 2014. These operating losses and foreign tax credits can be carried forward without expiration. Due to uncertainty surrounding their future utilization, valuation allowances of $0.2 million and $4.6 million have been established in respect of $1.2 million U.K. operating losses and $4.6 million foreign tax credits respectively.
At December 31, 2014, net operating loss carryforwards in Ireland were approximately $10.0 million. Although these losses may be carried forward indefinitely, a valuation allowance of $0.3 million has been established in respect of $2.5 million of these losses given management’s expectation that losses in a specific Irish entity will not be utilized in the future.
At December 31, 2014, net operating loss carryforwards in the U.S. were approximately $18.9 million. Such net operating loss carryforwards are currently available to offset future taxable income of the Company’s U.S. subsidiaries. Under applicable law, the U.S. net operating loss carryforwards expire between 2029 and 2034. On January 30, 2014, the Company’s U.S. mortgage operations underwent an ownership change for U.S. federal income tax purposes as a result of the Company’s acquisition of the CMG Entities. As a result of this ownership change, a limitation has been imposed upon the utilization of approximately $12.1 million of the Company’s existing U.S. net operating loss carryforwards. Utilization is limited to approximately $0.6 million per year in accordance with Section 382 of the Internal Revenue Code of 1986 as amended (“the Code”). Additionally, the Company has an alternative minimum tax (“AMT”) credit carryforward in the amount of $5.3 million which can be carried forward indefinitely.
Given the Company’s U.S. mortgage operations, the Company is eligible for a tax deduction, subject to certain limitations, under Section 832(e) of the Code for amounts required by state law or regulation to be set aside in statutory contingency reserves. The deduction is allowed only to the extent that the Company purchases non-interest bearing U.S. Mortgage Guaranty Tax and Loss Bonds (“T&L Bonds”) issued by the Treasury Department in an amount equal to the tax benefit derived from deducting any portion of the statutory contingency reserves.
Cumulative T&L bonds purchased and subsequent redemptions are reflected in the Company’s deferred tax asset and total approximately $21.2 million at December 31, 2014.
Deferred income tax liabilities have not been accrued with respect to all of the undistributed earnings of the Company's U.S. and Canadian subsidiaries. If the earnings were to be distributed, as dividends or otherwise, such amounts may be subject to withholding tax in the jurisdiction of the paying entity. No withholding taxes have been accrued with respect to the earnings of the Company's U.S. and Canadian subsidiaries, as it is the intention that all such earnings will be permanently reinvested. Additionally, determining the amount of the related deferred income tax liability is not practicable.
The Company recognizes interest and penalties relating to unrecognized tax benefits in the provision for income taxes. As of December 31, 2014, the Company’s total unrecognized tax benefits, including interest and penalties, were nil.
The Company or its subsidiaries or branches files income tax returns in the U.S. federal jurisdiction and various state, local and foreign jurisdictions. The Company has an open examination by the tax authorities in the U.S. for the 2009 and 2011 tax years. The Company believes that this examination will be concluded within the next 12 months. While the Company cannot estimate with certainty the outcome of this examination, the Company does not believe that adjustments from this examination will result in a significant change to the Company’s results of operation. The following table details open tax years that are potentially subject to examination by local tax authorities, in the following major jurisdictions:
Jurisdiction	Tax Years
United States	2009-2014
United Kingdom	2009-2014
Ireland	2010-2014
Canada	2010-2014
Switzerland	2011-2014
Denmark	2011-2014
As of December 31, 2014, the Company’s current income tax payable (included in “Other liabilities”) was $3.6 million.

Transactions_with_Related_Part

Transactions with Related Parties	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
Transactions with Related Parties	Transactions with Related Parties
	The Company made an investment of $50.0 million in Aeolus LP (“Aeolus”) in 2006. Aeolus operates as an unrated reinsurance platform that provides property catastrophe protection to insurers and reinsurers on both an ultimate net loss and industry loss warranty basis. The Company’s investment in Aeolus, included in “Investments accounted for using the equity method” on the balance sheet, represents an approximate 4% share in Aeolus and is accounted for using the equity method. The Company made its investment in Aeolus on the same economic terms as a fund affiliated with Warburg Pincus LLC. During 2010, all remaining shares of the Company owned by funds affiliated with Warburg Pincus LLC were distributed. In addition, one of the founders of Aeolus is Peter Appel, former President and CEO and a former director of the Company. During 2014, 2013 and 2012, the Company received distributions of $0.6 million, $2.1 million and $19.8 million, respectively, from Aeolus.
	As part of its investment philosophy, the Company invests a portion of its investment portfolio in alternative investment funds. As of December 31, 2014, the Company had aggregate commitments of $395.9 million to funds managed by The Carlyle Group (“Carlyle”). Of such amount, $260.2 million was unfunded as of December 31, 2014. The Company may make additional commitments to funds managed by Carlyle from time to time. During 2014, 2013 and 2012, the Company made aggregate capital contributions to funds managed by Carlyle of $51.7 million, $60.8 million and $43.0 million, respectively, and received aggregate cash distributions from funds managed by Carlyle of $33.6 million, $15.0 million and $4.5 million, respectively. Kewsong Lee, a director of ACGL, joined Carlyle as Managing Director and Deputy Chief Investment Officer for Corporate Private Equity on December 31, 2013.
	See note 4, “Variable Interest Entity and Noncontrolling Interests,” for information about Watford Re. Certain directors and executive officers of the Company own common and preference shares of Watford Re.

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies
	Concentrations of Credit Risk
	The creditworthiness of a counterparty is evaluated by the Company, taking into account credit ratings assigned by independent agencies. The credit approval process involves an assessment of factors, including, among others, the counterparty, country and industry credit exposure limits. Collateral may be required, at the discretion of the Company, on certain transactions based on the creditworthiness of the counterparty.
	The areas where significant concentrations of credit risk may exist include unpaid losses and loss adjustment expenses recoverable, prepaid reinsurance premiums and paid losses and loss adjustment expenses recoverable net of reinsurance balances payable (collectively, “reinsurance balances recoverable”), investments and cash and cash equivalent balances. The Company’s reinsurance balances recoverable at December 31, 2014 and 2013 amounted to $1.97 billion and $1.94 billion, respectively, and primarily resulted from reinsurance arrangements entered into in the course of its operations. A credit exposure exists with respect to reinsurance recoverables as they may become uncollectible. The Company manages its credit risk in its reinsurance relationships by transacting with reinsurers that it considers financially sound and, if necessary, the Company may hold collateral in the form of funds, trust accounts and/or irrevocable letters of credit. This collateral can be drawn on for amounts that remain unpaid beyond specified time periods on an individual reinsurer basis.
	In addition, the Company underwrites a significant amount of its business through brokers and a credit risk exists should any of these brokers be unable to fulfill their contractual obligations with respect to the payments of insurance and reinsurance balances owed to the Company. During 2014, approximately 13.4% and 15.0% of the Company’s consolidated gross written premiums were generated from or placed by Aon Corporation and its subsidiaries and Marsh & McLennan Companies and its subsidiaries, respectively, compared to approximately 16.0% and 17.0% for 2013, respectively, and 20.3% and 14.1% for 2012, respectively. No other broker and no one insured or reinsured accounted for more than 10% of gross premiums written for 2014, 2013 and 2012.
	The Company’s available for sale investment portfolio is managed in accordance with guidelines that have been tailored to meet specific investment strategies, including standards of diversification, which limit the allowable holdings of any single issue. There were no investments in any entity in excess of 10% of the Company’s shareholders’ equity at December 31, 2014 other than investments issued or guaranteed by the United States government or its agencies.
	Investment Commitments
	The Company’s investment commitments, which are primarily related to agreements entered into by the Company to invest in funds and separately managed accounts when called upon, were approximately $968.9 million at December 31, 2014.
	Letter of Credit and Revolving Credit Facilities
	As of December 31, 2014, the Company had a $300.0 million unsecured revolving loan and letter of credit facility and a $500.0 million secured letter of credit facility (the “Credit Agreement”). Under the terms of the Credit Agreement, Arch Re U.S. and Arch Re Bermuda are limited to issuing an aggregate of $100.0 million of unsecured letters of credit as part of the $300.0 million unsecured revolving loan. Borrowings of revolving loans may be made by ACGL at a variable rate based on LIBOR or an alternative base rate at the option of the Company. Secured letters of credit are available for issuance on behalf of the Company’s insurance and reinsurance subsidiaries. The Credit Agreement and related documents are structured such that each party that requests a letter of credit or borrowing does so only for itself and for only its own obligations. Issuance of letters of credit and borrowings under the Credit Agreement are subject to the Company’s compliance with certain covenants and conditions, including absence of a material adverse change. These covenants require, among other things, that the Company maintain a debt to total capital ratio of not greater than 0.35 to 1 and consolidated tangible net worth in excess of $3.95 billion plus 25% of future aggregate net income for each quarterly period (not including any future net losses) beginning after March 31, 2014 and 25% of future aggregate proceeds from the issuance of common or preferred equity and that the Company’s principal insurance and reinsurance subsidiaries maintain at least a “B++” rating from A.M. Best. In addition, certain of the Company’s subsidiaries which are party to the Credit Agreement are required to maintain minimum shareholders’ equity levels. The Company was in compliance with all covenants contained in the Credit Agreement at December 31, 2014. The Credit Agreement expires on June 30, 2019.
	In addition, the Company has access to secured letter of credit facilities for up to a total of $190.9 million, which are available on a limited basis and for limited purposes (together with the secured portion of the Credit Agreement and these letter of credit facilities, the “LOC Facilities”). The principal purpose of the LOC Facilities is to issue, as required, evergreen standby letters of credit in favor of primary insurance or reinsurance counterparties with which the Company has entered into reinsurance arrangements to ensure that such counterparties are permitted to take credit for reinsurance obtained from the Company’s reinsurance subsidiaries in United States jurisdictions where such subsidiaries are not licensed or otherwise admitted as an insurer, as required under insurance regulations in the United States, and to comply with requirements of Lloyd’s of London in connection with qualifying quota share and other arrangements. The amount of letters of credit issued is driven by, among other things, the timing and payment of catastrophe losses, loss development of existing reserves, the payment pattern of such reserves, the further expansion of the Company’s business and the loss experience of such business. When issued, certain letters of credit are secured by a portion of the Company’s investment portfolio. In addition, the LOC Facilities also require the maintenance of certain covenants, which the Company was in compliance with at December 31, 2014. At such date, the Company had approximately $423.2 million in outstanding letters of credit under the LOC Facilities, which were secured by investments with a fair value of $483.5 million, and had $100.0 million of borrowings outstanding under the Credit Agreement.
	As of December 31, 2014, Watford Re had a $200 million line of credit facility that expires on May 19, 2015. At December 31, 2014, Watford Re had $26.0 million in outstanding letters of credit under that credit facility. Watford Re was in compliance with all covenants contained in its credit facility at December 31, 2014.
	Leases and Purchase Obligations
	At December 31, 2014, the future minimum rental commitments, exclusive of escalation clauses and maintenance costs and net of rental income, for all of the Company’s operating leases are as follows:
	2015	$	23,343
	2016	23,670
	2017	22,539
	2018	20,803
	2019	17,705
	Thereafter	46,672
	Total	$	154,732
	All of these leases are for the rental of office space, with expiration terms that range from 2015 to 2024. Rental expense, net of income from subleases, was approximately $23.1 million, $18.7 million and $17.1 million for 2014, 2013 and 2012, respectively.
	The Company has also entered into certain agreements which commit the Company to purchase goods or services, primarily related to software and computerized systems. Such purchase obligations were approximately $22.2 million and $35.9 million at December 31, 2014 and 2013, respectively.
	Employment and Other Arrangements
	At December 31, 2014, the Company has entered into employment agreements with certain of its executive officers for periods extending up to March 2018. Such employment arrangements provide for compensation in the form of base salary, annual bonus, share-based awards, participation in the Company’s employee benefit programs and the reimbursements of expenses.

Senior_Notes

Senior Notes	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Senior Notes	Senior Notes
	On May 4, 2004, ACGL completed a public offering of $300.0 million principal amount of 7.35% senior notes due May 1, 2034 (“ACGL Senior Notes”). The ACGL Senior Notes are ACGL’s senior unsecured obligations and rank equally with all of its existing and future senior unsecured indebtedness. Interest payments on the ACGL Senior Notes are due on May 1st and November 1st of each year. ACGL may redeem the ACGL Senior Notes at any time and from time to time, in whole or in part, at a “make-whole” redemption price. The fair value of the ACGL Senior Notes at December 31, 2014 and 2013 was $462.4 million and $381.0 million, respectively.
	On December 13, 2013, Arch-U.S., a wholly-owned subsidiary of ACGL, completed a public offering of $500.0 million principal amount of 5.144% senior notes due November 1, 2043 (“Arch-U.S. Senior Notes”), fully and unconditionally guaranteed by ACGL (the “Guarantee”). The Arch-U.S. Senior Notes are unsecured and unsubordinated obligations of Arch-U.S. and ACGL, respectively, and rank equally and ratably with the other unsecured and unsubordinated indebtedness of Arch-U.S. and ACGL, respectively. Interest payments on the Arch-U.S. Senior Notes are due on May 1st and November 1st of each year. Arch-U.S. may redeem the Arch-U.S. Senior Notes at any time and from time to time, in whole or in part, at a “make-whole” redemption price. The fair value of the Arch-U.S. Senior Notes at December 31, 2014 and 2013 was $609.9 million and $514.1 million, respectively.

Goodwill_and_Intangible_Assets

Goodwill and Intangible Assets	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	Goodwill and Intangible Assets
	The following table shows an analysis of goodwill and intangible assets:
		Goodwill			Intangible assets with an indefinite life			Intangible assets with finite life			Total
	Net balance at December 31, 2012	$	—		$	16,666		$	21,593		$	38,259
	Acquisitions	—			—			—			—
	Amortization	—			—			(11,170		)	(11,170		)
	Foreign currency translation adjustment	—			—			230			230
	Net balance at December 31, 2013	—			16,666			10,653			27,319
	Acquisitions	14,965			16,858			76,435			108,258
	Amortization	—			—			(25,520		)	(25,520		)
	Foreign currency translation adjustment	(19		)	—			(499		)	(518		)
	Net balance at December 31, 2014	$	14,946		$	33,524		$	61,069		$	109,539
	Gross balance at December 31, 2014	$	14,965		$	33,524		$	124,908		$	173,397
	Accumulated amortization	—			—			(63,439		)	(63,439		)
	Foreign currency translation adjustment	(19		)	—			(400		)	(419		)
	Net balance at December 31, 2014	$	14,946		$	33,524		$	61,069		$	109,539
	The following table presents the components of intangible assets:
		Gross Balance			Accumulated			Foreign Currency Translation Adjustment			Net
					Amortization						Balance
	31-Dec-14
	Acquired insurance contracts	$	77,574		$	(47,710	)	$	(107	)	$	29,757
	Operating platform	29,900			(5,482		)				24,418
	Customer relationships	25,910			(12,283		)	(293		)	13,334
	Goodwill	14,965			—			(19		)	14,946
	Insurance licenses	33,524			—			—			33,524
	Unfavorable service contract	(9,533		)	2,229			—			(7,304		)
	Other	1,057			(193		)	—			864
	Total	$	173,397		$	(63,439	)	$	(419	)	$	109,539
	31-Dec-13
	Acquired insurance contracts	$	31,102		$	(28,208	)	$	—		$	2,894
	Customer relationships	17,371			(9,711		)	99			7,759
	Insurance licenses	16,666			—			—			16,666
	Total	$	65,139		$	(37,919	)	$	99		$	27,319
	The expected remaining amortization expense for intangible assets with a finite life is $21.8 million for 2015, $17.0 million for 2016, $13.3 million for 2017, $9.4 million for 2018 and $0.8 million for 2019. The estimated remaining useful lives of these assets range from two to eight years at December 31, 2014.
	The Company’s impairment reviews for goodwill and intangible assets did not result in the recognition of impairment losses for 2014, 2013 and 2012.

Shareholders_Equity

Shareholders' Equity	12 Months Ended
	Dec. 31, 2014
Stockholders' Equity Note [Abstract]
Shareholders' Equity	Shareholders’ Equity
	Authorized and Issued
	The authorized share capital of the Company consists of 600 million Common Shares, par value of $0.0033 per share, and 50 million Preferred Shares, par value of $0.01 per share.
	Common Shares
	The following table presents a roll-forward of changes in the Company’s issued and outstanding Common Shares:
		Year Ended December 31,
		2014		2013		2012
	Common Shares:
	Shares issued and outstanding, beginning of year	169,560,591		168,255,572		164,636,338
	Shares issued (1)	1,493,524		811,732		2,066,065
	Restricted shares issued, net of cancellations	618,293		493,287		1,553,169
	Shares issued, end of year	171,672,408		169,560,591		168,255,572
	Common shares in treasury, end of year	(44,304,474	)	(35,885,707	)	(34,412,959	)
	Shares issued and outstanding, end of year	127,367,934		133,674,884		133,842,613
	-1	Includes shares issued from the exercise of stock options and stock appreciation rights, and shares issued from the employee share purchase plan.
	Share Repurchases
	The board of directors of ACGL has authorized the investment in ACGL’s common shares through a share repurchase program. Authorizations have consisted of a $1.0 billion authorization in February 2007, a $500.0 million authorization in May 2008, a $1.0 billion authorization in November 2009, a $1.0 billion authorization in February 2011 and a $629.2 million authorization in November 2014. Since the inception of the share repurchase program through December 31, 2014, ACGL has repurchased approximately 118.1 million common shares for an aggregate purchase price of $3.24 billion. During 2014, ACGL repurchased 8.2 million common shares for an aggregate purchase price of $454.1 million, compared to 1.3 million common shares for an aggregate purchase price of $57.8 million during 2013 and 3.9 million common shares for an aggregate purchase price of $172.1 million during 2012. Weighted average shares outstanding for 2014 were reduced by 111.9 million shares, compared to 109.6 million shares for 2013 and 105.0 million shares for 2012. At December 31, 2014, approximately $887.1 million of share repurchases were available under the program. Repurchases under the program may be effected from time to time in open market or privately negotiated transactions. The timing and amount of the repurchase transactions under this program will depend on a variety of factors, including market conditions and corporate and regulatory considerations.
	Treasury Shares
	In May 2010, ACGL’s shareholders approved amendments to the bye-laws to permit ACGL to hold its own acquired shares as treasury shares in lieu of cancellation, as determined by ACGL’s board of directors. From May 5, 2010 to December 31, 2014, all repurchases of ACGL’s common shares in connection with the share repurchase plan noted above and other share-based transactions were held in the treasury under the cost method, and the cost of the common shares acquired is included in ‘Common shares held in treasury, at cost.’ At December 31, 2014, the Company held 44.3 million shares for an aggregate cost of $1.56 billion in treasury, at cost.
	Preferred Shares
	On April 2, 2012, the Company completed the underwritten public offering of $325.0 million of its 6.75% Series C non-cumulative preferred shares (“Preferred Shares”). Except in specified circumstances relating to certain tax or corporate events, the Preferred Shares are not redeemable prior to April 2, 2017. The net proceeds from the offering of $315.8 million and other available funds were used to redeem all of the Company’s $200.0 million of 8.0% Series A preferred shares and $125.0 million of 7.875% Series B preferred shares on May 2, 2012. The Series A preferred shares and Series B preferred shares were redeemed at a redemption price equal to $25.00 per share, plus all declared and unpaid dividends to (but excluding) the redemption date.
	Dividends on the Preferred Shares are non-cumulative. Consequently, in the event dividends are not declared on the Preferred Shares for any dividend period, holders of Preferred Shares will not be entitled to receive a dividend for such period, and such undeclared dividend will not accrue and will not be payable. Holders of Preferred Shares will be entitled to receive dividend payments only when, as and if declared by ACGL’s board of directors or a duly authorized committee of the board of directors. Any such dividends will be payable from the date of original issue on a non-cumulative basis, quarterly in arrears. To the extent declared, these dividends will accumulate, with respect to each dividend period, in an amount per share equal to 6.75% of the $25.00 liquidation preference per annum. The Company paid dividends of $21.9 million in 2014, compared to $21.9 million in 2013 and $28.4 million in 2012 to holders of the Company’s preferred shares.

ShareBased_Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-Based Compensation	Share-Based Compensation
	Long Term Incentive and Share Award Plans
	The 2012 Long Term Incentive and Share Award Plan (the “2012 Plan”) became effective as of May 9, 2012 (the “Effective Date”) following approval by shareholders of the Company. The 2012 Plan is intended to provide for competitive compensation opportunities, to encourage long-term service, to recognize individual contributions and reward achievement of performance goals and to promote the creation of long-term value for shareholders by aligning the interests of such persons with those of shareholders. The 2012 Plan provides for the grant to eligible employees and directors stock options, stock appreciation rights (“SARs”), restricted shares, restricted share units payable in common shares or cash, share awards in lieu of cash awards, dividend equivalents and other share-based awards. The 2012 Plan also provides the Company’s non-employee directors with the opportunity to receive the annual retainer fee for Board service in common shares. The 2012 Plan will terminate as to future awards on February 28, 2022.
	The 2007 Long Term Incentive and Share Award Plan (the “2007 Plan”) was merged with and into the 2012 Plan as of the Effective Date. As of the Effective Date, the 3,153,924 remaining shares available for issuance under the 2007 Plan were transferred into the 2012 Plan and as of such date no additional grants may be made under the 2007 Plan. Grants which were outstanding under the 2007 Plan as of the Effective Date will continue in accordance with their original terms (subject to such amendments as the compensation committee determines appropriate, consistent with the terms of the 2007 Plan) and will be issued or transferred under the 2012 Plan.
	The number of common shares reserved for grants of awards under the 2012 Plan, subject to anti-dilution adjustments in the event of certain changes in the Company’s capital structure is 7,433,924 which is the sum of (i) 4,280,000 and (ii) 3,153,924 shares remaining available for grants under the 2007 Plan. In addition, no more than 50% of such common shares may be issued in connection with full value awards (i.e., awards other than stock options or SARs) and no more than 2,000,000 common shares may be issued as incentive stock options under Section 422 of the Code. At December 31, 2014, 980,571 shares are available for grant under the 2012 Plan.
	On May 11, 2007, following shareholder approval, the Company adopted the 2007 Employee Share Purchase Plan (the “ESPP”). The purpose of the ESPP is to give employees of ACGL and its subsidiaries an opportunity to purchase common shares through payroll deductions, thereby encouraging employees to share in the economic growth and success of ACGL and its subsidiaries. The ESPP is designed to qualify as an “employee share purchase plan” under Section 423 of the Code. A total of 2,250,000 common shares are reserved for issuance under the ESPP. At December 31, 2014, approximately 762,293 shares remain available for issuance. The ESPP provides for consecutive six-month offering periods (or other periods of not more than 27 months as determined by the compensation committee) under which participating employees can elect to have up to 20% of their total compensation withheld and applied to the purchase of common shares of the Company at the end of the period. Unless otherwise determined by the compensation committee before an offering period commences, (1) the purchase price will be 85% of the fair market value of the common shares at the beginning of the offering period; and (2) the maximum number of common shares that may be purchased by an employee in any offering period is 3,000 shares. In addition, applicable Code limitations specify, in general, that a participant’s right to purchase stock under the ESPP cannot accumulate at a rate in excess of $25,000 (based on the value at the beginning of the applicable offering periods) per calendar year. The Company recorded $1.6 million of share-based compensation expense, net of a tax benefit of $0.1 million, related to the ESPP for 2014, compared to $1.4 million, net of a tax benefit of $0.1 million, for 2013 and $1.1 million, net of a tax benefit of $0.1 million, for 2012.
	In June 2002, following shareholder approval, the Company adopted the 2002 Long Term Incentive and Share Award Plan (the “2002 Plan”). An aggregate of 9,497,490 Common Shares has been reserved for issuance under the 2002 Plan. The 2002 Plan expired in January 2012.
	With respect to certain subsidiaries, the Company may withhold, or require a participant to remit to the Company, an amount sufficient to satisfy any federal, state or local withholding tax requirements associated with awards under the Company’s share award plans. This includes the authority to withhold or receive shares or other property and to make cash payments in respect thereof.
	Stock Options and Stock Appreciation Rights
	The Company generally issues stock options and SARs to eligible employees, with exercise prices equal to the fair market values of the Company’s Common Shares on the grant dates. Such grants generally vest over a three year period with one-third vesting on the first, second and third anniversaries of the grant date. In addition, in November 2012 the Company issued off-cycle stock options and SARs to certain employees, which will cliff vest on the fifth anniversary of the grant date. Option awards and SARs have a 10 year contractual life. Refer to Note 3(m) for details related to the Company’s accounting for stock options and SARs.
	The Company recorded after-tax share-based compensation expense of $12.5 million related to stock options and SARs for 2014, net of a tax benefit of $2.1 million, compared to $10.6 million for 2013, net of a tax benefit of $2.0 million, and $8.5 million for 2012, net of a tax benefit of $1.8 million. As of December 31, 2014, there was approximately $14.8 million of unrecognized compensation cost related to nonvested stock options and SARs. Such cost is expected to be recognized over a weighted average period of 1.8 years.
	For purposes of determining estimated market value, the Company has computed the estimated market values of share-based compensation related to stock options and SARs using the Black-Scholes option valuation model and has applied the assumptions set forth in the following table. As described above, stock options and SARs generally vest over a three year period with one-third vesting on the first, second and third anniversaries of the grant date. The expected life assumption (i.e., the estimated period of time between the date an option or SAR is granted and the date the option or SAR is exercised) was based on an expected term analysis which incorporated the Company’s historical exercise experience. The Company based its estimate of expected volatility for stock options and SARs granted during 2014 on daily historical trading data of its common shares from September 20, 2002, the date marking the completion of the Company’s transition as a worldwide insurance and reinsurance company. For stock options and SARs granted during 2013 and 2012, the Company based its volatility estimate under the same method used for 2014, using the period from September 20, 2002 through the last day of the applicable period.
		Year Ended December 31,
		2014			2013			2012
	Dividend yield	0	%		0	%		0	%
	Expected volatility	22.8	%		23.6	%		24.3	%
	Risk free interest rate	1.9	%		1	%		1	%
	Expected option life	6.0 years			6.0 years			6.25 years
	The Black-Scholes option pricing model requires the input of highly subjective assumptions. Because the Company’s employee stock options and SARs have characteristics significantly different from those of traded options, and because changes in the subjective input assumptions can materially affect the fair value estimate, in management’s opinion, the existing models may not provide a reliable single measure of the fair value of its employee stock options and SARs. In addition, management will continue to assess the assumptions and methodologies used to calculate estimated fair value of share-based compensation. Circumstances may change and additional data may become available over time, which could result in changes to these assumptions and methodologies, and which could materially impact the Company’s fair value determination.
	A summary of stock option and SAR activity under the Company’s Long Term Incentive and Share Award Plans during 2014 is presented below:
		Year Ended December 31, 2014
		Number of			Weighted Average Exercise Price
		Options / SARs
	Outstanding, beginning of year	8,338,480			$	28.82
	Granted	1,078,036			$	56.71
	Exercised	(1,542,828	)		$	18.62
	Forfeited or expired	(69,655	)		$	46.91
	Outstanding, end of year	7,804,033			$	34.52
	Exercisable, end of year	5,558,439			$	28.91
	The weighted average grant-date fair value of stock options and SARs granted during 2014, 2013 and 2012 was $14.77, $12.96 and $10.61, respectively. The aggregate intrinsic value of stock options and SARs exercised during 2014, 2013 and 2012 was approximately $58.4 million, $21.7 million, and $64.0 million, respectively and represents the difference between the exercise price of the stock options and SARs and the closing market price of the Company’s common shares on the exercise dates. Shares issued upon exercise of stock options and SARs were from the Company’s authorized but unissued share capital pool.
	The aggregate intrinsic value of the Company’s outstanding and exercisable stock options and SARs at December 31, 2014 was $191.8 million and $167.8 million, respectively. The weighted average remaining contractual life of the Company’s outstanding and exercisable stock options and SARs at December 31, 2014 was 5.7 years and 4.6 years, respectively. During 2014, the Company received proceeds of $11.0 million from the exercise of stock options and recognized a tax benefit of $8.9 million from the exercise of stock options and SARs.
	Restricted Common Shares and Restricted Units
	The Company also issues restricted share and unit awards to eligible employees and directors, for which the fair value is equal to the fair market values of the Company’s Common Shares on the grant dates. Restricted share and unit awards generally vest over a three year period with one-third vesting on the first, second and third anniversaries of the grant date. In addition, in November 2012 the Company issued off-cycle restricted share and unit awards to certain employees, which will cliff vest on the fifth anniversary of the grant date. Refer to Note 3(m) for details related to the Company’s accounting for restricted share and unit awards.
	The Company recorded $30.0 million of share-based compensation expense, net of a tax benefit of $8.4 million, related to restricted share and unit awards for 2014, compared to $27.4 million, net of a tax benefit of $7.8 million, for 2013 and $24.6 million, net of a tax benefit of $6.3 million, for 2012. As of December 31, 2014 , there were $48.5 million and $8.1 million, respectively, of unrecognized compensation costs related to unvested restricted share and unit awards which are expected to be recognized over a weighted average period of 1.7 years and 1.8 years, respectively.
	A summary of unvested restricted share and unit activity under the Company’s Long Term Incentive and Share Award Plans for 2014 is presented below:
		Year Ended December 31, 2014
		Restricted				Restricted
		Common				Unit
		Shares				Awards
	Unvested Shares:
	Unvested balance, beginning of year	1,943,125				308,322
	Granted	660,043				107,426
	Vested	(591,576		)		(88,210		)
	Forfeited	(41,750		)		(22,405		)
	Unvested balance, end of year	1,969,842				305,133
	Weighted Average Grant Date Fair Value:
	Unvested balance, beginning of year	$	43.55			$	43.32
	Granted	$	56.94			$	57.18
	Vested	$	41.89			$	41.86
	Forfeited	$	46.54			$	45.07
	Unvested balance, end of year	$	48.47			$	48.49
	During 2014, 2013 and 2012, the Company granted an aggregate of 767,469, 590,118 and 1,819,657 restricted share and restricted unit awards, respectively, with weighted average grant date fair values of $56.97, $53.21 and $40.75, respectively. During 2014, 2013 and 2012, the aggregate fair value of restricted shares and units that vested was $38.6 million, $41.0 million and $33.2 million, respectively. The aggregate intrinsic value of restricted units outstanding at December 31, 2014 was $26.4 million, and the aggregate intrinsic value of restricted units vested and deferred was $8.4 million.
	The issuance of share-based awards and amortization thereon has no effect on the Company’s consolidated shareholders’ equity.

Retirement_Plans

Retirement Plans	12 Months Ended
	Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Retirement Plans	Retirement Plans
	For purposes of providing employees with retirement benefits, the Company maintains defined contribution retirement plans. Contributions are based on the participants’ eligible compensation. For 2014, 2013 and 2012, the Company expensed $26.8 million, $21.5 million and $19.3 million, respectively, related to these retirement plans.

Legal_Proceedings

Legal Proceedings	12 Months Ended
	Dec. 31, 2014
Disclosure Legal Proceedings [Abstract]
Legal Proceedings	Legal Proceedings
	The Company, in common with the insurance industry in general, is subject to litigation and arbitration in the normal course of its business. As of December 31, 2014, the Company was not a party to any litigation or arbitration which is expected by management to have a material adverse effect on the Company’s results of operations and financial condition and liquidity.

Statutory_Information

Statutory Information	12 Months Ended
	Dec. 31, 2014
Disclosure Statutory Information [Abstract]
Statutory Information	Statutory Information
	The Company’s insurance and reinsurance subsidiaries are subject to insurance and/or reinsurance laws and regulations in the jurisdictions in which they operate. These regulations include certain restrictions on the amount of dividends or other distributions available to shareholders without prior approval of the insurance regulatory authorities.
	The actual and required statutory capital and surplus for the Company’s principal operating subsidiaries at December 31, 2014 and 2013 was as follows:
		December 31, 2014					December 31, 2013
		Actual (1)		Required (1)			Actual (1)			Required (1)
	Statutory capital and surplus (1):
	Bermuda	$	6,447,803	$	2,209,757		$	5,416,948		$	2,077,442
	Ireland	553,561		463,877			533,283			458,666
	United States	1,454,282		467,797			1,013,228			335,442
	United Kingdom	384,525		307,596			392,734			353,330
	Canada	79,123		58,837			77,877			74,951
	-1	Such amounts include ownership interests in affiliated insurance and reinsurance subsidiaries.
	The statutory net income (loss) for the Company’s principal operating subsidiaries for 2014, 2013 and 2012 was as follows:
		Year Ended December 31,
		2014		2013			2012
	Statutory net income (loss):
	Bermuda	$	692,676	$	719,267		$	631,483
	Ireland	43,197		(24,410		)	(1,940		)
	United States	46,110		62,605			(21,517		)
	United Kingdom	13,016		(11,353		)	(4,449		)
	Canada	1,517		(36,203		)	—
	Statutory accounting differs from U.S. GAAP in the reporting of certain items such as acquisition costs, deferred income taxes and investments.
	Bermuda
	Under The Insurance Act 1978, as amended, and related regulations of Bermuda (the “Insurance Act”), Arch Re Bermuda, the Company’s Bermuda reinsurance and insurance subsidiary, is registered as a Class 4 insurer and long-term insurer and is required to annually prepare and file statutory financial statements and a statutory financial return with the Bermuda Monetary Authority (“BMA”). The Insurance Act also requires Arch Re Bermuda to maintain minimum share capital and must ensure that the value of its general business assets exceeds the amount of its general business liabilities by an amount greater than the prescribed minimum solvency margins and enhanced capital requirement pertaining to its general business. At December 31, 2014, all such requirements were met.
	Arch Re Bermuda is also required to file a regulatory risk based capital model that measures risks and determines enhanced capital requirements and a target capital level. In addition, all Class 4 Bermuda insurers must prepare and file with the BMA audited GAAP basis annual financial statements, which must be made publicly available. Declarations of dividends from retained earnings and distributions from additional paid-in-capital are subject to these requirements being met. For all applicable periods presented herein, Arch Re Bermuda satisfied these requirements.
	The Bermuda Companies Act 1981 (the “Companies Act”) limits Arch Re Bermuda’s ability to pay dividends and distributions to shareholders if there are reasonable grounds for believing that: (a) Arch Re Bermuda is, or would after the payment be, unable to pay its liabilities as they become due; or (b) the realizable value of Arch Re Bermuda’s assets would thereby be less than the aggregate of its liabilities and its issued share capital and share premium accounts. Under the Insurance Act, Arch Re Bermuda is restricted with respect to the payment of dividends. Arch Re Bermuda is prohibited from declaring or paying in any financial year dividends of more than 25% of its total statutory capital and surplus (as shown on its previous financial year’s statutory balance sheet) unless it files, at least seven days before payment of such dividends, with the Bermuda Monetary Authority an affidavit stating that it will continue to meet the required margins. In addition, Arch Re Bermuda is prohibited, without prior approval of the Bermuda Monetary Authority, from reducing by 15% or more its total statutory capital, as set out in its previous year’s statutory financial statements. Accordingly, Arch Re Bermuda can pay approximately $1.36 billion to ACGL during 2015 without providing an affidavit to the BMA.
	Watford Re is registered as a Class 4 insurer and is required to annually prepare and file statutory financial statements and a statutory financial return with the BMA. The Insurance Act also requires Watford Re to maintain minimum share capital and must ensure that the value of its general business assets exceeds the amount of its general business liabilities by an amount greater than the prescribed minimum solvency margins and enhanced capital requirement pertaining to its general business. At December 31, 2014, all such requirements were met.
	Ireland
	Arch Re Europe was licensed and authorized by the Central Bank of Ireland (“CBOI”) as a non-life reinsurer in October 2008 and as a life reinsurer in November 2009 while Arch MI Europe was authorized as a non-life insurer in Ireland in December 2011. Irish authorized reinsurers and insurers, such as Arch Re Europe and Arch MI Europe, are also subject to the general body of Irish laws and regulations including the provisions of the Companies Acts 1963-2013. Arch Re Europe and Arch MI Europe must file and submit their annual audited financial statements in accordance with Irish generally accepted accounting principles and related reports to the Registrar of Companies (“Registrar”) under the Companies Acts 1963-2013 together with an annual return of certain core corporate information. Changes to core corporate information during the year must also be notified to the Registrar. These requirements are in addition to the regulatory returns required to be filed annually with the CBOI. Arch Re Europe and Arch MI Europe are required to maintain reserves, particularly in respect of underwriting liabilities and a solvency margin as provided for in the European Communities (Reinsurance) Regulations, 2006, related guidance and the European Communities Insurance Accounts Regulations, 1996. Assets constituting statutory reserves must comply with certain principles including obligations to secure sufficiency, liquidity, security, quality, profitability and currency matching of investments. Statutory reserves must be actuarially certified annually.
	Under Irish company law, Arch Re Europe and Arch MI Europe are permitted to make distributions only out of profits available for distribution. A company’s profits available for distribution are its accumulated, realized profits, so far as not previously utilized by distribution or capitalization, less its accumulated, realized losses, so far as not previously written off in a reduction or reorganization of capital duly made. Further, the CBOI has powers to intervene if a dividend payment were to lead to a breach of regulatory capital requirements. Dividends or distributions, if any, made by Arch Re Europe would result in an increase in available capital at Arch Re Bermuda.
	United States
	The Company’s U.S. insurance and reinsurance subsidiaries file financial statements prepared in accordance with statutory accounting practices prescribed or permitted by insurance regulators. Statutory net income and statutory surplus, as reported to the insurance regulatory authorities, differ in certain respects from the amounts prepared in accordance with GAAP. The main differences between statutory net income and GAAP net income relate to deferred acquisition costs and deferred income taxes. In addition to deferred acquisition costs and deferred income tax assets, other differences between statutory surplus and GAAP shareholder’s equity are unrealized appreciation or decline in value of investments and non-admitted assets.
	The Company’s U.S. insurance and reinsurance subsidiaries are subject to insurance laws and regulations in the jurisdictions in which they operate. The ability of the Company’s regulated insurance subsidiaries to pay dividends or make distributions is dependent on their ability to meet applicable regulatory standards. These regulations include restrictions that limit the amount of dividends or other distributions, such as loans or cash advances, available to shareholders without prior approval of the insurance regulatory authorities. Dividends or distributions, if any, made by Arch Re U.S. would result in an increase in available capital at Arch-U.S., the Company’s U.S. holding company. Arch Re U.S. can declare a maximum of approximately $110 million of dividends during 2015 subject to the approval of the Commissioner of the Delaware Department of Insurance (“Commissioner”). In addition, with respect to dividends in excess of the $110 million (extraordinary dividend), no payment can be made until (1) 30 days after the Commissioner has received notice of the declaration thereof and has not within such period disapproved such payments; or (2) the Commissioner shall have approved the payment within the 30-day period. Delaware insurance laws also require that the statutory surplus of Arch Re U.S. following any dividend or distribution be reasonable in relation to its outstanding liabilities and adequate to its financial needs.
	Arch MI U.S. is subject to detailed regulation both by its domiciliary and primary regulator, the Wisconsin Office of the Commissioner of Insurance, and by state insurance departments in each state in which it is licensed. In addition, Arch MI U.S. is currently approved by the GSEs as an eligible mortgage insurer. In addition to existing eligibility requirements applicable to all eligible mortgage insurers, the GSEs imposed conditions in connection with their approvals of Arch MI U.S. as a qualified mortgage insurer. These conditions require, among other things, that Arch MI U.S.: (i) maintain, through December 31, 2016, minimum capital funding of $400 million which may consist of statutory capital (policyholders’ surplus plus contingency reserves), dedicated reinsurance trust assets for any primary business reinsured and a value for purchased technology assets; (ii) maintain minimum statutory capital (defined as policyholders’ surplus plus contingency reserves) of no less than $260 million; (ii) maintain a risk-to-capital ratio of no greater than 18 to 1; and (iv) refrain from paying dividends to affiliates for three years commencing February 2014.
	United Kingdom
	The Company’s European insurance operations are conducted on two platforms: Arch Insurance Company Europe and Arch Syndicate 2012 (collectively, the insurance operations are referred to as “Arch Insurance Europe”). The Financial Services Authority (the “FSA”), which on April 1, 2013 was replaced by the Prudential Regulatory Authority (“PRA”) and the Financial Conduct Authority (“FCA”), regulated insurance and reinsurance companies and firms carrying on insurance mediation activities operating in the U.K. Arch Insurance Company Europe was licensed and authorized by the FSA to underwrite all classes of general insurance in the U.K. in May 2004. In 2009, AUAL was licensed and authorized by the FSA and the Lloyd’s Franchise Board. AUAL holds the relevant permissions for the classes of insurance business which are underwritten in the U.K. by Arch Syndicate 2012. Arch Syndicate 2012 has one member, Arch Syndicate Investments Ltd. Arch Risk Partners was licensed and authorized by the FSA in February 2012 to conduct insurance mediation activities. All U.K. companies are also subject to a range of statutory provisions, including the laws and regulations of the Companies Acts 2006 (as amended) (the “U.K. Companies Acts”).
	Like all U.K. companies, Arch Insurance Europe must file and submit their annual audited financial statements in accordance with International Financial Reporting Standards and related reports to the Registrar of Companies under the U.K. Companies Acts together with an annual return of certain core corporate information and changes from the prior year. This requirement is in addition to the regulatory returns required to be filed annually with the FSA (now the PRA or the FCA, as applicable) for Arch Insurance Company Europe, AUAL and Arch Risk Partners and, in the case of AUAL and ASIL, Lloyd’s.
	Arch Insurance Company Europe, AUAL (on behalf of itself, Arch Syndicate 2012 and ASIL) and Arch Risk Partners were each required to demonstrate to the FSA that each had adequate financial assets to meet the financial resources requirement for its category. However, since the FSA split into the PRA and FCA, Arch Insurance Company Europe and AUAL (on behalf of itself, Arch Syndicate 2012 and ASIL) are now each required to demonstrate the adequacy of its financial assets to the PRA, while Arch Risk Partners is required to demonstrate the adequacy of its financial assets to the FCA. On a periodic basis, Arch Insurance Europe was required to provide the FSA and Lloyd’s with its own risk-based assessment of its capital needs, taking into account comprehensive risk factors, including market, credit, operational, liquidity and group risks to generate a revised calculation of its expected liabilities which, in turn, enabled the FSA and Lloyd’s to provide individual capital guidance and requirements to Arch Insurance Europe. Following the split of the FSA into the PRA and FCA, similar requirements from the PRA have taken effect. Arch Insurance Europe’s surplus is above the risk-based capital threshold allowed by the FSA’s (now PRA's) individual capital assessment of Arch Insurance Europe. The FSA required, and now the PRA requires, that Arch Insurance Europe maintain a margin of solvency calculation based on the classes of business for which it is authorized and within its premium income projections applied to its worldwide general business.
	Under U.K. law, all U.K. companies are restricted from declaring a dividend to their shareholders unless they have “profits available for distribution.” The calculation as to whether a company has sufficient profits is based on its accumulated realized profits minus its accumulated realized losses. U.K. insurance regulatory laws do not prohibit the payment of dividends, but the FSA required, and now the PRA or FCA, as applicable, requires that insurance companies and insurance intermediaries maintain certain solvency margins and may restrict the payment of a dividend by Arch Insurance Company Europe, AUAL, ASIL or Arch Risk Partners. Dividends or distributions, if any, made by Arch Insurance Europe would result in an increase in available capital at Arch Re Europe, a subsidiary of Arch Re Bermuda.
	Canada
	Arch Insurance Canada, which commenced underwriting on January 1, 2013, and the Canadian branch of Arch Re U.S. (“Arch Re Canada”), which commenced underwriting on January 1, 2015, are subject to federal, as well as provincial and territorial, regulation in Canada. The Office of the Superintendent of Financial Institutions (“OSFI”) is the federal regulatory body that, under the Insurance Companies Act (Canada), regulates federal Canadian and non-Canadian insurance companies operating in Canada. The primary goal of OSFI is to supervise the safety and soundness of insurance companies with the aim of securing the appropriate level of protection of insureds by imposing risk management, solvency and capital requirements on such companies. Arch Insurance Canada and Arch Re Canada are subject to regulation in the provinces and territories in which they underwrite insurance/reinsurance, and the primary goal of insurance/reinsurance regulation at the provincial and territorial levels is to govern the market conduct of insurance/reinsurance companies. Arch Insurance Canada is licensed to carry on insurance business by OSFI and in each province and territory. Arch Re Canada is licensed to carry-on reinsurance business by OSFI and in the province of Ontario.
	Under the Insurance Companies Act (Canada), Arch Insurance Canada is required to maintain an adequate amount of capital in Canada, calculated in accordance with a test promulgated by OSFI called the Minimum Capital Test (“MCT”), and Arch Re Canada is required to maintain an adequate margin of assets over liabilities in Canada, calculated in accordance with a test promulgated by OSFI called the Branch Adequacy of Assets Test. Arch Insurance Canada and Arch Re Canada are required to file financial information with OSFI on an ongoing basis, including annual audited financial statements in accordance with IFRS. Dividends or distributions, if any, made by Arch Insurance Canada would result in an increase in available capital at Arch Insurance Company (see “—United States” section).

Unaudited_Condensed_Quarterly_

Unaudited Condensed Quarterly Financial Information	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Unaudited Condensed Quarterly Financial Information	Unaudited Condensed Quarterly Financial Information
	The following table summarizes the Company’s 2014 and 2013 unaudited condensed quarterly financial information (including the results of the ‘other’ segment):
		Fourth			Third			Second			First
		Quarter			Quarter			Quarter			Quarter
	Year Ended December 31, 2014
	Net premiums written	$	895,481		$	959,539		$	971,928		$	1,064,990
	Net premiums earned	923,165			903,651			907,152			859,780
	Net investment income	82,496			80,105			72,990			66,994
	Net realized gains	10,561			18,515			54,144			19,697
	Net impairment losses recognized in earnings	(3,837		)	(8,593		)	(14,749		)	(2,971		)
	Underwriting income	113,464			101,087			124,091			132,456
	Net income	207,134			223,264			211,717			179,145
	Preferred dividends	(5,485		)	(5,484		)	(5,485		)	(5,484		)
	Net income available to Arch common shareholders	209,679			223,191			202,531			177,016
	Net income per common share -- basic	$	1.65		$	1.69		$	1.53		$	1.34
	Net income per common share -- diluted	$	1.6		$	1.64		$	1.48		$	1.3
	Year Ended December 31, 2013
	Net premiums written	$	748,921		$	839,135		$	810,535		$	952,776
	Net premiums earned	839,366			795,000			758,816			752,770
	Net investment income	67,095			66,083			68,369			65,672
	Net realized gains (losses)	9,048			(6,022		)	12,652			58,340
	Net impairment losses recognized in earnings	(88		)	(728		)	(724		)	(2,246		)
	Underwriting income	128,318			110,992			96,029			116,398
	Net income	161,490			114,825			176,940			256,476
	Preferred dividends	(5,485		)	(5,484		)	(5,485		)	(5,484		)
	Net income available to Arch common shareholders	156,005			109,341			171,455			250,992
	Net income per common share -- basic	$	1.19		$	0.83		$	1.31		$	1.92
	Net income per common share -- diluted	$	1.14		$	0.8		$	1.26		$	1.85

Guarantor_Financial_Informatio

Guarantor Financial Information	12 Months Ended
	Dec. 31, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Guarantor Financial Information	Guarantor Financial Information
	The following tables present condensed consolidating balance sheets at December 31, 2014 and 2013 and condensed consolidating statements of income, comprehensive income and cash flows for 2014, 2013 and 2012 for ACGL, Arch-U.S., a 100% owned subsidiary of ACGL, and ACGL's other subsidiaries.
		31-Dec-14
	Condensed Consolidating Balance Sheet	ACGL (Parent Guarantor)			Arch-U.S. (Subsidiary Issuer)			Other ACGL Subsidiaries			Consolidating Adjustments and Eliminations			ACGL Consolidated
	Assets
	Total investments	$	107		$	62,867		$	15,268,275		$	—		$	15,331,249
	Cash	3,218			2,787			479,697			—			485,702
	Investments in subsidiaries	6,536,644			1,685,185			—			(8,221,829		)	—
	Due from subsidiaries and affiliates	48			7,517			370,429			(377,994		)	—
	Premiums receivable	—			—			1,331,511			(382,816		)	948,695
	Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	—			—			5,584,973			(3,772,128		)	1,812,845
	Contractholder receivables	—			—			1,309,192			—			1,309,192
	Prepaid reinsurance premiums	—			—			1,373,008			(995,930		)	377,078
	Deferred acquisition costs, net	—			—			414,525			—			414,525
	Other assets	7,590			49,576			1,689,670			(416,579		)	1,330,257
	Total assets	$	6,547,607		$	1,807,932		$	27,821,280		$	(14,167,276	)	$	22,009,543
	Liabilities
	Reserve for losses and loss adjustment expenses	$	—		$	—		$	12,784,030		$	(3,747,582	)	$	9,036,448
	Unearned premiums	—			—			3,227,508			(995,930		)	2,231,578
	Reinsurance balances payable	—			—			589,289			(369,977		)	219,312
	Contractholder payables	—			—			1,309,192			—			1,309,192
	Deposit accounting liabilities	—			—			587,050			(259,666		)	327,384
	Senior notes	300,000			500,000			—			—			800,000
	Revolving credit agreement borrowings	100,000			—			—			—			100,000
	Due to subsidiaries and affiliates	417			7,505			370,072			(377,994		)	—
	Other liabilities	17,137			49,403			994,741			(194,298		)	866,983
	Total liabilities	417,554			556,908			19,861,882			(5,945,447		)	14,890,897
	Redeemable noncontrolling interests (1)	—			—			219,512			—			219,512
	Shareholders' Equity
	Total shareholders' equity available to Arch	6,130,053			1,251,024			6,970,805			(8,221,829		)	6,130,053
	Non-redeemable noncontrolling interests (1)	—			—			769,081			—			769,081
	Total shareholders' equity	6,130,053			1,251,024			7,739,886			(8,221,829		)	6,899,134
	Total liabilities, noncontrolling interests and shareholders' equity	$	6,547,607		$	1,807,932		$	27,821,280		$	(14,167,276	)	$	22,009,543
	(1) See Note 4.
		31-Dec-13
	Condensed Consolidating Balance Sheet	ACGL (Parent Guarantor)			Arch-U.S. (Subsidiary Issuer)			Other ACGL Subsidiaries			Consolidating Adjustments and Eliminations			ACGL Consolidated
	Assets
	Total investments	$	2,530		$	408,957		$	13,200,247		$	—		$	13,611,734
	Cash	3,223			509			430,325			—			434,057
	Investments in subsidiaries	6,046,060			1,258,889			—			(7,304,949		)	—
	Due from subsidiaries and affiliates	2,251			—			405,110			(407,361		)	—
	Premiums receivable	—			—			1,085,369			(331,445		)	753,924
	Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	—			—			5,645,156			(3,840,826		)	1,804,330
	Contractholder receivables	—			—			1,064,246			—			1,064,246
	Prepaid reinsurance premiums	—			—			1,109,312			(780,969		)	328,343
	Deferred acquisition costs, net	—			—			342,314			—			342,314
	Other assets	6,598			60,342			1,714,651			(554,445		)	1,227,146
	Total assets	$	6,060,662		$	1,728,697		$	24,996,730		$	(13,219,995	)	$	19,566,094
	Liabilities
	Reserve for losses and loss adjustment expenses	$	—		$	—		$	12,625,766		$	(3,801,070	)	$	8,824,696
	Unearned premiums	—			—			2,677,334			(780,969		)	1,896,365
	Reinsurance balances payable	—			—			662,394			(466,227		)	196,167
	Contractholder payables	—			—			1,064,246			—			1,064,246
	Deposit accounting liabilities	—			—			758,490			(337,193		)	421,297
	Senior notes	300,000			500,000			—			—			800,000
	Revolving credit agreement borrowings	100,000			—			—			—			100,000
	Due to subsidiaries and affiliates	18			10,250			397,093			(407,361		)	—
	Other liabilities	13,148			33,206			691,699			(122,226		)	615,827
	Total liabilities	413,166			543,456			18,877,022			(5,915,046		)	13,918,598
	Redeemable noncontrolling interests	—			—			—			—			—
	Shareholders' Equity
	Total shareholders' equity available to Arch	5,647,496			1,185,241			6,119,708			(7,304,949		)	5,647,496
	Non-redeemable noncontrolling interests	—			—			—			—			—
	Total shareholders' equity	5,647,496			1,185,241			6,119,708			(7,304,949		)	5,647,496
Total liabilities, noncontrolling interests and shareholders' equity	$	6,060,662		$	1,728,697		$	24,996,730		$	(13,219,995	)	$	19,566,094
	Year Ended December 31, 2014
Condensed Consolidating Statement of Income and Comprehensive Income	ACGL (Parent Guarantor)			Arch-U.S. (Subsidiary Issuer)			Other ACGL Subsidiaries			Consolidating Adjustments and Eliminations			ACGL Consolidated
Revenues
Net premiums earned	$	—		$	—		$	3,593,748		$	—		$	3,593,748
Net investment income	—			—			326,831			(24,246		)	302,585
Net realized gains	—			5			102,912			—			102,917
Net impairment losses recognized in earnings	—			—			(30,150		)	—			(30,150		)
Other underwriting income	—			—			10,142			—			10,142
Equity in net income of investment funds accounted for using the equity method	—			—			19,883			—			19,883
Other income (loss)	—			—			(10,252		)	—			(10,252		)
Total revenues	—			5			4,013,114			(24,246		)	3,988,873
Expenses
Losses and loss adjustment expenses	—			—			1,919,250			—			1,919,250
Acquisition expenses	—			—			657,262			—			657,262
Other operating expenses	46,074			3,387			556,763			—			606,224
Interest expense	23,423			25,817			20,640			(24,246		)	45,634
Net foreign exchange gains	—			—			(53,222		)	(30,522		)	(83,744		)
Total expenses	69,497			29,204			3,100,693			(54,768		)	3,144,626
Income (loss) before income taxes	(69,497		)	(29,199		)	912,421			30,522			844,247
Income tax benefit (expense)	—			10,125			(33,112		)	—			(22,987		)
Income (loss) before equity in net income of subsidiaries	(69,497		)	(19,074		)	879,309			30,522			821,260
Equity in net income of subsidiaries	903,852			53,584			—			(957,436		)	—
Net income	834,355			34,510			879,309			(926,914		)	821,260
Amounts attributable to noncontrolling interests (1)	—			—			13,095			—			13,095
Net income available to Arch	834,355			34,510			892,404			(926,914		)	834,355
Preferred dividends	(21,938		)	—			—			—			(21,938		)
Net income available to Arch common shareholders	$	812,417		$	34,510		$	892,404		$	(926,914	)	$	812,417
Comprehensive income available to Arch	$	888,247		$	33,671		$	976,821		$	(1,010,492	)	$	888,247
(1) See Note 4.
	Year Ended December 31, 2013
Condensed Consolidating Statement of Income and Comprehensive Income	ACGL (Parent Guarantor)			Arch-U.S. (Subsidiary Issuer)			Other ACGL Subsidiaries			Consolidating Adjustments and Eliminations			ACGL Consolidated
Revenues
Net premiums earned	$	—		$	—		$	3,145,952		$	—		$	3,145,952
Net investment income	—			31			307,449			(40,261		)	267,219
Net realized gains	—			—			74,018			—			74,018
Net impairment losses recognized in earnings	—			—			(3,786		)	—			(3,786		)
Other underwriting income	—			—			7,639			—			7,639
Equity in net income of investment funds accounted for using the equity method	—			—			35,701			—			35,701
Other income (loss)	—			—			(586		)	—			(586		)
Total revenues	—			31			3,566,387			(40,261		)	3,526,157
Expenses
Losses and loss adjustment expenses	—			—			1,679,424			—			1,679,424
Acquisition expenses	—			—			564,103			—			564,103
Other operating expenses	38,702			2,691			459,337			—			500,730
Interest expense	23,272			1,316			42,733			(40,261		)	27,060
Net foreign exchange losses	—			—			2,145			10,190			12,335
Total expenses	61,974			4,007			2,747,742			(30,071		)	2,783,652
Income (loss) before income taxes	(61,974		)	(3,976		)	818,645			(10,190		)	742,505
Income tax benefit (expense)	—			1,383			(34,157		)	—			(32,774		)
Income (loss) before equity in net income of subsidiaries	(61,974		)	(2,593		)	784,488			(10,190		)	709,731
Equity in net income of subsidiaries	771,705			25,644			—			(797,349		)	—
Net income	709,731			23,051			784,488			(807,539		)	709,731
Amounts attributable to noncontrolling interests	—			—			—			—			—
Net income available to Arch	709,731			23,051			784,488			(807,539		)	709,731
Preferred dividends	(21,938		)	—			—			—			(21,938		)
Net income available to Arch common shareholders	$	687,793		$	23,051		$	784,488		$	(807,539	)	$	687,793
Comprehensive income (loss) available to Arch	$	497,678		$	(28,330	)	$	562,245		$	(533,915	)	$	497,678
	Year Ended December 31, 2012
Condensed Consolidating Statement of Income and Comprehensive Income	ACGL (Parent Guarantor)			Arch-U.S. (Subsidiary Issuer)			Other ACGL Subsidiaries			Consolidating Adjustments and Eliminations			ACGL Consolidated
Revenues
Net premiums earned	$	—		$	—		$	2,935,140		$	—		$	2,935,140
Net investment income	4			8			321,805			(26,922		)	294,895
Net realized gains	—			—			194,228			—			194,228
Net impairment losses recognized in earnings	—			—			(11,388		)	—			(11,388		)
Other underwriting income	—			—			8,090			—			8,090
Equity in net income of investment funds accounted for using the equity method	—			—			73,510			—			73,510
Other income (loss)	—			—			(12,094		)	—			(12,094		)
Total revenues	4			8			3,509,291			(26,922		)	3,482,381
Expenses
Losses and loss adjustment expenses	—			—			1,861,277			—			1,861,277
Acquisition expenses	—			—			508,884			—			508,884
Other operating expenses	35,570			1,218			428,565			—			465,353
Interest expense	23,496			—			31,951			(26,922		)	28,525
Net foreign exchange losses	—			—			21,787			7,168			28,955
Total expenses	59,066			1,218			2,852,464			(19,754		)	2,892,994
Income (loss) before income taxes	(59,062		)	(1,210		)	656,827			(7,168		)	589,387
Income tax benefit	—			424			3,586			—			4,010
Income (loss) before equity in net income of subsidiaries	(59,062		)	(786		)	660,413			(7,168		)	593,397
Equity in net income of subsidiaries	652,459			9,738			—			(662,197		)	—
Net income	593,397			8,952			660,413			(669,365		)	593,397
Amounts attributable to noncontrolling interests	—			—			—			—			—
Net income available to Arch	593,397			8,952			660,413			(669,365		)	593,397
Preferred dividends	(25,079		)	—			—			—			(25,079		)
Loss on repurchase of preferred shares	(10,612		)	—			—			—			(10,612		)
Net income available to Arch common shareholders	$	557,706		$	8,952		$	660,413		$	(669,365	)	$	557,706
Comprehensive income available to Arch	$	726,491		$	2,796		$	786,338		$	(789,134	)	$	726,491
	Year Ended December 31, 2014
Condensed Consolidating Statement	ACGL (Parent Guarantor)			Arch-U.S. (Subsidiary Issuer)			Other ACGL Subsidiaries			Consolidating Adjustments and Eliminations			ACGL Consolidated
of Cash Flows
Operating Activities
Net Cash Provided By (Used For)	$	467,091		$	8,142		$	1,077,597		$	(515,700	)	$	1,037,130
Operating Activities
Investing Activities
Purchases of fixed maturity investments	—			(78,509		)	(28,666,770		)	—			(28,745,279		)
Purchases of equity securities	—			—			(520,817		)	—			(520,817		)
Purchases of other investments	—			—			(2,257,481		)	—			(2,257,481		)
Proceeds from the sales of fixed maturity investments	—			16,011			26,807,181			—			26,823,192
Proceeds from the sales of equity securities	—			—			411,362			—			411,362
Proceeds from the sales of other investments	—			—			1,643,000			—			1,643,000
Proceeds from redemptions and maturities of fixed maturity investments	—			—			762,995			—			762,995
Net (purchases) sales of short-term investments	2,423			408,591			166,112			—			577,126
Change in investment of securities lending collateral	—			—			57,470			—			57,470
Contributions to subsidiaries	—			(341,707		)	(128,825		)	470,532			—
Issuance of intercompany loans	—			(7,500		)	(7,500		)	15,000			—
Repayments of intercompany loans	—			—			38,964			(38,964		)	—
Purchase of business, net of cash acquired	—			—			(237,106		)	—			(237,106		)
Purchases of furniture, equipment and other assets	(271		)	—			(19,612		)	—			(19,883		)
Net Cash Provided By (Used For)	2,152			(3,114		)	(1,951,027		)	446,568			(1,505,421		)
Investing Activities
Financing Activities
Purchases of common shares under share repurchase program	(454,137		)	—			—			—			(454,137		)
Proceeds from common shares issued, net	6,827			—			470,532			(470,532		)	6,827
Proceeds from intercompany loans	—			7,500			7,500			(15,000		)	—
Repayments of intercompany loans	—			(10,250		)	(28,714		)	38,964			—
Change in securities lending collateral	—			—			(57,470		)	—			(57,470		)
Third party investment in non-redeemable noncontrolling interests (1)	—			—			796,903			—			796,903
Third party investment in redeemable noncontrolling interests (1)	—			—			219,233			—			219,233
Dividends paid to redeemable noncontrolling interests (1)	—			—			(14,448		)	—			(14,448		)
Dividends paid to parent (2)	—			—			(515,700		)	515,700			—
Other	—			—			64,973			—			64,973
Preferred dividends paid	(21,938		)	—			—			—			(21,938		)
Net Cash Provided By (Used For)	(469,248		)	(2,750		)	942,809			69,132			539,943
Financing Activities
Effects of exchange rates changes on foreign currency cash	—			—			(20,007		)	—			(20,007		)
Increase (decrease) in cash	(5		)	2,278			49,372			—			51,645
Cash beginning of year	3,223			509			430,325			—			434,057
Cash end of year	$	3,218		$	2,787		$	479,697		$	—		$	485,702
(1) See Note 4.
(2) Included in net cash provided by (used for) operating activities in the ACGL (Parent Guarantor) column.
	Year Ended December 31, 2013
Condensed Consolidating Statement	ACGL (Parent Guarantor)			Arch-U.S. (Subsidiary Issuer)			Other ACGL Subsidiaries			Consolidating Adjustments and Eliminations			ACGL Consolidated
of Cash Flows
Operating Activities
Net Cash Provided By (Used For)	$	68,562		$	(6,569	)	$	908,375		$	(119,500	)	$	850,868
Operating Activities
Investing Activities
Purchases of fixed maturity investments	—			—			(18,174,988		)	—			(18,174,988		)
Purchases of equity securities	—			—			(535,857		)	—			(535,857		)
Purchases of other investments	—			—			(1,326,729		)	—			(1,326,729		)
Proceeds from the sales of fixed maturity investments	—			—			17,196,614			—			17,196,614
Proceeds from the sales of equity securities	—			—			462,787			—			462,787
Proceeds from the sales of other investments	—			—			1,162,707			—			1,162,707
Proceeds from redemptions and maturities of fixed maturity investments	—			—			731,708			—			731,708
Net (purchases) sales of short-term investments	5,799			(400,162		)	(356,250		)	—			(750,613		)
Change in investment of securities lending collateral	—			—			(55,643		)	—			(55,643		)
Contributions to subsidiaries	(160		)	(97,850		)	(20,250		)	118,260			—
Issuance of intercompany loans	—			—			(10,250		)	10,250			—
Purchases of furniture, equipment and other assets	(712		)	—			(16,787		)	—			(17,499		)
Net Cash Provided By (Used For)	4,927			(498,012		)	(942,938		)	128,510			(1,307,513		)
Investing Activities
Financing Activities
Purchases of common shares under share repurchase program	(57,796		)	—			—			—			(57,796		)
Proceeds from common shares issued, net	3,051			—			118,260			(118,260		)	3,051
Proceeds from intercompany loans	—			10,250			—			(10,250		)	—
Proceeds from borrowings	—			494,228			—			—			494,228
Change in securities lending collateral	—			—			55,643			—			55,643
Dividends paid to parent (1)	—			—			(119,500		)	119,500			—
Other	—			—			50,830			—			50,830
Preferred dividends paid	(21,938		)	—			—			—			(21,938		)
Net Cash Provided By (Used For)	(76,683		)	504,478			105,233			(9,010		)	524,018
Financing Activities
Effects of exchange rates changes on foreign currency cash	—			—			(4,357		)	—			(4,357		)
Increase (decrease) in cash	(3,194		)	(103		)	66,313			—			63,016
Cash beginning of year	6,417			612			364,012			—			371,041
Cash end of year	$	3,223		$	509		$	430,325		$	—		$	434,057
(1) Included in net cash provided by (used for) operating activities in the ACGL (Parent Guarantor) column.
	Year Ended December 31, 2012
Condensed Consolidating Statement	ACGL (Parent Guarantor)			Arch-U.S. (Subsidiary Issuer)			Other ACGL Subsidiaries			Consolidating Adjustments and Eliminations			ACGL Consolidated
of Cash Flows
Operating Activities
Net Cash Provided By (Used For)	$	209,500		$	(8,086	)	$	978,679		$	(258,490	)	$	921,603
Operating Activities
Investing Activities
Purchases of fixed maturity investments	—			—			(17,568,592		)	—			(17,568,592		)
Purchases of equity securities	—			—			(268,999		)	—			(268,999		)
Purchases of other investments	—			—			(1,000,049		)	—			(1,000,049		)
Proceeds from the sales of fixed maturity investments	—			—			16,366,306			—			16,366,306
Proceeds from the sales of equity securities	—			—			313,617			—			313,617
Proceeds from the sales of other investments	—			—			443,630			—			443,630
Proceeds from redemptions and maturities of fixed maturity investments	—			—			1,115,594			—			1,115,594
Net (purchases) sales of short-term investments	(5,094		)	1,986			189,027			—			185,919
Change in investment of securities lending collateral	—			—			6,190			—			6,190
Contributions to subsidiaries	—			—			(38,576		)	38,576			—
Purchase of business, net of cash acquired	—			—			28,948			—			28,948
Purchases of furniture, equipment and other assets	(65		)	—			(18,467		)	—			(18,532		)
Net Cash Provided By (Used For)	(5,159		)	1,986			(431,371		)	38,576			(395,968		)
Investing Activities
Financing Activities
Proceeds from issuance of Series C preferred shares issued, net	315,763			—			—			—			315,763
Repurchase of Series A and B preferred shares	(325,000		)	—			—			—			(325,000		)
Purchases of common shares under share repurchase program	(172,056		)	—			—			—			(172,056		)
Proceeds from common shares issued, net	7,033			—			38,576			(38,576		)	7,033
Repayments of borrowings	—			—			(310,868		)	—			(310,868		)
Change in securities lending collateral	—			—			(6,190		)	—			(6,190		)
Dividends paid to parent (1)	—			—			(258,490		)	258,490			—
Other	—			—			6,664			—			6,664
Preferred dividends paid	(28,381		)	—			—			—			(28,381		)
Net Cash Provided By (Used For)	(202,641		)	—			(530,308		)	219,914			(513,035		)
Financing Activities
Effects of exchange rates changes on foreign currency cash	—			—			6,742			—			6,742
Increase (decrease) in cash	1,700			(6,100		)	23,742			—			19,342
Cash beginning of year	4,717			6,712			340,270			—			351,699
Cash end of year	$	6,417		$	612		$	364,012		$	—		$	371,041
(1) Included in net cash provided by (used for) operating activities in the ACGL (Parent Guarantor) column.

Schedule_III_Supplementary_Ins

Schedule III - Supplementary Insurance Information	12 Months Ended
	Dec. 31, 2014
Supplementary Insurance Information [Abstract]
Supplementary Insurance Information	SCHEDULE III
	ARCH CAPITAL GROUP LTD. AND SUBSIDIARIES
	SUPPLEMENTARY INSURANCE INFORMATION
	(U.S. dollars in thousands)
		Deferred Acquisition Costs,		Reserves for Losses and Loss Adjustment Expenses		Unearned Premiums		Net Premiums Earned		Net Investment Income (1)	Net Losses and Loss Adjustment Expenses Incurred		Amortization of Deferred Acquisition Costs		Other Operating Expenses (2)		Net Premiums Written
		Net
	December 31, 2014
	Insurance		$178,545		$6,161,500		$1,237,099		$2,017,370	NM		$1,260,953		$316,308		$335,157		$2,146,654
	Reinsurance	148,130		2,690,070		666,233		1,279,328		NM	532,450		261,438		147,964		1,265,991
	Mortgage	38,321		118,550		148,232		193,573		NM	55,674		49,400		66,891		204,837
	Other	49,529		66,328		180,014		103,477		NM	70,173		30,116		6,268		274,456
	Total		$414,525		$9,036,448		$2,231,578		$3,593,748	NM		$1,919,250		$657,262		$556,280		$3,891,938
	December 31, 2013
	Insurance	158,121		6,137,121		1,192,188		1,876,014		NM	1,188,445		311,904		315,387		1,948,796
	Reinsurance	167,642		2,680,288		612,725		1,218,672		NM	486,236		234,373		134,563		1,313,001
	Mortgage	16,551		7,287		91,452		51,266		NM	4,743		17,826		8,377		89,570
	Total		$342,314		$8,824,696		$1,896,365		$3,145,952	NM		$1,679,424		$564,103		$458,327		$3,351,367
	31-Dec-12
	Insurance		$141,962		$6,149,247		$1,077,211		$1,800,343	NM		$1,283,841		$298,983		$307,489		$1,825,334
	Reinsurance	106,677		2,781,424		517,619		1,118,127		NM	574,821		204,903		118,245		1,158,790
	Mortgage	14,183		2,621		53,148		16,670		NM	2,615		4,998		4,301		68,111
	Total		$262,822		$8,933,292		$1,647,978		$2,935,140	NM		$1,861,277		$508,884		$430,035		$3,052,235
	-1	The Company does not manage its assets by segment and, accordingly, net investment income is not allocated to each underwriting segment. See Note 5 for information related to the ‘other’ segment.
	-2	Certain other operating expenses relate to the Company’s corporate segment (non-underwriting). Such amounts are not reflected in the table above. See Note 5.

Schedule_IV_Reinsurance

Schedule IV - Reinsurance	12 Months Ended
	Dec. 31, 2014
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]
Reinsurance	SCHEDULE IV
	ARCH CAPITAL GROUP LTD. AND SUBSIDIARIES
	REINSURANCE
	(U.S. dollars in thousands)
		Gross Amount		Ceded to Other Companies (1)			Assumed From Other Companies (1)		Net		Percentage of Amount Assumed to Net
									Amount
	Year Ended December 31, 2014
	Premiums Written:
	Insurance	$	2,974,996	$	(862,015	)	$	33,673	$	2,146,654	1.6	%
	Reinsurance	22,405		(261,255		)	1,504,841		1,265,991		118.9	%
	Mortgage	103,545		(22,519		)	123,811		204,837		60.4	%
	Other	—		(14,171		)	288,627		274,456		105.2	%
	Total	$	3,100,946	$	(948,678	)	$	1,739,670	$	3,891,938	44.7	%
	Year Ended December 31, 2013
	Premiums Written:
	Insurance	$	2,682,446	$	(763,713	)	$	30,063	$	1,948,796	1.5	%
	Reinsurance	72,136		(86,620		)	1,327,485		1,313,001		101.1	%
	Mortgage	—		—			89,570		89,570		100	%
	Total	$	2,754,582	$	(845,256	)	$	1,442,041	$	3,351,367	43	%
	Year Ended December 31, 2012
	Premiums Written:
	Insurance	$	2,562,788	$	(768,625	)	$	31,171	$	1,825,334	1.7	%
	Reinsurance	111,076		(55,099		)	1,102,813		1,158,790		95.2	%
	Mortgage	—		—			68,111		68,111		100	%
	Total	$	2,673,864	$	(816,926	)	$	1,195,297	$	3,052,235	39.2	%
	-1	Certain amounts included in the gross premiums written of each segment are related to intersegment transactions and are included in the gross premiums written of each segment. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.

Schedule_VI_Supplementary_Info

Schedule VI - Supplementary Information For Property and Casualty Insurance Underwriters	12 Months Ended
	Dec. 31, 2014
Supplemental Information for Property, Casualty Insurance Underwriters [Abstract]
Supplementary Information for Property and Casualty Insurance Underwriters	SCHEDULE VI
	ARCH CAPITAL GROUP LTD. AND SUBSIDIARIES
	SUPPLEMENTARY INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
	(U.S. dollars in thousands)
	Column A	Column B		Column C		Column D		Column E		Column F		Column G		Column H					Column I		Column J		Column K
	Affiliation with Registrant	Deferred Acquisition Costs,		Reserves for Losses and Loss Adjustment Expenses		Discount, if any, deducted in Column C		Unearned Premiums		Net		Net Investment Income		Net Losses and Loss Adjustment Expenses Incurred Related to					Amortization		Net Paid Losses and Loss Adjustment Expenses		Net
		Net								Premiums Earned				(a) Current Year		(b)			of Deferred Acquisition Costs				Premiums Written
																Prior Years
	Consolidated Subsidiaries
	2014	$	414,525	$	9,036,448	$	14,732	$	2,231,578	$	3,593,748	$	302,585	$	2,246,152	$	(326,902	)	$	657,262	$	1,697,736	$	3,891,938
	2013	342,314		8,824,696		12,539		1,896,365		3,145,952		267,219		1,943,466		(264,042		)	564,103		1,708,817		3,351,367
	2012	262,822		8,933,292		10,485		1,647,978		2,935,140		294,895		2,082,805		(221,528		)	508,884		1,465,379		3,052,235

Significant_Accounting_Policie1

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Basis of presentation	The consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of ACGL and its subsidiaries, including Arch Re Bermuda, Arch Reinsurance Company (“Arch Re U.S.”), Arch-U.S., Arch Insurance Company, Arch Specialty Insurance Company, Arch Excess & Surplus Insurance Company, Arch Indemnity Insurance Company, Arch Insurance Canada Ltd. (“Arch Insurance Canada”), Arch Risk Transfer Services Ltd., Arch Reinsurance Europe Underwriting Limited (“Arch Re Europe”), Arch MI U.S., Arch Mortgage Guaranty Company, Arch Mortgage Insurance Limited (“Arch MI Europe”), Arch Insurance Company (Europe) Limited (“Arch Insurance Company Europe”), Lloyd’s of London syndicate 2012 and related companies (“Arch Syndicate 2012”) and Watford Re. All significant intercompany transactions and balances have been eliminated in consolidation.
Use of estimates	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates and assumptions.
Premium revenues	Insurance. Insurance premiums written are generally recorded at the policy inception and are primarily earned on a pro rata basis over the terms of the policies for all products, usually 12 months. Premiums written include estimates in the Company’s programs, specialty lines, lenders products business and for participation in involuntary pools. Such premium estimates are derived from multiple sources which include the historical experience of the underlying business, similar business and available industry information. Unearned premium reserves represent the portion of premiums written that relates to the unexpired terms of in-force insurance policies.
	Reinsurance. Reinsurance premiums written include amounts reported by brokers and ceding companies, supplemented by the Company’s own estimates of premiums where reports have not been received. The determination of premium estimates requires a review of the Company’s experience with the ceding companies, familiarity with each market, the timing of the reported information, an analysis and understanding of the characteristics of each line of business, and management’s judgment of the impact of various factors, including premium or loss trends, on the volume of business written and ceded to the Company. On an ongoing basis, the Company’s underwriters review the amounts reported by these third parties for reasonableness based on their experience and knowledge of the subject class of business, taking into account the Company’s historical experience with the brokers or ceding companies. In addition, reinsurance contracts under which the Company assumes business generally contain specific provisions which allow the Company to perform audits of the ceding company to ensure compliance with the terms and conditions of the contract, including accurate and timely reporting of information. Based on a review of all available information, management establishes premium estimates where reports have not been received. Premium estimates are updated when new information is received and differences between such estimates and actual amounts are recorded in the period in which estimates are changed or the actual amounts are determined.
	Reinsurance premiums written are recorded based on the type of contracts the Company writes. Premiums on the Company’s excess of loss and pro rata reinsurance contracts are estimated when the business is underwritten. For excess of loss contracts, premiums are recorded as written based on the terms of the contract. Estimates of premiums written under pro rata contracts are recorded in the period in which the underlying risks are expected to incept and are based on information provided by the brokers and the ceding companies. For multi-year reinsurance treaties which are payable in annual installments, generally, only the initial annual installment is included as premiums written at policy inception due to the ability of the reinsured to commute or cancel coverage during the term of the policy. The remaining annual installments are included as premiums written at each successive anniversary date within the multi-year term.
	Reinstatement premiums for the Company’s insurance and reinsurance operations are recognized at the time a loss event occurs, where coverage limits for the remaining life of the contract are reinstated under pre-defined contract terms. Reinstatement premiums, if obligatory, are fully earned when recognized. The accrual of reinstatement premiums is based on an estimate of losses and loss adjustment expenses, which reflects management’s judgment.
	Premium estimates are reviewed by management at least quarterly. Such review includes a comparison of actual reported premiums to expected ultimate premiums along with a review of the aging and collection of premium estimates. Based on management’s review, the appropriateness of the premium estimates is evaluated, and any adjustment to these estimates is recorded in the period in which it becomes known. Adjustments to premium estimates could be material and such adjustments could directly and significantly impact earnings favorably or unfavorably in the period they are determined because the estimated premium may be fully or substantially earned. A significant portion of amounts included as premiums receivable, which represent estimated premiums written, net of commissions, are not currently due based on the terms of the underlying contracts.
	Reinsurance premiums written, irrespective of the class of business, are generally earned on a pro rata basis over the terms of the underlying policies or reinsurance contracts. Contracts and policies written on a “losses occurring” basis cover claims that may occur during the term of the contract or policy, which is typically 12 months. Accordingly, the premium is earned evenly over the term. Contracts which are written on a “risks attaching” basis cover claims which attach to the underlying insurance policies written during the terms of such contracts. Premiums earned on such contracts usually extend beyond the original term of the reinsurance contract, typically resulting in recognition of premiums earned over a 24-month period. Certain of the Company’s reinsurance contracts include provisions that adjust premiums or acquisition expenses based upon the experience under the contracts. Premiums written and earned, as well as related acquisition expenses, are recorded based upon the projected experience under such contracts.
	The Company also writes certain reinsurance business that is intended to provide insurers with risk management solutions that complement traditional reinsurance. Under these contracts, the Company assumes a measured amount of insurance risk in exchange for an anticipated margin, which is typically lower than on traditional reinsurance contracts. The terms and conditions of these contracts may include additional or return premiums based on loss experience, loss corridors, sublimits and caps. Examples of such business include aggregate stop-loss coverages, financial quota share coverages and multi-year retrospectively rated excess of loss coverages. If these contracts are deemed to transfer risk, they are accounted for as reinsurance.
	Mortgage. Mortgage guaranty insurance policies are contracts that are generally non-cancelable by the insurer, are renewable at a fixed price, and provide for payment of premiums on a monthly, annual or single basis. Upon renewal, the Company is not able to re-underwrite or re-price its policies. Consistent with industry accounting practices, premiums written on a monthly basis are earned as coverage is provided. Premiums written on an annual basis are amortized on a monthly pro rata basis over the year of coverage. Primary mortgage insurance premiums written on policies covering more than one year are referred to as single premiums. A portion of the revenue from single premiums is recognized in premiums earned in the current period, and the remaining portion is deferred as unearned premiums and earned over the estimated expiration of risk of the policy. If single premium policies related to insured loans are canceled due to repayment by the borrower and the policy is a non-refundable product, the remaining unearned premium related to each canceled policy is recognized as earned premium upon notification of the cancellation.
	Unearned premiums represent the portion of premiums written that is applicable to the estimated unexpired risk of insured loans. A portion of premium payments may be refundable if the insured cancels coverage, which generally occurs when the loan is repaid, the loan amortizes to a sufficiently low amount to trigger a lender permitted or legally required cancellation, or the value of the property has increased sufficiently in accordance with the terms of the contract. Premium refunds reduce premiums earned in the consolidated statements of income. Generally, only unearned premiums are refundable. However, when the Company pays a claim on a delinquent loan, all servicer paid premiums received on the delinquent loan covering any period after the default date will be refunded, in accordance with the terms of the contract.
Deferred acquisition costs	Acquisition Costs. Acquisition expenses and other expenses related to the Company’s underwriting operations that vary with, and are directly related to, the successful acquisition or renewal of business are deferred and amortized based on the type of contract. For property and casualty insurance and reinsurance contracts, deferred acquisition costs are amortized over the period in which the related premiums are earned. Consistent with mortgage insurance industry accounting practice, amortization of acquisition costs related to the mortgage insurance contracts for each underwriting year’s book of business is recorded in proportion to estimated gross profits. Estimated gross profits are comprised of earned premiums and losses and loss adjustment expenses. For each underwriting year, the Company estimates the rate of amortization to reflect actual experience and any changes to persistency or loss development.
	Acquisition expenses, net of ceding commissions received from reinsurers, consist principally of commissions and premium taxes paid to obtain the Company’s business. Other operating expenses also include expenses that vary with, and are directly related to, the successful acquisition of business. Deferred acquisition costs are carried at their estimated realizable value and take into account anticipated losses and loss adjustment expenses, based on historical and current experience, and anticipated investment income.
	A premium deficiency occurs if the sum of anticipated losses and loss adjustment expenses, unamortized acquisition costs and maintenance costs exceed unearned premiums (including expected future premiums) and anticipated investment income. A premium deficiency reserve (“PDR”) is recorded by charging any unamortized acquisition costs to expense to the extent required in order to eliminate the deficiency. If the premium deficiency exceeds unamortized acquisition costs then a liability is accrued for the excess deficiency.
	To assess the need for a PDR on mortgage exposures, the Company develops loss projections based on modeled loan defaults related to its current policies in force. This projection is based on recent trends in default experience, severity and rates of defaulted loans moving to claim, as well as recent trends in the rate at which loans are prepaid. Evaluating the expected profitability of the Company’s existing mortgage insurance business and the need for a PDR for its mortgage business involves significant reliance upon assumptions and estimates with regard to the likelihood, magnitude and timing of potential losses and premium revenues.
	No premium deficiency charges were recorded by the Company during 2014, 2013 or 2012.
Deposit accounting	Certain assumed reinsurance contracts, which pursuant to Financial Accounting Standards Board (“FASB”) guidance regarding the accounting and reporting for reinsurance and short-duration and long-duration contracts, that are deemed not to transfer insurance risk, are accounted for using the deposit method of accounting as prescribed in the FASB guidance. However, it is possible that the Company could incur financial losses on such contracts. Management exercises significant judgment in the assumptions used in determining whether assumed contracts should be accounted for as reinsurance contracts or deposit contracts. Under the FASB guidance, for those contracts that contain only significant underwriting risk, the estimated profit margin is deferred and amortized over the contract period and such amount is included in the Company’s underwriting results. When the estimated profit margin is explicit, the margin is reflected as other underwriting income and any adverse financial results on such contracts are reflected as incurred losses. When the estimated profit margin is implicit, the margin is reflected as an offset to paid losses and any adverse financial results on such contracts are reflected as incurred losses. Additional judgments are required when applying the accounting guidance with respect to the revenue recognition criteria for contracts deemed to transfer only significant underwriting risk. For those contracts that contain only significant timing risk, an accretion rate is established at inception of the contract based on actuarial estimates whereby the deposit accounting liability is increased to the estimated amount payable over the contract term. The accretion on the deposit is based on the expected rate of return required to fund the expected future payment obligations. Periodically the Company reassesses the estimated ultimate liability and the related expected rate of return. The accretion of the deposit accounting liability as well as changes to the estimated ultimate liability and the accretion rate are reflected as part of interest expense in the Company’s results of operations.
	Deposit accounting liabilities totaled $327.4 million and $421.3 million, respectively, at December 31, 2014 and 2013. Under some of these contracts, the ceding company retains the related assets on a funds-held basis. In those instances, the Company records the asset as a deposit accounting asset. Deposit accounting assets of $284.4 million and $370.5 million, respectively, at December 31, 2014 and 2013 are included in “Other assets” on the Company’s balance sheet. Interest income produced by those assets are recorded as part of net investment income in the Company's results of operations.
Retroactive accounting	Retroactive reinsurance reimburses a ceding company for liabilities incurred as a result of past insurable events covered by the underlying policies reinsured. In certain instances, reinsurance contracts cover losses both on a prospective basis and on a retroactive basis and, accordingly, the Company bifurcates the prospective and retrospective elements of these reinsurance contracts and accounts for each element separately. Underwriting income generated in connection with retroactive reinsurance contracts is deferred and amortized into income over the settlement period while losses are charged to income immediately. Subsequent changes in estimated or actual cash flows under such retroactive reinsurance contracts are accounted for by adjusting the previously deferred amount to the balance that would have existed had the revised estimate been available at the inception of the reinsurance transaction, with a corresponding charge or credit to income.
Reinsurance ceded	In the normal course of business, the Company purchases reinsurance to increase capacity and to limit the impact of individual losses and events on its underwriting results by reinsuring certain levels of risk with other insurance enterprises or reinsurers. The Company uses pro rata, excess of loss and facultative reinsurance contracts. Reinsurance ceding commissions are recognized as a reduction to acquisition costs on a pro rata basis over the period of risk while the portion of such commissions that will be earned in the future is deferred. The accompanying consolidated statement of income reflects premiums and losses and loss adjustment expenses and acquisition costs, net of reinsurance ceded. See Note 7 for information on the Company's reinsurance usage. Ceded unearned premiums are reported as prepaid reinsurance premiums and estimated amounts of reinsurance recoverable on unpaid losses are reported as unpaid losses and loss adjustment expenses recoverable. Reinsurance premiums ceded and unpaid losses and loss adjustment expenses recoverable are estimated in a manner consistent with that of the original policies issued and the terms of the reinsurance contracts. If the reinsurers are unable to satisfy their obligations under the agreements, the Company’s insurance or reinsurance subsidiaries would be liable for such defaulted amounts.
Cash	Cash includes cash equivalents, which are investments with original maturities of three months or less that are not managed by external or internal investment advisors.
Investments	The Company currently classifies substantially all of its fixed maturity investments, equity securities and short-term investments as “available for sale” and, accordingly, they are carried at estimated fair value (also known as fair value) with the changes in fair value recorded as an unrealized gain or loss component of accumulated other comprehensive income in shareholders’ equity. The fair value of fixed maturity securities and equity securities is generally determined from quotations received from nationally recognized pricing services, or when such prices are not available, by reference to broker or underwriter bid indications. Short-term investments comprise securities due to mature within one year of the date of issue. Short-term investments include certain cash equivalents which are part of investment portfolios under the management of external and internal investment managers.
	The Company participates in a securities lending program as a mechanism for generating additional interest income on its fixed income portfolio. Under the security lending agreements, certain of its fixed income portfolio securities are loaned to third parties, primarily major brokerage firms, for short periods of time through a lending agent. Such securities have been reclassified as “Fixed maturities and short-term investments pledged under securities lending agreements, at fair value.” The Company maintains legal control over the securities it lends, retains the earnings and cash flows associated with the loaned securities and receives a fee from the borrower for the temporary use of the securities. Collateral received, primarily in the form of cash, is required at a rate of 102% of the fair value of the loaned securities including accrued investment income and is monitored and maintained by the lending agent. Such collateral is reinvested and is reflected as “Investment of funds received under securities lending agreements, at fair value.”
	The Company’s investment portfolio includes certain funds that, due to their ownership structure, are accounted for by the Company using the equity method. In applying the equity method, these investments are initially recorded at cost and are subsequently adjusted based on the Company’s proportionate share of the net income or loss of the funds (which include changes in the fair value of the underlying securities in the funds). Such investments are generally recorded on a one to three month lag based on the availability of reports from the investment funds. Changes in the carrying value of such investments are recorded in net income as “Equity in net income (loss) of investment funds accounted for using the equity method.” As such, fluctuations in the carrying value of the investment funds accounted for using the equity method may increase the volatility of the Company’s reported results of operations.
	Other investments include funds and separately managed accounts with holdings in Asian and emerging markets, fixed maturities, term loans and other investment strategies. The fair value for certain of the Company’s other investments are determined using net asset values (“NAVs”) as advised by external fund managers. The NAV is based on the fund manager’s valuation of the underlying holdings in accordance with the fund’s governing documents. Certain of the funds are accounted for as available for sale equity securities, regardless of the nature of the investments held within the fund.
	The Company elected to carry certain fixed maturity securities, equity securities and other investments at fair value under the fair value option afforded by accounting guidance regarding the fair value option for financial assets and liabilities. Changes in fair value of investments accounted for using the fair value option are included in “Net realized gains (losses).” The primary reasons for electing the fair value option were to address simplification and cost-benefit considerations.
	The Company invests in limited partner equity interests issued by third party variable interest entities (“VIE”). Such amounts are included in investments accounted for using the equity method, other investments available for sale and investments accounted for using the fair value option. A VIE refers to entities that have characteristics such as (i) insufficient equity at risk to allow the entity to finance its activities without additional financial support or (ii) instances where the equity investors, as a group, do not have the characteristic of a controlling financial interest. If the Company is determined to be the primary beneficiary, it is required to consolidate the VIE. The primary beneficiary is defined as the variable interest holder that is determined to have the controlling financial interest as a result of having both (i) the power to direct the activities of a VIE that most significantly impact the economic performance of the VIE and (ii) the obligation to absorb losses or right to receive benefits from the VIE that could potentially be significant to the VIE. At inception of the VIE as well as on an ongoing basis, the Company determines whether it is the primary beneficiary based on an analysis of the Company’s level of involvement in the VIE, the contractual terms, and the overall structure of the VIE. The Company's maximum exposure to loss with respect to these investments is limited to the investment carrying amounts reported in the Company's consolidated balance sheet and any unfunded commitment.
	The Company performs quarterly reviews of its investments to determine whether declines in fair value below the cost basis are considered other-than-temporary in accordance with applicable accounting guidance regarding the recognition and presentation of other-than-temporary impairments (“OTTI”). The process of determining whether a security is other-than-temporarily impaired requires judgment and involves analyzing many factors. These factors include (i) an analysis of the liquidity, business prospects and overall financial condition of the issuer, (ii) the time period in which there was a significant decline in value, (iii) the significance of the decline and (iv) the analysis of specific credit events. The Company evaluates the unrealized losses of its equity securities by issuer and forecasts a reasonable period of time by which the fair value of the securities would increase and the Company would recover its cost. If the Company is unable to forecast a reasonable period of time in which to recover the cost of its equity securities, a net impairment loss in earnings equivalent to the entire unrealized loss is recognized.
	When there are credit-related losses associated with debt securities for which the Company does not have an intent to sell and it is more likely than not that it will not be required to sell the security before recovery of its cost basis, the amount of the OTTI related to a credit loss is recognized in earnings and the amount of the OTTI related to other factors (e.g., interest rates, market conditions, etc.) is recorded as a component of other comprehensive income (loss). The amount of the credit loss of an impaired debt security is the difference between the amortized cost and the greater of (i) the present value of expected future cash flows and (ii) the fair value of the security. In instances where no credit loss exists but it is more likely than not that the Company will have to sell the debt security prior to the anticipated recovery, the decline in fair value below amortized cost is recognized as an OTTI in earnings. In periods after the recognition of an OTTI on debt securities, the Company accounts for such securities as if they had been purchased on the measurement date of the OTTI at an amortized cost basis equal to the previous amortized cost basis less the OTTI recognized in earnings. For debt securities for which OTTI were recognized in earnings, the difference between the new amortized cost basis and the cash flows expected to be collected will be accreted or amortized into net investment income. See Note 8, “Investment Information—Other-Than-Temporary Impairments” for additional information.
	Net investment income includes interest and dividend income together with amortization of market premiums and discounts and is net of investment management and custody fees. Anticipated prepayments and expected maturities are used in applying the interest method for certain investments such as mortgage and other asset-backed securities. When actual prepayments differ significantly from anticipated prepayments, the effective yield is recalculated to reflect actual payments to date and anticipated future payments. The net investment in such securities is adjusted to the amount that would have existed had the new effective yield been applied since the acquisition of the security. Such adjustments, if any, are included in net investment income when determined. Equity in net income (loss) of investment funds accounted for using the equity method includes changes in the fair value of certain alternative investments accounted for under the equity method.
	Investment gains or losses realized on the sale of investments, except for certain fund investments, are determined on a first-in, first-out basis and are reflected in net income. Investments gains or losses realized on the sale of certain fund investments are determined on an average cost basis. Unrealized appreciation or decline in the value of securities, which are carried at fair value, is excluded from net income and recorded as a separate component of accumulated other comprehensive income, net of applicable deferred income tax.
Derivative instruments	The Company recognizes all derivative instruments, including embedded derivative instruments, at fair value in the Consolidated Balance Sheets. The Company employs the use of derivative instruments within its operations to mitigate risks arising from assets and liabilities held in foreign currencies as well as part of its overall investment strategy. For such instruments, changes in assets and liabilities measured at fair value are recorded as “Net realized gains” in the consolidated statements of income. In addition, the Company’s derivative instruments include amounts related to underwriting activities where an insurance or reinsurance contract meets the accounting definition of a derivative instrument. For such contracts, changes in fair value are reflected in “Other underwriting income” in the consolidated statements of income as the underlying contract originates from the Company’s underwriting operations. For the periods ended 2014, 2013, and 2012, the Company did not designate any derivative instruments as hedges under the relevant accounting guidance. See Note 11 for information on the Company’s derivative instruments.
Reserve for losses and loss adjustment expenses	Insurance and Reinsurance. The reserve for losses and loss adjustment expenses consists of estimates of unpaid reported losses and loss adjustment expenses and estimates for losses incurred but not reported. The reserve for unpaid reported losses and loss adjustment expenses, established by management based on reports from ceding companies and claims from insureds, excludes estimates of amounts related to losses under high deductible policies, and represents the estimated ultimate cost of events or conditions that have been reported to or specifically identified by the Company. Such reserves are supplemented by management’s estimates of reserves for losses incurred for which reports or claims have not been received. The Company’s reserves are based on a combination of reserving methods, incorporating both Company and industry loss development patterns. The Company selects the initial expected loss and loss adjustment expense ratios based on information derived by its underwriters and actuaries during the initial pricing of the business, supplemented by industry data where appropriate. Such ratios consider, among other things, rate changes and changes in terms and conditions that have been observed in the market. These estimates are reviewed regularly and, as experience develops and new information becomes known, the reserves are adjusted as necessary. Such adjustments, if any, are reflected in income in the period in which they are determined. As actual loss information has been reported, the Company has developed its own loss experience and its reserving methods include other actuarial techniques. Over time, such techniques have been given further weight in its reserving process based on the continuing maturation of the Company’s reserves. Inherent in the estimates of ultimate losses and loss adjustment expenses are expected trends in claims severity and frequency and other factors which may vary significantly as claims are settled. Accordingly, ultimate losses and loss adjustment expenses may differ materially from the amounts recorded in the accompanying consolidated financial statements. Losses and loss adjustment expenses are recorded on an undiscounted basis, except for excess workers’ compensation and employers’ liability business written by the Company’s insurance operations.
	Mortgage. The reserves for mortgage guaranty insurance losses and loss adjustment expenses are the estimated claim settlement costs on notices of default that have been received by the Company, as well as loan defaults that have been incurred but have not been reported by the lenders. Consistent with industry accounting practice, the Company does not establish loss reserves for future claims on insured loans that are not currently in default (defined as two consecutive missed payments). The Company establishes loss reserves on a case-by-case basis when insured loans are identified as currently in default using estimated claim rates and average claim sizes for each report year, net of any salvage recoverable. The Company also reserves for defaults that have occurred but have not yet been reported to the Company prior to the close of an accounting period. To determine this reserve, the Company estimates the number of defaults not yet reported using historical information regarding late reported delinquencies and applies estimated claim rates and claim sizes for the estimated defaults not yet reported.
	The establishment of reserves across the Company’s segments is an inherently uncertain process, are necessarily based on estimates, and the ultimate net cost may vary from such estimates. The methods for making such estimates and for establishing the resulting liability are reviewed and updated using the most current information available. Any resulting adjustments, which may be material, are reflected in current operations.
Contractholder receivables and payables	Certain insurance policies written by the Company’s insurance operations feature large deductibles, primarily in its construction and national accounts lines of business. Under such contracts, the Company is obligated to pay the claimant for the full amount of the claim. The Company is subsequently reimbursed by the policyholder for the deductible amount. These amounts are included on a gross basis in the consolidated balance sheet in contractholder payables and contractholder receivables, respectively. In the event that the Company is unable to collect from the policyholder, the Company would record an increase to losses and loss adjustment expenses related to such policy.
Foreign exchange	Assets and liabilities of foreign operations whose functional currency is not the U.S. Dollar are translated at the prevailing exchange rates at each balance sheet date. Revenues and expenses of such foreign operations are translated at average exchange rates during the year. The net effect of the translation adjustments for foreign operations is included in accumulated other comprehensive income, net of applicable deferred income tax. Monetary assets and liabilities, such as premiums receivable and the reserve for losses and loss adjustment expenses, denominated in foreign currencies are revalued at the exchange rate in effect at the balance sheet date with the resulting foreign exchange gains and losses included in net income. Accounts that are classified as non-monetary, such as deferred acquisition costs and the unearned premium reserves, are not revalued. In the case of foreign currency denominated fixed maturity securities which are classified as “available for sale,” the change in exchange rates between the local currency in which the investments are denominated and the Company’s functional currency at each balance sheet date is included in unrealized appreciation or decline in value of securities, a component of accumulated other comprehensive income, net of applicable deferred income tax.
Income taxes	Deferred income taxes reflect the expected future tax consequences of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and amounts used for income tax purposes. A valuation allowance is recorded if it is more likely than not that some or all of a deferred tax asset may not be realized. The Company considers future taxable income and feasible tax planning strategies in assessing the need for a valuation allowance. In the event the Company determines that it will not be able to realize all or part of its deferred income tax assets in the future, an adjustment to the deferred income tax assets would be charged to income in the period in which such determination is made. In addition, if the Company subsequently assesses that the valuation allowance is no longer needed, a benefit would be recorded to income in the period in which such determination is made. See Note 14 for more information.
	The Company recognizes a tax benefit where it concludes that it is more likely than not that the tax benefit will be sustained on audit by the taxing authority based solely on the technical merits of the associated tax position. If the recognition threshold is met, the Company recognizes a tax benefit measured at the largest amount of the tax benefit that, in the Company’s judgment, is greater than 50% likely to be realized. The Company records interest and penalties related to unrecognized tax benefits in the provision for income taxes.
Share-based payment arrangements	The Company applies a fair value based measurement method in accounting for its share-based payment arrangements with eligible employees and directors. Compensation expense is estimated based on the fair value of the award at the grant date and is recognized in net income over the requisite service period with a corresponding increase in shareholders’ equity. No value is attributed to awards that employees forfeit because they fail to satisfy vesting conditions. As such, the number of shares granted is reduced by assumed forfeitures and adjusted based on actual forfeitures until vesting. The Company’s share-based payment arrangements generally vest over a three year period with one-third vesting on the first, second and third anniversaries of the grant date. The share-based compensation expense associated with such awards that have graded vesting features and vest based on service conditions only is calculated on a straight-line basis over the requisite service period for the entire award. For awards granted to retirement-eligible employees where no service is required for the employee to retain the award, the grant date fair value is immediately recognized as compensation expense at the grant date because the employee is able to retain the award without continuing to provide service. For employees near retirement eligibility, attribution of compensation cost is over the period from the grant date to the retirement eligibility date. In November 2012, the Company issued off-cycle share-based awards, which will cliff vest on the fifth anniversary of the grant date. The expense for such grant is being amortized on a straight-line basis over the five-year requisite service period. The off-cycle awards have similar terms as the annual award agreements, however with respect to retirement eligible employees service is required for the employee to retain the award. Retirement-eligible employees retiring prior to the fifth anniversary of the grant date will vest in a pro-rated portion of the award commensurate with the service performed. Such employees will receive the vested portion at the end of the five-year cliff period. These charges had no impact on the Company’s cash flows or total shareholders’ equity. See Note 20 for information relating to the Company’s share-based payment awards.
Guaranty fund and other related assessments	Liabilities for guaranty fund and other related assessments in the Company’s insurance and reinsurance operations are accrued when the Company receives notice that an amount is payable, or earlier if a reasonable estimate of the assessment can be made.
Treasury shares	Treasury shares are common shares purchased by the Company and not subsequently canceled. These shares are recorded at cost and result in a reduction of the Company’s shareholders’ equity in its Consolidated Balance Sheets.
Goodwill and intangible assets	Goodwill represents the excess of the purchase price of an acquisition over the fair value of the net assets acquired and is assigned to the applicable reporting unit at acquisition. Goodwill is not amortized and is evaluated for impairment on an annual basis. Impairment tests may be performed more frequently if the facts and circumstances indicate a possible impairment. In performing impairment tests, the Company may first assess qualitative factors to determine whether it is more likely than not (that is, more than a 50% probability) that the fair value of a reporting unit exceeds its carrying amount as a basis for determining whether it is necessary to perform the two-step goodwill impairment test described in the accounting guidance.
	Indefinite-lived intangible assets, such as insurance licenses, are not amortized and are evaluated for impairment similar to goodwill. Finite-lived intangible assets and liabilities include the value of acquired insurance and reinsurance contracts, which are estimated based on the present value of future expected cash flows and amortized in ‘acquisition expenses’ in proportion to the estimated profits expected to be realized. Other finite-lived intangible assets or liabilities, including favorable or unfavorable contracts, are amortized in ‘other operating expenses’ over their useful lives. Finite-lived intangible assets and liabilities are periodically reviewed for indicators of impairment. An impairment is recognized when the carrying amount is not recoverable from its undiscounted cash flows and is measured as the difference between the carrying amount and fair value.
	If goodwill or intangible assets are impaired, such assets are written down to their fair values with the related expense recorded in the Company’s results of operations. See Note 18 for information relating to the Company’s goodwill and intangible assets.
Recent accounting pronouncements	A new accounting standard issued in the 2014 second quarter will change the manner in which most companies recognize revenue. The standard requires that revenue reflect the transfer of goods or services to customers based on the consideration or payment the company expects to be entitled to in exchange for those goods or services; however, the standard does not change the accounting for insurance contracts or financial instruments. The new standard also requires enhanced disclosures about revenue. This accounting guidance is effective in the 2017 first quarter and may be applied on a full retrospective or modified retrospective approach. The Company is currently assessing the impact the implementation of this standard will have on its consolidated financial statements.
	A new accounting standard was issued in the 2015 first quarter providing targeted improvements to consolidation guidance for limited partnerships and other similarly structured entities. The new standard addresses instances where a reporting entity consolidates another entity when the reporting entity is simply acting on the behalf of others, amongst other related issues. While the standard is targeted, the application is relevant for all companies that are required to assess whether or not to consolidate certain entities. The standard is effective in the 2016 first quarter and early adoption is permitted. The Company is currently assessing the impact the implementation of this standard will have on its consolidated financial statements.

Business_Acquired_Tables

Business Acquired (Tables)	12 Months Ended
	Dec. 31, 2014
Business Combinations [Abstract]
Summary of fair value of net assets acquired and allocation of purchase price	The following table summarizes the fair value of net assets acquired and allocation of purchase price, measured as of the acquisition date:
		Total		Useful Life
	Purchase price
	Cash paid	$	245,157
	Contingent consideration liability	41,762
	Total purchase price (a)	$	286,919
	Assets acquired
	Cash	$	9,579
	Investments, at fair value	312,093
	Intangible asset -- acquired insurance contracts	46,473		5 years
	Intangible asset -- operating platform	29,900		5 years
	Intangible asset -- favorable lease contract	1,056		5 years
	Intangible asset -- insurance licenses	16,858		Indefinite
	Other assets acquired	21,691
	Total assets acquired	437,650
	Liabilities acquired
	Reserves for losses and loss adjustment expenses	$	121,572
	Unearned premiums	26,261
	Intangible liability -- unfavorable service contract	9,533		9 years
	Other liabilities acquired	7,217
	Total liabilities acquired	164,583
	Net assets acquired (b)	$	273,067
	Goodwill (a)-(b)	$	13,852

Variable_Interest_Entity_and_N1

Variable Interest Entity and Noncontrolling Interests (Tables)	12 Months Ended
	Dec. 31, 2014
Variable Interest Entity and Noncontrolling Interest Disclosure [Abstract]
Activity in non-redeemable noncontrolling interests	The following table sets forth activity in the non-redeemable noncontrolling interests:
		Year Ended December 31, 2014
	Balance, beginning of year	$	—
	Sale of shares to noncontrolling interests	796,904
	Amounts attributable to noncontrolling interests	(27,823		)
	Balance, end of year	$	769,081

Segment_Information_Tables

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Analysis of underwriting income by segment and reconciliation to net income available to common shareholders	The following tables summarize the Company’s underwriting income or loss by segment, together with a reconciliation of underwriting income to net income available to Arch common shareholders, summary information regarding net premiums written and earned by major line of business and net premiums written by location:
		Year Ended December 31, 2014
		Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
	Gross premiums written (1)	$	3,008,669		$	1,527,245		$	227,356		$	4,760,394		$	288,627		$	4,840,616
	Premiums ceded	(862,015		)	(261,254		)	(22,519		)	(1,142,912		)	(14,171		)	(948,678		)
	Net premiums written	2,146,654			1,265,991			204,837			3,617,482			274,456			3,891,938
	Change in unearned premiums	(129,284		)	13,337			(11,264		)	(127,211		)	(170,979		)	(298,190		)
	Net premiums earned	2,017,370			1,279,328			193,573			3,490,271			103,477			3,593,748
	Other underwriting income	2,135			3,167			4,840			10,142			—			10,142
	Losses and loss adjustment expenses	(1,260,953		)	(532,450		)	(55,674		)	(1,849,077		)	(70,173		)	(1,919,250		)
	Acquisition expenses, net	(316,308		)	(261,438		)	(49,400		)	(627,146		)	(30,116		)	(657,262		)
	Other operating expenses	(335,157		)	(147,964		)	(66,891		)	(550,012		)	(6,268		)	(556,280		)
	Underwriting income (loss)	$	107,087		$	340,643		$	26,448		474,178			(3,080		)	471,098
	Net investment income										284,336			18,249			302,585
	Net realized gains (losses)										133,384			(30,467		)	102,917
	Net impairment losses recognized in earnings										(30,150		)	—			(30,150		)
	Equity in net income of investment funds accounted for using the equity method										19,883			—			19,883
	Other income (loss)										(10,252		)	—			(10,252		)
	Other expenses										(47,615		)	(2,329		)	(49,944		)
	Interest expense										(45,634		)	—			(45,634		)
	Net foreign exchange gains (losses)										82,658			1,086			83,744
	Income before income taxes										860,788			(16,541		)	844,247
	Income tax expense										(22,987		)	—			(22,987		)
	Net income										837,801			(16,541		)	821,260
	Dividends attributable to redeemable noncontrolling interests										—			(14,728		)	(14,728		)
	Amounts attributable to noncontrolling interests										—			27,823			27,823
	Net income available to Arch										837,801			(3,446		)	834,355
	Preferred dividends										(21,938		)	—			(21,938		)
	Net income available to Arch common shareholders										$	815,863		$	(3,446	)	$	812,417
	Underwriting Ratios
	Loss ratio	62.5		%	41.6		%	28.8		%	53		%	67.8		%	53.4		%
	Acquisition expense ratio	15.7		%	20.4		%	25.5		%	18		%	29.1		%	18.3		%
	Other operating expense ratio	16.6		%	11.6		%	34.6		%	15.8		%	6.1		%	15.5		%
	Combined ratio	94.8		%	73.6		%	88.9		%	86.8		%	103		%	87.2		%
	Goodwill and intangible assets	$	28,331		$	3,333		$	77,875		$	109,539		$	—		$	109,539
	Total investable assets										$	14,609,969		$	1,163,240		$	15,773,209
	Total assets										20,527,027			1,482,516			22,009,543
	Total liabilities										14,492,237			398,660			14,890,897
	-1	Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.
		Year Ended December 31, 2013
		Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
	Gross premiums written (1)	$	2,712,509		$	1,399,621		$	89,570		$	4,196,623		$	—		$	4,196,623
	Premiums ceded	(763,713		)	(86,620		)	—			(845,256		)	—			(845,256		)
	Net premiums written	1,948,796			1,313,001			89,570			3,351,367			—			3,351,367
	Change in unearned premiums	(72,782		)	(94,329		)	(38,304		)	(205,415		)	—			(205,415		)
	Net premiums earned	1,876,014			1,218,672			51,266			3,145,952			—			3,145,952
	Other underwriting income	2,122			5,258			259			7,639			—			7,639
	Losses and loss adjustment expenses	(1,188,445		)	(486,236		)	(4,743		)	(1,679,424		)	—			(1,679,424		)
	Acquisition expenses, net	(311,904		)	(234,373		)	(17,826		)	(564,103		)	—			(564,103		)
	Other operating expenses	(315,387		)	(134,563		)	(8,377		)	(458,327		)	—			(458,327		)
	Underwriting income (loss)	$	62,400		$	368,758		$	20,579		451,737			—			451,737
	Net investment income										267,219			—			267,219
	Net realized gains										74,018			—			74,018
	Net impairment losses recognized in earnings										(3,786		)	—			(3,786		)
	Equity in net income (loss) of investment funds accounted for using the equity method										35,701			—			35,701
	Other income (loss)										(586		)	—			(586		)
	Other expenses										(42,403		)	—			(42,403		)
	Interest expense										(27,060		)	—			(27,060		)
	Net foreign exchange gains (losses)										(12,335		)	—			(12,335		)
	Income before income taxes										742,505			—			742,505
	Income tax expense										(32,774		)	—			(32,774		)
	Net income										709,731			—			709,731
	Dividends attributable to redeemable noncontrolling interests										—			—			—
	Amounts attributable to noncontrolling interests										—			—			—
Net income available to Arch										709,731			—			709,731
Preferred dividends										(21,938		)	—			(21,938		)
Net income available to Arch common shareholders										$	687,793		$	—		$	687,793
Underwriting Ratios
Loss ratio	63.3		%	39.9		%	9.3		%	53.4		%	—			53.4		%
Acquisition expense ratio	16.6		%	19.2		%	34.8		%	17.9		%	—			17.9		%
Other operating expense ratio	16.8		%	11		%	16.3		%	14.6		%	—			14.6		%
Combined ratio	96.7		%	70.1		%	60.4		%	85.9		%	—			85.9		%
Goodwill and intangible assets	$	20,419		$	6,900		$	—		$	27,319		$	—		$	27,319
Total investable assets										$	14,049,525		$	—		$	14,049,525
Total assets										19,566,094			—			19,566,094
Total liabilities										13,918,598			—			13,918,598
-1	Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.
	Year Ended December 31, 2012
	Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
Gross premiums written (1)	$	2,593,959		$	1,213,889		$	68,111		$	3,869,161		$	—		$	3,869,161
Premiums ceded	(768,625		)	(55,099		)	—			(816,926		)	—			(816,926		)
Net premiums written	1,825,334			1,158,790			68,111			3,052,235			—			3,052,235
Change in unearned premiums	(24,991		)	(40,663		)	(51,441		)	(117,095		)	—			(117,095		)
Net premiums earned	1,800,343			1,118,127			16,670			2,935,140			—			2,935,140
Other underwriting income	2,335			5,755			—			8,090			—			8,090
Losses and loss adjustment expenses	(1,283,841		)	(574,821		)	(2,615		)	(1,861,277		)	—			(1,861,277		)
Acquisition expenses, net	(298,983		)	(204,903		)	(4,998		)	(508,884		)	—			(508,884		)
Other operating expenses	(307,489		)	(118,245		)	(4,301		)	(430,035		)	—			(430,035		)
Underwriting income (loss)	$	(87,635	)	$	225,913		$	4,756		143,034			—			143,034
Net investment income										294,895			—			294,895
Net realized gains										194,228			—			194,228
Net impairment losses recognized in earnings										(11,388		)	—			(11,388		)
Equity in net income (loss) of investment funds accounted for using the equity method										73,510			—			73,510
Other income (loss)										(12,094		)	—			(12,094		)
Other expenses										(35,318		)	—			(35,318		)
Interest expense										(28,525		)	—			(28,525		)
Net foreign exchange gains (losses)										(28,955		)	—			(28,955		)
Income before income taxes										589,387			—			589,387
Income tax benefit										4,010			—			4,010
Net income										593,397			—			593,397
Dividends attributable to redeemable noncontrolling interests										—			—			—
Amounts attributable to noncontrolling interests										—			—			—
Net income available to Arch										593,397			—			593,397
Preferred dividends										(25,079		)	—			(25,079		)
Loss on repurchase of preferred shares										(10,612		)	—			(10,612		)
Net income available to Arch common shareholders										$	557,706		$	—		$	557,706
Underwriting Ratios
Loss ratio	71.3		%	51.4		%	15.7		%	63.4		%	—		%	63.4		%
Acquisition expense ratio	16.6		%	18.3		%	30		%	17.3		%	—		%	17.3		%
Other operating expense ratio	17.1		%	10.6		%	25.8		%	14.7		%	—		%	14.7		%
Combined ratio	105		%	80.3		%	71.5		%	95.4		%	—		%	95.4		%
Goodwill and intangible assets	$	21,357		$	16,902		$	—		$	38,259		$	—		$	38,259
Total investable assets										$	13,045,134		$	—		$	13,045,134
Total assets										17,816,762			—			17,816,762
Total liabilities										12,647,884			—			12,647,884
-1	Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.
Summary of information regarding net premiums written and earned by major line of business and net premiums written by location	The following tables provide summary information regarding net premiums written and earned by major line of business and net premiums written by location:
	INSURANCE SEGMENT	Year Ended December 31,
		2014			2013			2012
	Net premiums written (1)
	Programs	$	480,580		$	419,673		$	340,130
	Professional lines (2)	476,604			476,193			548,423
	Construction and national accounts	286,994			271,110			211,130
	Property, energy, marine and aviation	244,640			280,551			294,690
	Excess and surplus casualty (3)	212,519			149,286			112,307
	Travel, accident and health	145,732			104,903			106,871
	Lenders products	100,407			101,576			99,724
	Other (4)	199,178			145,504			112,059
	Total	$	2,146,654		$	1,948,796		$	1,825,334
	Net premiums earned (1)
	Programs	$	460,392		$	386,840		$	318,740
	Professional lines (2)	456,508			491,791			536,971
	Construction and national accounts	277,811			250,729			209,217
	Property, energy, marine and aviation	244,974			304,294			313,081
	Excess and surplus casualty (3)	182,024			118,395			113,597
	Travel, accident and health	127,691			97,135			98,686
	Lenders products	94,438			99,847			103,478
	Other (4)	173,532			126,983			106,573
	Total	$	2,017,370		$	1,876,014		$	1,800,343
	Net premiums written by client location (1)
	United States	$	1,726,181		$	1,526,156		$	1,314,577
	Europe	240,136			226,254			271,278
	Asia and Pacific	79,564			95,970			120,492
	Other	100,773			100,416			118,987
	Total	$	2,146,654		$	1,948,796		$	1,825,334
	Net premiums written by underwriting location (1)
	United States	$	1,688,887		$	1,478,930		$	1,254,623
	Europe	394,430			389,763			472,132
	Other	63,337			80,103			98,579
	Total	$	2,146,654		$	1,948,796		$	1,825,334
	-1	Insurance segment results include premiums written and earned assumed through intersegment transactions and exclude premiums written and earned ceded through intersegment transactions.
	-2	Includes professional liability, executive assurance and healthcare business.
	-3	Includes casualty and contract binding business.
	-4	Includes alternative markets, excess workers' compensation and surety business.
	REINSURANCE SEGMENT	Year Ended December 31,
		2014			2013			2012
	Net premiums written (1)
	Other specialty (2)	$	405,126		$	417,865		$	308,104
	Property excluding property catastrophe (3)	343,043			292,536			265,783
	Casualty (4)	317,996			306,304			205,925
	Property catastrophe	137,471			220,749			283,677
	Marine and aviation	50,444			64,380			84,649
	Other (5)	11,911			11,167			10,652
	Total	$	1,265,991		$	1,313,001		$	1,158,790
	Net premiums earned (1)
	Other specialty (2)	$	424,725		$	387,630		$	309,101
	Property excluding property catastrophe (3)	303,496			274,719			254,338
	Casualty (4)	327,518			241,774			188,963
	Property catastrophe	150,761			232,423			280,185
	Marine and aviation	61,118			70,105			76,145
	Other (5)	11,710			12,021			9,395
	Total	$	1,279,328		$	1,218,672		$	1,118,127
	Net premiums written (1)
	Pro rata	$	663,135		$	692,024		$	530,763
	Excess of loss	602,856			620,977			628,027
	Total	$	1,265,991		$	1,313,001		$	1,158,790
	Net premiums earned (1)
	Pro rata	$	686,201		$	608,586		$	499,094
	Excess of loss	593,127			610,086			619,033
	Total	$	1,279,328		$	1,218,672		$	1,118,127
Net premiums written by client location (1)
United States	$	589,255		$	706,388		$	568,043
Europe	355,735			327,059			341,653
Asia and Pacific	142,626			94,252			97,879
Bermuda	77,620			87,047			72,864
Other	100,755			98,255			78,351
Total	$	1,265,991		$	1,313,001		$	1,158,790
Net premiums written by underwriting location (1)
United States	$	492,891		$	507,183		$	379,239
Bermuda	394,351			459,467			527,909
Europe	343,823			309,129			225,470
Other	34,926			37,222			26,172
Total	$	1,265,991		$	1,313,001		$	1,158,790
-1	Reinsurance segment results include premiums written and earned assumed through intersegment transactions and exclude premiums written and earned ceded through intersegment transactions.
-2	Includes non-excess motor, surety, accident and health, workers’ compensation catastrophe, agriculture, trade credit and other.
-3	Includes facultative business.
-4	Includes executive assurance, professional liability, workers’ compensation, excess motor, healthcare and other.
-5	Includes life, casualty clash and other.
	Year Ended December 31,
MORTGAGE SEGMENT	2014			2013			2012
Net premiums written by client location
United States	$	184,333		$	63,692		$	61,571
Other	20,504			25,878			6,540
Total	$	204,837		$	89,570		$	68,111
Net premiums written by underwriting location
United States	$	98,809		$	—		$	—
Other	106,028			89,570			68,111
Total	$	204,837		$	89,570		$	68,111
OTHER SEGMENT	Year Ended December 31,
	2014			2013			2012
Net premiums written (1)
Casualty (2)	$	179,054		$	—		$	—
Other specialty (3)	66,524			—			—
Property catastrophe	9,280			—			—
Marine and aviation	1,251			—			—
Property excluding property catastrophe	564			—			—
Other (4)	17,783			—			—
Total	$	274,456		$	—		$	—
Net premiums earned (1)
Casualty (2)	$	72,395		$	—		$	—
Other specialty (3)	24,157			—			—
Property catastrophe	5,115			—			—
Marine and aviation	789			—			—
Property excluding property catastrophe	182			—			—
Other (4)	839			—			—
Total	$	103,477		$	—		$	—
Net premiums written by client location (1)
United States	$	186,795		$	—		$	—
Europe	54,131			—			—
Bermuda	27,977			—			—
Other	5,553			—			—
Total	$	274,456		$	—		$	—
-1	Other segment results include premiums written and earned assumed through intersegment transactions and exclude premiums written and earned ceded through intersegment transactions.
-2	Includes professional liability, excess motor, programs and other.
-3	Includes non-excess motor and other.
-4	Includes mortgage and other.

Reserve_for_Losses_and_Loss_Ad1

Reserve for Losses and Loss Adjustment Expenses (Tables)	12 Months Ended
	Dec. 31, 2014
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Analysis of losses and loss adjustment expenses and reconciliation of beginning and ending reserve balances	The following table represents an analysis of losses and loss adjustment expenses and a reconciliation of the beginning and ending reserve for losses and loss adjustment expenses:
		Year Ended December 31,
		2014			2013			2012
	Reserve for losses and loss adjustment expenses at beginning of year	$	8,824,696		$	8,933,292		$	8,456,210
	Unpaid losses and loss adjustment expenses recoverable	1,748,250			1,829,070			1,818,047
	Net reserve for losses and loss adjustment expenses at beginning of year	7,076,446			7,104,222			6,638,163
	Net incurred losses and loss adjustment expenses relating to losses occurring in:
	Current year	2,246,152			1,943,466			2,082,805
	Prior years	(326,902		)	(264,042		)	(221,528		)
	Total net incurred losses and loss adjustment expenses	1,919,250			1,679,424			1,861,277
	Net losses and loss adjustment expense reserves of acquired business (1)	120,671			—			31,977
	Foreign exchange (gains) losses	(160,486		)	1,617			38,184
	Net paid losses and loss adjustment expenses relating to losses occurring in:
	Current year	(347,270		)	(288,114		)	(295,984		)
	Prior years	(1,350,466		)	(1,420,703		)	(1,169,395		)
	Total net paid losses and loss adjustment expenses	(1,697,736		)	(1,708,817		)	(1,465,379		)
	Net reserve for losses and loss adjustment expenses at end of year	7,258,145			7,076,446			7,104,222
	Unpaid losses and loss adjustment expenses recoverable	1,778,303			1,748,250			1,829,070
	Reserve for losses and loss adjustment expenses at end of year	$	9,036,448		$	8,824,696		$	8,933,292
	-1	2014 amount relates to the Company’s acquisition of Arch MI U.S. while the 2012 amount relates to the Company’s acquisition of the trade credit and surety operations of Ariel Reinsurance Company.

Reinsurance_Tables

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2014
Reinsurance Disclosures [Abstract]
Effects of reinsurance	The effects of reinsurance on the Company’s written and earned premiums and losses and loss adjustment expenses with unaffiliated reinsurers were as follows:
		Year Ended December 31,
		2014			2013			2012
	Premiums Written
	Direct	$	3,100,946		$	2,754,582		$	2,673,864
	Assumed	1,739,670			1,442,041			1,195,297
	Ceded	(948,678		)	(845,256		)	(816,926		)
	Net	$	3,891,938		$	3,351,367		$	3,052,235
	Premiums Earned
	Direct	$	2,914,755		$	2,666,104		$	2,572,078
	Assumed	1,574,049			1,296,048			1,165,371
	Ceded	(895,056		)	(816,200		)	(802,309		)
	Net	$	3,593,748		$	3,145,952		$	2,935,140
	Losses and Loss Adjustment Expenses
	Direct	$	1,763,492		$	1,603,369		$	1,725,707
	Assumed	621,346			450,618			578,081
	Ceded	(465,588		)	(374,563		)	(442,511		)
	Net	$	1,919,250		$	1,679,424		$	1,861,277

Investment_Information_Tables

Investment Information (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure Investment Information [Abstract]
Summary of fair value and cost or amortized cost of available for sale securities	The following table summarizes the fair value and cost or amortized cost of the Company’s securities classified as available for sale:
		Estimated			Gross			Gross			Cost or			OTTI
		Fair			Unrealized			Unrealized			Amortized			Unrealized
		Value			Gains			Losses			Cost			Losses (2)
	31-Dec-14
	Fixed maturities and fixed maturities pledged
	under securities lending agreements (1):
	Corporate bonds	$	3,108,513		$	37,928		$	(38,974	)	$	3,109,559		$	(317	)
	Mortgage backed securities	943,343			18,843			(3,842		)	928,342			(3,307		)
	Municipal bonds	1,494,122			31,227			(1,044		)	1,463,939			—
	Commercial mortgage backed securities	1,114,528			14,594			(3,822		)	1,103,756			—
	U.S. government and government agencies	1,447,972			8,345			(1,760		)	1,441,387			—
	Non-U.S. government securities	1,015,153			21,311			(37,203		)	1,031,045			—
	Asset backed securities	1,677,941			8,425			(6,089		)	1,675,605			(22		)
	Total	10,801,572			140,673			(92,734		)	10,753,633			(3,646		)
	Equity securities	658,182			109,012			(13,364		)	562,534			—
	Other investments	296,224			31,839			(362		)	264,747			—
	Short-term investments	797,226			738			(5,270		)	801,758			—
	Total	$	12,553,204		$	282,262		$	(111,730	)	$	12,382,672		$	(3,646	)
	31-Dec-13
	Fixed maturities and fixed maturities pledged
	under securities lending agreements (1):
	Corporate bonds	$	2,267,263		$	35,289		$	(35,537	)	$	2,267,511		$	(16	)
	Mortgage backed securities	1,133,095			16,270			(22,209		)	1,139,034			(9,269		)
	Municipal bonds	1,481,738			29,378			(9,730		)	1,462,090			(17		)
	Commercial mortgage backed securities	1,074,497			13,972			(15,224		)	1,075,749			(199		)
	U.S. government and government agencies	1,301,809			3,779			(11,242		)	1,309,272			(19		)
	Non-U.S. government securities	1,085,861			14,729			(19,363		)	1,090,495			—
	Asset backed securities	1,332,594			20,033			(13,795		)	1,326,356			(3,422		)
	Total	9,676,857			133,450			(127,100		)	9,670,507			(12,942		)
	Equity securities	496,824			69,487			(5,938		)	433,275			—
	Other investments	498,310			28,082			(18,459		)	488,687			—
	Short-term investments	1,478,367			1,654			(871		)	1,477,584			—
	Total	$	12,150,358		$	232,673		$	(152,368	)	$	12,070,053		$	(12,942	)
	-1	In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged. For purposes of this table, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged. See “—Securities Lending Agreements.”
	-2	Represents the total other-than-temporary impairments (“OTTI”) recognized in accumulated other comprehensive income (“AOCI”). It does not include the change in fair value subsequent to the impairment measurement date. At December 31, 2014, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $0.9 million, compared to a net unrealized gain of $6.0 million at December 31, 2013.
Summary of available for sale securities in a continual unrealized loss position	The following table summarizes, for all available for sale securities in an unrealized loss position, the fair value and gross unrealized loss by length of time the security has been in a continual unrealized loss position:
		Less than 12 Months						12 Months or More						Total
		Estimated			Gross			Estimated			Gross			Estimated			Gross
		Fair			Unrealized			Fair			Unrealized			Fair			Unrealized
		Value			Losses			Value			Losses			Value			Losses
	31-Dec-14
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	1,309,637		$	(32,903	)	$	148,963		$	(6,071	)	$	1,458,600		$	(38,974	)
	Mortgage backed securities	293,624			(1,476		)	59,107			(2,366		)	352,731			(3,842		)
	Municipal bonds	210,614			(588		)	13,643			(456		)	224,257			(1,044		)
	Commercial mortgage backed securities	232,147			(770		)	125,894			(3,052		)	358,041			(3,822		)
	U.S. government and government agencies	618,381			(1,626		)	3,438			(134		)	621,819			(1,760		)
	Non-U.S. government securities	510,766			(31,172		)	46,910			(6,031		)	557,676			(37,203		)
	Asset backed securities	612,950			(2,486		)	243,452			(3,603		)	856,402			(6,089		)
	Total	3,788,119			(71,021		)	641,407			(21,713		)	4,429,526			(92,734		)
	Equity securities	181,002			(13,364		)	—			—			181,002			(13,364		)
	Other investments	59,638			(362		)	—			—			59,638			(362		)
	Short-term investments	79,271			(5,270		)	—			—			79,271			(5,270		)
	Total	$	4,108,030		$	(90,017	)	$	641,407		$	(21,713	)	$	4,749,437		$	(111,730	)
	31-Dec-13
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	1,183,625		$	(32,837	)	$	46,673		$	(2,700	)	$	1,230,298		$	(35,537	)
	Mortgage backed securities	778,693			(20,253		)	43,634			(1,956		)	822,327			(22,209		)
	Municipal bonds	589,009			(9,422		)	6,092			(308		)	595,101			(9,730		)
	Commercial mortgage backed securities	677,617			(15,110		)	1,612			(114		)	679,229			(15,224		)
	U.S. government and government agencies	1,144,809			(11,242		)	—			—			1,144,809			(11,242		)
	Non-U.S. government securities	821,506			(15,776		)	24,334			(3,587		)	845,840			(19,363		)
	Asset backed securities	692,362			(10,431		)	88,629			(3,364		)	780,991			(13,795		)
	Total	5,887,621			(115,071		)	210,974			(12,029		)	6,098,595			(127,100		)
	Equity securities	76,563			(5,938		)	—			—			76,563			(5,938		)
	Other investments	165,891			(15,775		)	47,316			(2,684		)	213,207			(18,459		)
	Short-term investments	28,170			(871		)	—			—			28,170			(871		)
	Total	$	6,158,245		$	(137,655	)	$	258,290		$	(14,713	)	$	6,416,535		$	(152,368	)
	-1	In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged. For purposes of this table, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged. See “—Securities Lending Agreements.”
Contractual maturities of the Company's fixed maturities and fixed maturities pledged under securities lending arrangements
		December 31, 2014						December 31, 2013
	Maturity	Estimated Fair Value			Amortized Cost			Estimated Fair Value			Amortized Cost
	Due in one year or less	$	235,930		$	233,794		$	235,330		$	232,652
	Due after one year through five years	4,074,562			4,077,408			3,738,500			3,718,920
	Due after five years through 10 years	2,475,726			2,461,356			1,966,536			1,979,510
	Due after 10 years	279,542			273,372			196,305			198,286
		7,065,760			7,045,930			6,136,671			6,129,368
	Mortgage backed securities	943,343			928,342			1,133,095			1,139,034
	Commercial mortgage backed securities	1,114,528			1,103,756			1,074,497			1,075,749
	Asset backed securities	1,677,941			1,675,605			1,332,594			1,326,356
	Total	$	10,801,572		$	10,753,633		$	9,676,857		$	9,670,507
Summary of other investments, including available for sale and fair value option components	The following table summarizes the Company’s other investments, including available for sale and fair value option components:
		December 31,
		2014			2013
	Available for sale securities:
	Asian and emerging markets	$	236,586		$	331,984
	Investment grade fixed income	59,638			159,115
	Other	—			7,211
	Total available for sale	296,224			498,310
	Fair value option:
	Term loan investments (par value: $1,076,011 and $360,204)	1,073,649			399,591
	Mezzanine debt funds	121,341			102,172
	Credit related funds	114,436			63,271
	Investment grade fixed income	69,108			75,062
	Asian and emerging markets	25,800			14,054
	Other (1)	147,573			119,130
	Total fair value option	1,551,907			773,280
	Total	$	1,848,131		$	1,271,590
	-1	Includes fund investments with strategies in mortgage servicing rights, transportation and infrastructure assets and other.
Summary of assets and liabilities accounted for using the fair value option	The following table summarizes the Company’s assets and liabilities which are accounted for using the fair value option:
		December 31,
		2014			2013
	Fixed maturities	$	632,024		$	448,254
	Other investments	1,551,907			773,280
	Short term investments	251,601			—
	Investments accounted for using the fair value option	$	2,435,532		$	1,221,534
Components of net investment income	The components of net investment income were derived from the following sources:
		Year Ended December 31,
		2014			2013			2012
	Fixed maturities	$	265,219		$	249,833		$	281,140
	Term loan investments	39,940			20,608			15,283
	Equity securities (dividends)	13,005			8,919			7,963
	Short-term investments	1,888			1,259			1,980
	Other (1)	28,869			19,710			14,196
	Gross investment income	348,921			300,329			320,562
	Investment expenses	(46,336		)	(33,110		)	(25,667		)
	Net investment income	$	302,585		$	267,219		$	294,895
	-1	Includes dividends on investment funds and other items.
Summary of net realized gains (losses)	Net realized gains (losses) were as follows, excluding the other-than-temporary impairment provisions:
		Year Ended December 31,
		2014			2013			2012
	Available for sale securities:
	Gross gains on investment sales	$	248,031		$	239,421		$	247,291
	Gross losses on investment sales	(145,808		)	(203,077		)	(71,722		)
	Change in fair value of assets and liabilities accounted for using the fair value option:
	Fixed maturities	(20,743		)	54			13,195
	Other investments	2,672			17,503			7,383
	Equity securities	—			704			(73		)
	Short term investments	(395		)	—			—
	Derivative instruments (1)	43,934			20,912			(1,326		)
	Other (2)	(24,774		)	(1,499		)	(520		)
	Net realized gains	$	102,917		$	74,018		$	194,228
	-1	See Note 11 for information on the Company’s derivative instruments.
	-2	Includes accretion of contingent consideration liability amounts related to the acquisition of the CMG Entities (see Note 2 for further details).
Summary of OTTI recognized in earnings by asset class	The Company performs quarterly reviews of its available for sale investments in order to determine whether declines in fair value below the amortized cost basis were considered other-than-temporary in accordance with applicable guidance. The following table details the net impairment losses recognized in earnings by asset class:
		Year Ended December 31,
		2014			2013			2012
	Fixed maturities:
	Corporate bonds	$	(2,062	)	$	(88	)	$	(1,512	)
	Mortgage backed securities	(1,525		)	(295		)	(2,491		)
	Asset backed securities	(40		)	(128		)	(127		)
	Non-U.S. government securities	—			—			(261		)
	Commercial mortgage backed securities	(7		)	—			(211		)
	U.S. government and government agencies	—			—			(10		)
	Total	(3,634		)	(511		)	(4,612		)
	Other investments	(25,410		)	—			—
	Equity securities	(1,106		)	(3,275		)	(6,689		)
	Investment of funds received under securities lending agreements	—			—			(87		)
	Net impairment losses recognized in earnings	$	(30,150	)	$	(3,786	)	$	(11,388	)
Rollforward of the amount related to credit losses recognized in earnings for which a portion was recognized in AOCI	The following table provides a roll forward of the amount related to credit losses recognized in earnings for which a portion of an OTTI was recognized in accumulated other comprehensive income:
		Year Ended December 31,
		2014			2013			2012
	Balance at start of year	$	60,062		$	62,001		$	66,545
	Credit loss impairments recognized on securities not previously impaired	691			423			1,962
	Credit loss impairments recognized on securities previously impaired	162			88			2,735
	Reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security	—			—			—
	Reductions for securities sold during the period	(40,719		)	(2,450		)	(9,241		)
	Balance at end of year	$	20,196		$	60,062		$	62,001
Summary of restricted assets	The following table details the value of the Company’s restricted assets:
		December 31,
		2014			2013
	Assets used for collateral or guarantees:
	Affiliated transactions	$	4,138,527		$	4,060,533
	Third party agreements	970,120			856,890
	Deposits with U.S. regulatory authorities	337,981			302,809
	Trust funds	72,461			75,264
	Deposits with non-U.S. regulatory authorities	—			6,546
	Total restricted assets	$	5,519,089		$	5,302,042

Fair_Value_Tables

Fair Value (Tables)	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair value hierarchy	The following table presents the Company’s financial assets and liabilities measured at fair value by level at December 31, 2014:
					Fair Value Measurement Using:
		Estimated			Quoted Prices in Active Markets for Identical Assets			Significant Other Observable Inputs			Significant Unobservable Inputs
		Fair Value			(Level 1)			(Level 2)			(Level 3)
	Assets measured at fair value:
	Available for sale securities:
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	3,108,513		$	—		$	3,108,513		$	—
	Mortgage backed securities	943,343			—			943,343			—
	Municipal bonds	1,494,122			—			1,494,122			—
	Commercial mortgage backed securities	1,114,528			—			1,114,528			—
	U.S. government and government agencies	1,447,972			1,447,972			—			—
	Non-U.S. government securities	1,015,153			—			1,015,153			—
	Asset backed securities	1,677,941			—			1,620,441			57,500
	Total	10,801,572			1,447,972			9,296,100			57,500
	Equity securities	658,182			658,182			—			—
	Other investments	296,224			—			192,687			103,537
	Short-term investments	797,226			759,621			37,605			—
	Fair value option:
	Investments accounted for using the fair value option:
	Corporate bonds	497,101			—			497,101			—
	Non-U.S. government bonds	88,411			—			88,411			—
	Mortgage backed securities	22,190			—			22,190			—
	Asset backed securities	24,322			—			24,322			—
	Other investments	1,551,907			—			1,085,557			466,350
	Short-term investments	251,601			250,580			1,021			—
	Total	2,435,532			250,580			1,718,602			466,350
	Total assets measured at fair value	$	14,988,736		$	3,116,355		$	11,244,994		$	627,387
	Liabilities measured at fair value:
	Contingent consideration liability	$	(61,845	)	$	—		$	—		$	(61,845	)
	Total liabilities measured at fair value	$	(61,845	)	$	—		$	—		$	(61,845	)
	-1	In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged. For purposes of this table, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged.
	The following table presents the Company’s financial assets and liabilities measured at fair value by level at December 31, 2013:
					Fair Value Measurement Using:
		Estimated			Quoted Prices in Active Markets for Identical Assets			Significant Other Observable Inputs			Significant Unobservable Inputs
		Fair Value			(Level 1)			(Level 2)			(Level 3)
	Assets measured at fair value:
	Available for sale securities:
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	2,267,263		$	—		$	2,265,218		$	2,045
	Mortgage backed securities	1,133,095			—			1,133,095			—
	Municipal bonds	1,481,738			—			1,481,738			—
	Commercial mortgage backed securities	1,074,497			—			1,074,497			—
	U.S. government and government agencies	1,301,809			1,301,809			—			—
	Non-U.S. government securities	1,085,861			—			1,085,861			—
	Asset backed securities	1,332,594			—			1,332,594			—
	Total	9,676,857			1,301,809			8,373,003			2,045
	Equity securities	496,824			496,738			86			—
	Other investments	498,310			—			327,890			170,420
	Short-term investments	1,478,367			1,427,744			50,623			—
	Fair value option:
	Investments accounted for using the fair value option:
	Corporate bonds	334,065			—			334,065			—
	Non-U.S. government bonds	73,156			—			73,156			—
	Mortgage backed securities	41,033			—			41,033			—
	Other investments	773,280			—			395,755			377,525
	Total	1,221,534			—			844,009			377,525
	Total assets measured at fair value	$	13,371,892		$	3,226,291		$	9,595,611		$	549,990
	-1	In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged. For purposes of this table, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged.
Rollforward of Level 3 investments	The following table presents a reconciliation of the beginning and ending balances for all investments measured at fair value on a recurring basis using Level 3 inputs for 2014, 2013 and 2012:
		Fair Value Measurements Using:
		Significant Unobservable Inputs (Level 3)
					Assets												Liabilities
		Available for sale									Fair value option
		Asset Backed Securities			Corporate Bonds			Other Investments			Other Investments			Total			Contingent
																	Consideration
																	Liability
	Year Ended December 31, 2014
	Balance at beginning of year	$	—		$	2,045		$	170,420		$	377,525		$	549,990		$	—
	Total gains or (losses) (realized/unrealized)
	Included in earnings (1)	—			—			(39		)	16,818			16,779			(20,083		)
	Included in other comprehensive income	—			—			(680		)	932			252			—
	Purchases, issuances and settlements
	Purchases	—			—			—			185,224			185,224			—
	Issuances	—			—			—			—			—			(41,762		)
	Sales	—			(2,045		)	(66,164		)	—			(68,209		)	—
	Settlements	—			—			—			(114,149		)	(114,149		)	—
	Transfers in and/or out of Level 3	57,500			—			—			—			57,500			—
	Balance at end of year	$	57,500		$	—		$	103,537		$	466,350		$	627,387		$	(61,845	)
	Year Ended December 31, 2013
	Balance at beginning of year	$	—		$	98,404		$	184,202		$	195,350		$	477,956		$	—
	Total gains or (losses) (realized/unrealized)
	Included in earnings (1)	—			4,679			8,915			11,743			25,337			—
	Included in other comprehensive income	—			(3,051		)	473			—			(2,578		)	—
	Purchases, issuances and settlements
	Purchases	—			—			25,000			275,067			300,067			—
	Issuances	—			—			—			—			—			—
	Sales	—			(96,655		)	—			(20,156		)	(116,811		)	—
	Settlements	—			(1,332		)	(48,170		)	(84,479		)	(133,981		)	—
	Transfers in and/or out of Level 3	—			—			—			—			—			—
	Balance at end of year	$	—		$	2,045		$	170,420		$	377,525		$	549,990		$	—
	-1	Gains or losses on corporate bonds were included in net realized gains (losses) while gains or losses on other investments were included in net realized gains (losses) or net investment income. Amounts related to the contingent consideration liability were included in net realized gains (losses).

Derivative_Instruments_Tables

Derivative Instruments (Tables)	12 Months Ended
	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Fair value and notional amount of derivatives	The following table summarizes information on the fair values and notional values of the Company’s derivative instruments. The fair value of TBAs is included in “fixed maturities available for sale, at fair value.”
		Asset			Liability Derivatives			Net
		Derivatives						Derivatives
		Fair Value			Fair Value			Fair Value			Notional
											Value (1)
	31-Dec-14
	Futures contracts	$	2,156		$	(1,907	)	$	249		$	2,549,027
	Foreign currency forward contracts	10,511			(1,145		)	9,366			397,106
	TBAs	10,592			—			10,592			10,056
	Other	3,209			(2,345		)	864			735,684
	Total	$	26,468		$	(5,397	)	$	21,071
	31-Dec-13
	Futures contracts	$	461		$	(110	)	$	351		$	475,967
	Foreign currency forward contracts	5,023			(3,090		)	1,933			330,746
	TBAs	33,455			(21,731		)	11,724			56,160
	Other	920			(1,541		)	(621		)	347,916
	Total	$	39,859		$	(26,472	)	$	13,387
	-1	Represents the absolute notional value of all outstanding contracts, consisting of long and short positions.
Summary of net realized gains (losses) recorded in the consolidated statements of income	The following table summarizes net realized gains (losses) recorded on the Company’s derivative instruments in the consolidated statements of income:
	Derivatives not designated	Year Ended December 31,
	as hedging instruments	2014			2013			2012
	Futures contracts	$	30,444		$	10,742		$	(3,307	)
	Foreign currency forward contracts	13,430			9,762			(214		)
	TBAs	(342		)	(1,623		)	4,413
	Other	402			2,031			(2,218		)
	Total	$	43,934		$	20,912		$	(1,326	)

Other_Comprehensive_Income_Tab

Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2014
Comprehensive Income Note Disclosure [Abstract]
Schedule of changes in each component of AOCI	The following table presents the changes in each component of accumulated other comprehensive income (“AOCI”), net of tax:
		Unrealized Appreciation on Available-For-Sale Investments				Foreign Currency Translation Adjustments				Total
	Year Ended December 31, 2014
	Beginning balance	$	80,692			$	(5,728	)		$	74,964
	Other comprehensive income (loss) before reclassifications	146,010				(27,014		)		118,996
	Amounts reclassified from accumulated other comprehensive income	(65,104		)		—				(65,104		)
	Net current period other comprehensive income (loss)	80,906				(27,014		)		53,892
	Ending balance	$	161,598			$	(32,742	)		$	128,856
	Year Ended December 31, 2013
	Beginning balance	$	289,956			$	(2,939	)		$	287,017
	Other comprehensive income (loss) before reclassifications	(176,578		)		(2,789		)		(179,367		)
	Amounts reclassified from accumulated other comprehensive income	(32,686		)		—				(32,686		)
	Net current period other comprehensive income (loss)	(209,264		)		(2,789		)		(212,053		)
	Ending balance	$	80,692			$	(5,728	)		$	74,964
	Year Ended December 31, 2012
	Beginning balance	$	174,636			$	(20,713	)		$	153,923
	Other comprehensive income (loss) before reclassifications	273,144				17,774				290,918
	Amounts reclassified from accumulated other comprehensive income	(157,824		)		—				(157,824		)
	Net current period other comprehensive income (loss)	115,320				17,774				133,094
	Ending balance	$	289,956			$	(2,939	)		$	287,017
Details about amounts reclassified from AOCI	The following table presents details about amounts reclassified from accumulated other comprehensive income:
			Consolidated Statement of Income		Amounts Reclassed from AOCI
			Line Item That Includes		Year Ended December 31,
	Details About AOCI Components		Reclassification		2014				2013				2012
	Unrealized appreciation on available-for-sale investments
			Net realized gains		$	102,223			$	39,308			$	180,144
			Other-than-temporary impairment losses		(30,470		)		(3,961		)		(12,175		)
			Total before tax		71,753				35,347				167,969
			Income tax expense		(6,649		)		(2,661		)		(10,145		)
			Net of tax		$	65,104			$	32,686			$	157,824
Schedule of comprehensive income (loss)	Following are the related tax effects allocated to each component of other comprehensive income (loss):
		Before				Tax				Net
		Tax				Expense				of Tax
		Amount				(Benefit)				Amount
	Year Ended December 31, 2014
	Unrealized appreciation (decline) in value of available-for-sale investments:
	Unrealized holding losses arising during period	$	161,685			$	15,355			$	146,330
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	(320		)		—				(320		)
	Less reclassification of net realized gains included in net income	71,753				6,649				65,104
	Foreign currency translation adjustments	(27,014		)		—				(27,014		)
	Other comprehensive income (loss)	$	62,598			$	8,706			$	53,892
	Year Ended December 31, 2013
	Unrealized appreciation (decline) in value of available-for-sale investments:
	Unrealized holding gains arising during period	$	(201,386	)		$	(24,983	)		$	(176,403	)
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	(175		)		—				(175		)
	Less reclassification of net realized gains included in net income	35,347				2,661				32,686
	Foreign currency translation adjustments	(2,789		)		—				(2,789		)
	Other comprehensive income (loss)	$	(239,697	)		$	(27,644	)		$	(212,053	)
	Year Ended December 31, 2012
	Unrealized appreciation (decline) in value of available-for-sale investments:
	Unrealized holding gains arising during period	$	278,917			$	4,986			$	273,931
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	(787		)		—				(787		)
	Less reclassification of net realized gains included in net income	167,969				10,145				157,824
	Foreign currency translation adjustments	18,748				974				17,774
	Other comprehensive income (loss)	$	128,909			$	(4,185	)		$	133,094

Earnings_Per_Common_Share_Tabl

Earnings Per Common Share (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Schedule of computation of basic and diluted earnings per common share	The following table sets forth the computation of basic and diluted earnings per common share:
		Year Ended December 31,
		2014			2013			2012
	Numerator:
	Net income	$	821,260		$	709,731		$	593,397
	Amounts attributable to noncontrolling interests	13,095			—			—
	Net income available to Arch	834,355			709,731			593,397
	Preferred dividends	(21,938		)	(21,938		)	(25,079		)
	Loss on repurchase of preferred shares	—			—			(10,612		)
	Net income available to Arch common shareholders	$	812,417		$	687,793		$	557,706
	Denominator:
	Weighted average common shares outstanding – basic	130,817,610			131,355,392			134,446,158
	Effect of dilutive common share equivalents:
	Nonvested restricted shares	1,128,540			1,090,100			857,174
	Stock options (1)	2,976,172			3,331,691			2,955,515
	Weighted average common shares and common share	134,922,322			135,777,183			138,258,847
	equivalents outstanding – diluted
	Earnings per common share:
	Basic	$	6.21		$	5.24		$	4.15
	Diluted	$	6.02		$	5.07		$	4.03
	-1	Certain stock options were not included in the computation of diluted earnings per share where the exercise price of the stock options exceeded the average market price and would have been anti-dilutive or where, when applying the treasury stock method to in-the-money options, the sum of the proceeds, including unrecognized compensation, exceeded the average market price and would have been anti-dilutive. For 2014, 2013 and 2012, the number of stock options excluded were 1,435,955, 892,439 and 839,414, respectively.

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Components of income taxes attributable to operations	The components of income taxes attributable to operations were as follows:
		Year Ended December 31,
		2014			2013			2012
	Current expense (benefit):
	United States	$	29,749		$	27,537		$	8,267
	Non-U.S.	801			5,159			737
		30,550			32,696			9,004
	Deferred expense (benefit):
	United States	(8,168		)	1,937			(9,779		)
	Non-U.S.	605			(1,859		)	(3,235		)
		(7,563		)	78			(13,014		)
	Income tax expense (benefit)	$	22,987		$	32,774		$	(4,010	)
Schedule of income or loss before income taxes by jurisdiction	The Company’s income or loss before income taxes was earned in the following jurisdictions:
		Year Ended December 31,
		2014			2013			2012
	Income (Loss) Before Income Taxes:
	Bermuda	$	794,926		$	717,661		$	609,710
	United States	53,055			87,032			9,600
	Other	(3,734		)	(62,188		)	(29,923		)
	Total	$	844,247		$	742,505		$	589,387
Reconciliation of the differences between the provision for income taxes and the expected tax provision at the weighted average tax rate	A reconciliation of the difference between the provision for income taxes and the expected tax provision at the weighted average tax rate follows:
		Year Ended December 31,
		2014			2013			2012
	Expected income tax expense (benefit) computed on pre-tax income	$	18,178		$	17,506		$	(3,426	)
	at weighted average income tax rate
	Addition (reduction) in income tax expense (benefit) resulting from:
	Tax-exempt investment income	(8,048		)	(8,255		)	(9,257		)
	Meals and entertainment	711			599			688
	State taxes, net of U.S. federal tax benefit	1,281			431			270
	Foreign branch taxes	464			703			544
	Prior year adjustment	320			2,810			(1,581		)
	Foreign exchange gains & losses	746			(1,254		)	(436		)
	Changes in applicable tax rate	(51		)	2,007			1,193
	Dividend withholding taxes	3,276			4,619			2,511
	Change in valuation allowance	(736		)	11,795			4,281
	Contingent consideration	6,763			—			—
	Other	83			1,813			1,203
	Income tax expense (benefit)	$	22,987		$	32,774		$	(4,010	)
Significant components of deferred income tax assets and liabilities	Significant components of the Company’s deferred income tax assets and liabilities were as follows:
		December 31,
		2014			2013
	Deferred income tax assets:
	Net operating loss	$	12,017		$	15,294
	AMT credit carryforward	5,309			14,456
	Discounting of net loss reserves	63,880			53,293
	Net unearned premium reserve	34,087			30,698
	Compensation liabilities	27,765			26,481
	Foreign tax credit carryforward	4,628			4,343
	Interest expense	5,208			6,049
	Goodwill	885			—
	Tax and loss bonds	21,247			—
	Other, net	13,408			13,347
	Deferred tax assets before valuation allowance	188,434			163,961
	Valuation allowance	(12,624		)	(15,548		)
	Deferred tax assets net of valuation allowance	175,810			148,413
	Deferred income tax liabilities:
	Depreciation and amortization	(7,579		)	(3,810		)
	Deferred acquisition costs, net	(2,744		)	(3,369		)
	Deposit accounting liability	(4,170		)	(4,581		)
	Contingency reserve	(18,480		)	—
	Net unrealized appreciation of investments	(11,695		)	(6,354		)
	Other, net	(150		)	(1,738		)
	Total deferred tax liabilities	(44,818		)	(19,852		)
	Net deferred income tax assets	$	130,992		$	128,561
Summary of open tax years potentially subject to examination, by jurisdiction	The following table details open tax years that are potentially subject to examination by local tax authorities, in the following major jurisdictions:
	Jurisdiction	Tax Years
	United States	2009-2014
	United Kingdom	2009-2014
	Ireland	2010-2014
	Canada	2010-2014
	Switzerland	2011-2014
	Denmark	2011-2014

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Summary of future minimum lease payments	At December 31, 2014, the future minimum rental commitments, exclusive of escalation clauses and maintenance costs and net of rental income, for all of the Company’s operating leases are as follows:
	2015	$	23,343
	2016	23,670
	2017	22,539
	2018	20,803
	2019	17,705
	Thereafter	46,672
	Total	$	154,732

Goodwill_and_Intangible_Assets1

Goodwill and Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Analysis of goodwill and intangible assets	The following table shows an analysis of goodwill and intangible assets:
		Goodwill			Intangible assets with an indefinite life			Intangible assets with finite life			Total
	Net balance at December 31, 2012	$	—		$	16,666		$	21,593		$	38,259
	Acquisitions	—			—			—			—
	Amortization	—			—			(11,170		)	(11,170		)
	Foreign currency translation adjustment	—			—			230			230
	Net balance at December 31, 2013	—			16,666			10,653			27,319
	Acquisitions	14,965			16,858			76,435			108,258
	Amortization	—			—			(25,520		)	(25,520		)
	Foreign currency translation adjustment	(19		)	—			(499		)	(518		)
	Net balance at December 31, 2014	$	14,946		$	33,524		$	61,069		$	109,539
	Gross balance at December 31, 2014	$	14,965		$	33,524		$	124,908		$	173,397
	Accumulated amortization	—			—			(63,439		)	(63,439		)
	Foreign currency translation adjustment	(19		)	—			(400		)	(419		)
	Net balance at December 31, 2014	$	14,946		$	33,524		$	61,069		$	109,539
Summary of components of intangible assets	The following table presents the components of intangible assets:
		Gross Balance			Accumulated			Foreign Currency Translation Adjustment			Net
					Amortization						Balance
	31-Dec-14
	Acquired insurance contracts	$	77,574		$	(47,710	)	$	(107	)	$	29,757
	Operating platform	29,900			(5,482		)				24,418
	Customer relationships	25,910			(12,283		)	(293		)	13,334
	Goodwill	14,965			—			(19		)	14,946
	Insurance licenses	33,524			—			—			33,524
	Unfavorable service contract	(9,533		)	2,229			—			(7,304		)
	Other	1,057			(193		)	—			864
	Total	$	173,397		$	(63,439	)	$	(419	)	$	109,539
	31-Dec-13
	Acquired insurance contracts	$	31,102		$	(28,208	)	$	—		$	2,894
	Customer relationships	17,371			(9,711		)	99			7,759
	Insurance licenses	16,666			—			—			16,666
	Total	$	65,139		$	(37,919	)	$	99		$	27,319

Shareholders_Equity_Tables

Shareholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2014
Stockholders' Equity Note [Abstract]
Roll-forward of changes in issued and outstanding Common Shares	The following table presents a roll-forward of changes in the Company’s issued and outstanding Common Shares:
		Year Ended December 31,
		2014		2013		2012
	Common Shares:
	Shares issued and outstanding, beginning of year	169,560,591		168,255,572		164,636,338
	Shares issued (1)	1,493,524		811,732		2,066,065
	Restricted shares issued, net of cancellations	618,293		493,287		1,553,169
	Shares issued, end of year	171,672,408		169,560,591		168,255,572
	Common shares in treasury, end of year	(44,304,474	)	(35,885,707	)	(34,412,959	)
	Shares issued and outstanding, end of year	127,367,934		133,674,884		133,842,613
	-1	Includes shares issued from the exercise of stock options and stock appreciation rights, and shares issued from the employee share purchase plan.

ShareBased_Compensation_Tables

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Valuation assumptions
		Year Ended December 31,
		2014			2013			2012
	Dividend yield	0	%		0	%		0	%
	Expected volatility	22.8	%		23.6	%		24.3	%
	Risk free interest rate	1.9	%		1	%		1	%
	Expected option life	6.0 years			6.0 years			6.25 years
Option activity	A summary of stock option and SAR activity under the Company’s Long Term Incentive and Share Award Plans during 2014 is presented below:
		Year Ended December 31, 2014
		Number of			Weighted Average Exercise Price
		Options / SARs
	Outstanding, beginning of year	8,338,480			$	28.82
	Granted	1,078,036			$	56.71
	Exercised	(1,542,828	)		$	18.62
	Forfeited or expired	(69,655	)		$	46.91
	Outstanding, end of year	7,804,033			$	34.52
	Exercisable, end of year	5,558,439			$	28.91
Unvested restricted share and unit activity	A summary of unvested restricted share and unit activity under the Company’s Long Term Incentive and Share Award Plans for 2014 is presented below:
		Year Ended December 31, 2014
		Restricted				Restricted
		Common				Unit
		Shares				Awards
	Unvested Shares:
	Unvested balance, beginning of year	1,943,125				308,322
	Granted	660,043				107,426
	Vested	(591,576		)		(88,210		)
	Forfeited	(41,750		)		(22,405		)
	Unvested balance, end of year	1,969,842				305,133
	Weighted Average Grant Date Fair Value:
	Unvested balance, beginning of year	$	43.55			$	43.32
	Granted	$	56.94			$	57.18
	Vested	$	41.89			$	41.86
	Forfeited	$	46.54			$	45.07
	Unvested balance, end of year	$	48.47			$	48.49

Statutory_Information_Tables

Statutory Information (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure Statutory Information [Abstract]
Summary of statutory capital, surplus and net income	The actual and required statutory capital and surplus for the Company’s principal operating subsidiaries at December 31, 2014 and 2013 was as follows:
		December 31, 2014					December 31, 2013
		Actual (1)		Required (1)			Actual (1)			Required (1)
	Statutory capital and surplus (1):
	Bermuda	$	6,447,803	$	2,209,757		$	5,416,948		$	2,077,442
	Ireland	553,561		463,877			533,283			458,666
	United States	1,454,282		467,797			1,013,228			335,442
	United Kingdom	384,525		307,596			392,734			353,330
	Canada	79,123		58,837			77,877			74,951
	-1	Such amounts include ownership interests in affiliated insurance and reinsurance subsidiaries.
	The statutory net income (loss) for the Company’s principal operating subsidiaries for 2014, 2013 and 2012 was as follows:
		Year Ended December 31,
		2014		2013			2012
	Statutory net income (loss):
	Bermuda	$	692,676	$	719,267		$	631,483
	Ireland	43,197		(24,410		)	(1,940		)
	United States	46,110		62,605			(21,517		)
	United Kingdom	13,016		(11,353		)	(4,449		)
	Canada	1,517		(36,203		)	—

Unaudited_Condensed_Quarterly_1

Unaudited Condensed Quarterly Financial Information (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Summary of condensed quarterly financial information	The following table summarizes the Company’s 2014 and 2013 unaudited condensed quarterly financial information (including the results of the ‘other’ segment):
		Fourth			Third			Second			First
		Quarter			Quarter			Quarter			Quarter
	Year Ended December 31, 2014
	Net premiums written	$	895,481		$	959,539		$	971,928		$	1,064,990
	Net premiums earned	923,165			903,651			907,152			859,780
	Net investment income	82,496			80,105			72,990			66,994
	Net realized gains	10,561			18,515			54,144			19,697
	Net impairment losses recognized in earnings	(3,837		)	(8,593		)	(14,749		)	(2,971		)
	Underwriting income	113,464			101,087			124,091			132,456
	Net income	207,134			223,264			211,717			179,145
	Preferred dividends	(5,485		)	(5,484		)	(5,485		)	(5,484		)
	Net income available to Arch common shareholders	209,679			223,191			202,531			177,016
	Net income per common share -- basic	$	1.65		$	1.69		$	1.53		$	1.34
	Net income per common share -- diluted	$	1.6		$	1.64		$	1.48		$	1.3
	Year Ended December 31, 2013
	Net premiums written	$	748,921		$	839,135		$	810,535		$	952,776
	Net premiums earned	839,366			795,000			758,816			752,770
	Net investment income	67,095			66,083			68,369			65,672
	Net realized gains (losses)	9,048			(6,022		)	12,652			58,340
	Net impairment losses recognized in earnings	(88		)	(728		)	(724		)	(2,246		)
	Underwriting income	128,318			110,992			96,029			116,398
	Net income	161,490			114,825			176,940			256,476
	Preferred dividends	(5,485		)	(5,484		)	(5,485		)	(5,484		)
	Net income available to Arch common shareholders	156,005			109,341			171,455			250,992
	Net income per common share -- basic	$	1.19		$	0.83		$	1.31		$	1.92
	Net income per common share -- diluted	$	1.14		$	0.8		$	1.26		$	1.85

Guarantor_Financial_Informatio1

Guarantor Financial Information (Tables)	12 Months Ended
	Dec. 31, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Condensed consolidating balance sheet
		31-Dec-14
	Condensed Consolidating Balance Sheet	ACGL (Parent Guarantor)			Arch-U.S. (Subsidiary Issuer)			Other ACGL Subsidiaries			Consolidating Adjustments and Eliminations			ACGL Consolidated
	Assets
	Total investments	$	107		$	62,867		$	15,268,275		$	—		$	15,331,249
	Cash	3,218			2,787			479,697			—			485,702
	Investments in subsidiaries	6,536,644			1,685,185			—			(8,221,829		)	—
	Due from subsidiaries and affiliates	48			7,517			370,429			(377,994		)	—
	Premiums receivable	—			—			1,331,511			(382,816		)	948,695
	Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	—			—			5,584,973			(3,772,128		)	1,812,845
	Contractholder receivables	—			—			1,309,192			—			1,309,192
	Prepaid reinsurance premiums	—			—			1,373,008			(995,930		)	377,078
	Deferred acquisition costs, net	—			—			414,525			—			414,525
	Other assets	7,590			49,576			1,689,670			(416,579		)	1,330,257
	Total assets	$	6,547,607		$	1,807,932		$	27,821,280		$	(14,167,276	)	$	22,009,543
	Liabilities
	Reserve for losses and loss adjustment expenses	$	—		$	—		$	12,784,030		$	(3,747,582	)	$	9,036,448
	Unearned premiums	—			—			3,227,508			(995,930		)	2,231,578
	Reinsurance balances payable	—			—			589,289			(369,977		)	219,312
	Contractholder payables	—			—			1,309,192			—			1,309,192
	Deposit accounting liabilities	—			—			587,050			(259,666		)	327,384
	Senior notes	300,000			500,000			—			—			800,000
	Revolving credit agreement borrowings	100,000			—			—			—			100,000
	Due to subsidiaries and affiliates	417			7,505			370,072			(377,994		)	—
	Other liabilities	17,137			49,403			994,741			(194,298		)	866,983
	Total liabilities	417,554			556,908			19,861,882			(5,945,447		)	14,890,897
	Redeemable noncontrolling interests (1)	—			—			219,512			—			219,512
	Shareholders' Equity
	Total shareholders' equity available to Arch	6,130,053			1,251,024			6,970,805			(8,221,829		)	6,130,053
	Non-redeemable noncontrolling interests (1)	—			—			769,081			—			769,081
	Total shareholders' equity	6,130,053			1,251,024			7,739,886			(8,221,829		)	6,899,134
	Total liabilities, noncontrolling interests and shareholders' equity	$	6,547,607		$	1,807,932		$	27,821,280		$	(14,167,276	)	$	22,009,543
	(1) See Note 4.
		31-Dec-13
	Condensed Consolidating Balance Sheet	ACGL (Parent Guarantor)			Arch-U.S. (Subsidiary Issuer)			Other ACGL Subsidiaries			Consolidating Adjustments and Eliminations			ACGL Consolidated
	Assets
	Total investments	$	2,530		$	408,957		$	13,200,247		$	—		$	13,611,734
	Cash	3,223			509			430,325			—			434,057
	Investments in subsidiaries	6,046,060			1,258,889			—			(7,304,949		)	—
	Due from subsidiaries and affiliates	2,251			—			405,110			(407,361		)	—
	Premiums receivable	—			—			1,085,369			(331,445		)	753,924
	Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	—			—			5,645,156			(3,840,826		)	1,804,330
	Contractholder receivables	—			—			1,064,246			—			1,064,246
	Prepaid reinsurance premiums	—			—			1,109,312			(780,969		)	328,343
	Deferred acquisition costs, net	—			—			342,314			—			342,314
	Other assets	6,598			60,342			1,714,651			(554,445		)	1,227,146
	Total assets	$	6,060,662		$	1,728,697		$	24,996,730		$	(13,219,995	)	$	19,566,094
	Liabilities
	Reserve for losses and loss adjustment expenses	$	—		$	—		$	12,625,766		$	(3,801,070	)	$	8,824,696
	Unearned premiums	—			—			2,677,334			(780,969		)	1,896,365
	Reinsurance balances payable	—			—			662,394			(466,227		)	196,167
	Contractholder payables	—			—			1,064,246			—			1,064,246
	Deposit accounting liabilities	—			—			758,490			(337,193		)	421,297
	Senior notes	300,000			500,000			—			—			800,000
	Revolving credit agreement borrowings	100,000			—			—			—			100,000
	Due to subsidiaries and affiliates	18			10,250			397,093			(407,361		)	—
	Other liabilities	13,148			33,206			691,699			(122,226		)	615,827
	Total liabilities	413,166			543,456			18,877,022			(5,915,046		)	13,918,598
	Redeemable noncontrolling interests	—			—			—			—			—
	Shareholders' Equity
	Total shareholders' equity available to Arch	5,647,496			1,185,241			6,119,708			(7,304,949		)	5,647,496
	Non-redeemable noncontrolling interests	—			—			—			—			—
	Total shareholders' equity	5,647,496			1,185,241			6,119,708			(7,304,949		)	5,647,496
	Total liabilities, noncontrolling interests and shareholders' equity	$	6,060,662		$	1,728,697		$	24,996,730		$	(13,219,995	)	$	19,566,094
Condensed consolidating statement of income and comprehensive income
		Year Ended December 31, 2014
	Condensed Consolidating Statement of Income and Comprehensive Income	ACGL (Parent Guarantor)			Arch-U.S. (Subsidiary Issuer)			Other ACGL Subsidiaries			Consolidating Adjustments and Eliminations			ACGL Consolidated
	Revenues
	Net premiums earned	$	—		$	—		$	3,593,748		$	—		$	3,593,748
	Net investment income	—			—			326,831			(24,246		)	302,585
	Net realized gains	—			5			102,912			—			102,917
	Net impairment losses recognized in earnings	—			—			(30,150		)	—			(30,150		)
	Other underwriting income	—			—			10,142			—			10,142
	Equity in net income of investment funds accounted for using the equity method	—			—			19,883			—			19,883
	Other income (loss)	—			—			(10,252		)	—			(10,252		)
	Total revenues	—			5			4,013,114			(24,246		)	3,988,873
	Expenses
	Losses and loss adjustment expenses	—			—			1,919,250			—			1,919,250
	Acquisition expenses	—			—			657,262			—			657,262
	Other operating expenses	46,074			3,387			556,763			—			606,224
	Interest expense	23,423			25,817			20,640			(24,246		)	45,634
	Net foreign exchange gains	—			—			(53,222		)	(30,522		)	(83,744		)
	Total expenses	69,497			29,204			3,100,693			(54,768		)	3,144,626
	Income (loss) before income taxes	(69,497		)	(29,199		)	912,421			30,522			844,247
	Income tax benefit (expense)	—			10,125			(33,112		)	—			(22,987		)
	Income (loss) before equity in net income of subsidiaries	(69,497		)	(19,074		)	879,309			30,522			821,260
	Equity in net income of subsidiaries	903,852			53,584			—			(957,436		)	—
	Net income	834,355			34,510			879,309			(926,914		)	821,260
	Amounts attributable to noncontrolling interests (1)	—			—			13,095			—			13,095
	Net income available to Arch	834,355			34,510			892,404			(926,914		)	834,355
	Preferred dividends	(21,938		)	—			—			—			(21,938		)
	Net income available to Arch common shareholders	$	812,417		$	34,510		$	892,404		$	(926,914	)	$	812,417
	Comprehensive income available to Arch	$	888,247		$	33,671		$	976,821		$	(1,010,492	)	$	888,247
	(1) See Note 4.
		Year Ended December 31, 2013
	Condensed Consolidating Statement of Income and Comprehensive Income	ACGL (Parent Guarantor)			Arch-U.S. (Subsidiary Issuer)			Other ACGL Subsidiaries			Consolidating Adjustments and Eliminations			ACGL Consolidated
	Revenues
	Net premiums earned	$	—		$	—		$	3,145,952		$	—		$	3,145,952
	Net investment income	—			31			307,449			(40,261		)	267,219
	Net realized gains	—			—			74,018			—			74,018
	Net impairment losses recognized in earnings	—			—			(3,786		)	—			(3,786		)
	Other underwriting income	—			—			7,639			—			7,639
	Equity in net income of investment funds accounted for using the equity method	—			—			35,701			—			35,701
	Other income (loss)	—			—			(586		)	—			(586		)
	Total revenues	—			31			3,566,387			(40,261		)	3,526,157
	Expenses
	Losses and loss adjustment expenses	—			—			1,679,424			—			1,679,424
	Acquisition expenses	—			—			564,103			—			564,103
	Other operating expenses	38,702			2,691			459,337			—			500,730
	Interest expense	23,272			1,316			42,733			(40,261		)	27,060
	Net foreign exchange losses	—			—			2,145			10,190			12,335
	Total expenses	61,974			4,007			2,747,742			(30,071		)	2,783,652
	Income (loss) before income taxes	(61,974		)	(3,976		)	818,645			(10,190		)	742,505
	Income tax benefit (expense)	—			1,383			(34,157		)	—			(32,774		)
	Income (loss) before equity in net income of subsidiaries	(61,974		)	(2,593		)	784,488			(10,190		)	709,731
	Equity in net income of subsidiaries	771,705			25,644			—			(797,349		)	—
	Net income	709,731			23,051			784,488			(807,539		)	709,731
	Amounts attributable to noncontrolling interests	—			—			—			—			—
	Net income available to Arch	709,731			23,051			784,488			(807,539		)	709,731
	Preferred dividends	(21,938		)	—			—			—			(21,938		)
	Net income available to Arch common shareholders	$	687,793		$	23,051		$	784,488		$	(807,539	)	$	687,793
	Comprehensive income (loss) available to Arch	$	497,678		$	(28,330	)	$	562,245		$	(533,915	)	$	497,678
		Year Ended December 31, 2012
	Condensed Consolidating Statement of Income and Comprehensive Income	ACGL (Parent Guarantor)			Arch-U.S. (Subsidiary Issuer)			Other ACGL Subsidiaries			Consolidating Adjustments and Eliminations			ACGL Consolidated
	Revenues
	Net premiums earned	$	—		$	—		$	2,935,140		$	—		$	2,935,140
	Net investment income	4			8			321,805			(26,922		)	294,895
	Net realized gains	—			—			194,228			—			194,228
Net impairment losses recognized in earnings	—			—			(11,388		)	—			(11,388		)
Other underwriting income	—			—			8,090			—			8,090
Equity in net income of investment funds accounted for using the equity method	—			—			73,510			—			73,510
Other income (loss)	—			—			(12,094		)	—			(12,094		)
Total revenues	4			8			3,509,291			(26,922		)	3,482,381
Expenses
Losses and loss adjustment expenses	—			—			1,861,277			—			1,861,277
Acquisition expenses	—			—			508,884			—			508,884
Other operating expenses	35,570			1,218			428,565			—			465,353
Interest expense	23,496			—			31,951			(26,922		)	28,525
Net foreign exchange losses	—			—			21,787			7,168			28,955
Total expenses	59,066			1,218			2,852,464			(19,754		)	2,892,994
Income (loss) before income taxes	(59,062		)	(1,210		)	656,827			(7,168		)	589,387
Income tax benefit	—			424			3,586			—			4,010
Income (loss) before equity in net income of subsidiaries	(59,062		)	(786		)	660,413			(7,168		)	593,397
Equity in net income of subsidiaries	652,459			9,738			—			(662,197		)	—
Net income	593,397			8,952			660,413			(669,365		)	593,397
Amounts attributable to noncontrolling interests	—			—			—			—			—
Net income available to Arch	593,397			8,952			660,413			(669,365		)	593,397
Preferred dividends	(25,079		)	—			—			—			(25,079		)
Loss on repurchase of preferred shares	(10,612		)	—			—			—			(10,612		)
Net income available to Arch common shareholders	$	557,706		$	8,952		$	660,413		$	(669,365	)	$	557,706
Comprehensive income available to Arch	$	726,491		$	2,796		$	786,338		$	(789,134	)	$	726,491
Condensed consolidating statement of cash flows
		Year Ended December 31, 2014
	Condensed Consolidating Statement	ACGL (Parent Guarantor)			Arch-U.S. (Subsidiary Issuer)			Other ACGL Subsidiaries			Consolidating Adjustments and Eliminations			ACGL Consolidated
	of Cash Flows
	Operating Activities
	Net Cash Provided By (Used For)	$	467,091		$	8,142		$	1,077,597		$	(515,700	)	$	1,037,130
	Operating Activities
	Investing Activities
	Purchases of fixed maturity investments	—			(78,509		)	(28,666,770		)	—			(28,745,279		)
	Purchases of equity securities	—			—			(520,817		)	—			(520,817		)
	Purchases of other investments	—			—			(2,257,481		)	—			(2,257,481		)
	Proceeds from the sales of fixed maturity investments	—			16,011			26,807,181			—			26,823,192
	Proceeds from the sales of equity securities	—			—			411,362			—			411,362
	Proceeds from the sales of other investments	—			—			1,643,000			—			1,643,000
	Proceeds from redemptions and maturities of fixed maturity investments	—			—			762,995			—			762,995
	Net (purchases) sales of short-term investments	2,423			408,591			166,112			—			577,126
	Change in investment of securities lending collateral	—			—			57,470			—			57,470
	Contributions to subsidiaries	—			(341,707		)	(128,825		)	470,532			—
	Issuance of intercompany loans	—			(7,500		)	(7,500		)	15,000			—
	Repayments of intercompany loans	—			—			38,964			(38,964		)	—
	Purchase of business, net of cash acquired	—			—			(237,106		)	—			(237,106		)
	Purchases of furniture, equipment and other assets	(271		)	—			(19,612		)	—			(19,883		)
	Net Cash Provided By (Used For)	2,152			(3,114		)	(1,951,027		)	446,568			(1,505,421		)
	Investing Activities
	Financing Activities
	Purchases of common shares under share repurchase program	(454,137		)	—			—			—			(454,137		)
	Proceeds from common shares issued, net	6,827			—			470,532			(470,532		)	6,827
	Proceeds from intercompany loans	—			7,500			7,500			(15,000		)	—
	Repayments of intercompany loans	—			(10,250		)	(28,714		)	38,964			—
	Change in securities lending collateral	—			—			(57,470		)	—			(57,470		)
	Third party investment in non-redeemable noncontrolling interests (1)	—			—			796,903			—			796,903
	Third party investment in redeemable noncontrolling interests (1)	—			—			219,233			—			219,233
	Dividends paid to redeemable noncontrolling interests (1)	—			—			(14,448		)	—			(14,448		)
	Dividends paid to parent (2)	—			—			(515,700		)	515,700			—
	Other	—			—			64,973			—			64,973
	Preferred dividends paid	(21,938		)	—			—			—			(21,938		)
	Net Cash Provided By (Used For)	(469,248		)	(2,750		)	942,809			69,132			539,943
	Financing Activities
	Effects of exchange rates changes on foreign currency cash	—			—			(20,007		)	—			(20,007		)
	Increase (decrease) in cash	(5		)	2,278			49,372			—			51,645
	Cash beginning of year	3,223			509			430,325			—			434,057
	Cash end of year	$	3,218		$	2,787		$	479,697		$	—		$	485,702
	(1) See Note 4.
	(2) Included in net cash provided by (used for) operating activities in the ACGL (Parent Guarantor) column.
		Year Ended December 31, 2013
	Condensed Consolidating Statement	ACGL (Parent Guarantor)			Arch-U.S. (Subsidiary Issuer)			Other ACGL Subsidiaries			Consolidating Adjustments and Eliminations			ACGL Consolidated
	of Cash Flows
	Operating Activities
	Net Cash Provided By (Used For)	$	68,562		$	(6,569	)	$	908,375		$	(119,500	)	$	850,868
	Operating Activities
	Investing Activities
	Purchases of fixed maturity investments	—			—			(18,174,988		)	—			(18,174,988		)
	Purchases of equity securities	—			—			(535,857		)	—			(535,857		)
	Purchases of other investments	—			—			(1,326,729		)	—			(1,326,729		)
	Proceeds from the sales of fixed maturity investments	—			—			17,196,614			—			17,196,614
	Proceeds from the sales of equity securities	—			—			462,787			—			462,787
	Proceeds from the sales of other investments	—			—			1,162,707			—			1,162,707
	Proceeds from redemptions and maturities of fixed maturity investments	—			—			731,708			—			731,708
	Net (purchases) sales of short-term investments	5,799			(400,162		)	(356,250		)	—			(750,613		)
	Change in investment of securities lending collateral	—			—			(55,643		)	—			(55,643		)
	Contributions to subsidiaries	(160		)	(97,850		)	(20,250		)	118,260			—
	Issuance of intercompany loans	—			—			(10,250		)	10,250			—
	Purchases of furniture, equipment and other assets	(712		)	—			(16,787		)	—			(17,499		)
	Net Cash Provided By (Used For)	4,927			(498,012		)	(942,938		)	128,510			(1,307,513		)
Investing Activities
Financing Activities
Purchases of common shares under share repurchase program	(57,796		)	—			—			—			(57,796		)
Proceeds from common shares issued, net	3,051			—			118,260			(118,260		)	3,051
Proceeds from intercompany loans	—			10,250			—			(10,250		)	—
Proceeds from borrowings	—			494,228			—			—			494,228
Change in securities lending collateral	—			—			55,643			—			55,643
Dividends paid to parent (1)	—			—			(119,500		)	119,500			—
Other	—			—			50,830			—			50,830
Preferred dividends paid	(21,938		)	—			—			—			(21,938		)
Net Cash Provided By (Used For)	(76,683		)	504,478			105,233			(9,010		)	524,018
Financing Activities
Effects of exchange rates changes on foreign currency cash	—			—			(4,357		)	—			(4,357		)
Increase (decrease) in cash	(3,194		)	(103		)	66,313			—			63,016
Cash beginning of year	6,417			612			364,012			—			371,041
Cash end of year	$	3,223		$	509		$	430,325		$	—		$	434,057
(1) Included in net cash provided by (used for) operating activities in the ACGL (Parent Guarantor) column.
	Year Ended December 31, 2012
Condensed Consolidating Statement	ACGL (Parent Guarantor)			Arch-U.S. (Subsidiary Issuer)			Other ACGL Subsidiaries			Consolidating Adjustments and Eliminations			ACGL Consolidated
of Cash Flows
Operating Activities
Net Cash Provided By (Used For)	$	209,500		$	(8,086	)	$	978,679		$	(258,490	)	$	921,603
Operating Activities
Investing Activities
Purchases of fixed maturity investments	—			—			(17,568,592		)	—			(17,568,592		)
Purchases of equity securities	—			—			(268,999		)	—			(268,999		)
Purchases of other investments	—			—			(1,000,049		)	—			(1,000,049		)
Proceeds from the sales of fixed maturity investments	—			—			16,366,306			—			16,366,306
Proceeds from the sales of equity securities	—			—			313,617			—			313,617
Proceeds from the sales of other investments	—			—			443,630			—			443,630
Proceeds from redemptions and maturities of fixed maturity investments	—			—			1,115,594			—			1,115,594
Net (purchases) sales of short-term investments	(5,094		)	1,986			189,027			—			185,919
Change in investment of securities lending collateral	—			—			6,190			—			6,190
Contributions to subsidiaries	—			—			(38,576		)	38,576			—
Purchase of business, net of cash acquired	—			—			28,948			—			28,948
Purchases of furniture, equipment and other assets	(65		)	—			(18,467		)	—			(18,532		)
Net Cash Provided By (Used For)	(5,159		)	1,986			(431,371		)	38,576			(395,968		)
Investing Activities
Financing Activities
Proceeds from issuance of Series C preferred shares issued, net	315,763			—			—			—			315,763
Repurchase of Series A and B preferred shares	(325,000		)	—			—			—			(325,000		)
Purchases of common shares under share repurchase program	(172,056		)	—			—			—			(172,056		)
Proceeds from common shares issued, net	7,033			—			38,576			(38,576		)	7,033
Repayments of borrowings	—			—			(310,868		)	—			(310,868		)
Change in securities lending collateral	—			—			(6,190		)	—			(6,190		)
Dividends paid to parent (1)	—			—			(258,490		)	258,490			—
Other	—			—			6,664			—			6,664
Preferred dividends paid	(28,381		)	—			—			—			(28,381		)
Net Cash Provided By (Used For)	(202,641		)	—			(530,308		)	219,914			(513,035		)
Financing Activities
Effects of exchange rates changes on foreign currency cash	—			—			6,742			—			6,742
Increase (decrease) in cash	1,700			(6,100		)	23,742			—			19,342
Cash beginning of year	4,717			6,712			340,270			—			351,699
Cash end of year	$	6,417		$	612		$	364,012		$	—		$	371,041
(1) Included in net cash provided by (used for) operating activities in the ACGL (Parent Guarantor) column.

General_Details

General (Details) (USD $)	1 Months Ended	0 Months Ended
In Millions, except Per Share data, unless otherwise specified	Oct. 30, 2001	Mar. 20, 2014	Dec. 31, 2014	Dec. 31, 2013
Variable Interest Entity [Line Items]
Proceeds from from common shares issued	$763.20
Common shares, par value per share			$0.00	$0.00
Watford Holdings Ltd | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Percentage ownership		11.00%
Initial investment contribution amount		$100

Business_Acquired_Summary_of_t

Business Acquired - Summary of the fair value of net assets acquired and allocation of purchase price (Details) (USD $)	0 Months Ended
In Thousands, unless otherwise specified	Jan. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Liabilities acquired
Goodwill		$14,946	$0	$0
CMG Mortgage Insurance Company
Purchase price
Cash paid	245,157
Contingent consideration liability	41,762
Total purchase price	286,919
Assets acquired
Cash	9,579
Investments, at fair value	312,093
Other assets acquired	21,691
Total assets acquired	437,650
Liabilities acquired
Reserves for losses and loss adjustment expenses	121,572
Unearned premiums	26,261
Intangible liability - unfavorable service contract	9,533
Other liabilities acquired	7,217
Total liabilities acquired	164,583
Net assets acquired	273,067
Goodwill	13,852
Useful lives of acquired intangibles
Finite-lived intangible liabilities, useful life	9 years
CMG Mortgage Insurance Company | Insurance licenses
Assets acquired
Indefinite-lived intangible assets	16,858
CMG Mortgage Insurance Company | Acquired insurance contracts
Assets acquired
Finite-lived intangible assets	46,473
Useful lives of acquired intangibles
Finite-lived intangible assets, useful life	5 years
CMG Mortgage Insurance Company | Operating platform
Assets acquired
Finite-lived intangible assets	29,900
Useful lives of acquired intangibles
Finite-lived intangible assets, useful life	5 years
CMG Mortgage Insurance Company | Favorable lease contract
Assets acquired
Finite-lived intangible assets	$1,056
Useful lives of acquired intangibles
Finite-lived intangible assets, useful life	5 years

Business_Acquired_Narrative_De

Business Acquired - Narrative (Details) (USD $)	0 Months Ended	12 Months Ended		11 Months Ended
	Jan. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2012
Business Acquisition [Line Items]
Discount rate, percentage	15.00%
Amortization of intangible assets		$25,520,000	$11,170,000
Goodwill		14,946,000	0	14,946,000	0
Financial and Nonfinancial Liabilities, Fair Value Disclosure		61,845,000		61,845,000
CMG Mortgage Insurance Company
Business Acquisition [Line Items]
Percentage of voting interests acquired	100.00%
Quota share agreement, percentage				100.00%
Quota share reinsurance agreement limit				25,000,000,000
Cash paid	245,157,000
Contingent consideration, upper bound	136,900,000
Contingent consideration liability	41,762,000
Amortization of intangible assets				20,100,000
Goodwill	13,852,000
Goodwill, deferred tax asset		48,000,000		48,000,000
Financial and Nonfinancial Liabilities, Fair Value Disclosure		61,845,000		61,845,000
CMG Mortgage Insurance Company | New mortgage insurance business (post closing)
Business Acquisition [Line Items]
Quota share agreement, percentage				20.00%
CMG Mortgage Insurance Company | Minimum
Business Acquisition [Line Items]
Extension period				1 year
CMG Mortgage Insurance Company | Maximum
Business Acquisition [Line Items]
Percentage of book value				150.00%
Extension period				3 years
CMG Mortgage Insurance Company | Stock Purchase Agreement (SPA)
Business Acquisition [Line Items]
Cash paid				160,600,000
Percentage of book value				80.00%
CMG Mortgage Insurance Company | Asset Purchase Agreement (APA)
Business Acquisition [Line Items]
Cash paid				$84,600,000

Significant_Accounting_Policie2

Significant Accounting Policies (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Significant Accounting Policies [Line Items]
Premium revenue recognition period	12 months
Deposit accounting liabilities	$327,384,000	$421,297,000
Deposit accounting assets	$284,400,000	$370,500,000
Securities lending rate of collateral required (percentage)	102.00%
Equity method investments, time lag for reporting - low end of range	1 month
Equity method investments, time lag for reporting - high end of range	3 months
Vesting period, share-based awards	3 years
Requisite service period, share-based awards	5 years
Losses occurring
Significant Accounting Policies [Line Items]
Premium revenue recognition period	12 months
Risks attaching
Significant Accounting Policies [Line Items]
Premium revenue recognition period	24 months

Variable_Interest_Entity_and_N2

Variable Interest Entity and Noncontrolling Interests - Variable interest entity (Details) (USD $)	1 Months Ended	12 Months Ended			1 Months Ended
Share data in Millions, unless otherwise specified	Oct. 30, 2001	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2014	Dec. 31, 2011
Variable Interest Entity [Line Items]
Proceeds from from common shares issued, net	$763,200,000
Proceeds on issuance of preference shares		0	0	315,763,000
Unearned premiums		2,231,578,000	1,896,365,000
Reserve for losses and loss adjustment expenses		9,036,448,000	8,824,696,000	8,933,292,000		8,456,210,000
Net cash provided by (used in) operating activities		1,037,130,000	850,868,000	921,603,000
Net cash provided by (used in) financing activities		539,943,000	524,018,000	-513,035,000
Net cash provided by (used in) investing activities		1,505,421,000	1,307,513,000	395,968,000
Variable Interest Entity, Primary Beneficiary | Watford Re
Variable Interest Entity [Line Items]
Initial capitalization amount					1,100,000,000
Proceeds from from common shares issued, net					907,300,000
Net proceeds from issuance of common shares					895,600,000
Proceeds on issuance of preference shares					226,600,000
Net proceeds on issuance of preference shares					219,200,000
Initial investment contribution amount					100,000,000
Ownership percentage					11.00%
Number of warrants received in capital raising					1.7
Unearned premiums		180,000,000
Reserve for losses and loss adjustment expenses		66,300,000
Net cash provided by (used in) operating activities		39,300,000
Net cash provided by (used in) financing activities		1,160,000,000
Net cash provided by (used in) investing activities		$1,180,000,000

Variable_Interest_Entity_and_N3

Variable Interest Entity and Noncontrolling Interests - Non-redeemable noncontrolling interest (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Noncontrolling Interest [Line Items]
Non-redeemable noncontrolling interest, beginning of year	$0	[1]	$0	[1]
Amounts attributable to noncontrolling interests	-27,823		0		0
Non-redeemable noncontrolling interest, end of year	769,081	[1]	0	[1]	0	[1]
Watford Re | Noncontrolling Interest [Member]
Noncontrolling Interest [Line Items]
Non-redeemable noncontrolling interest, beginning of year	0
Sale of shares to noncontrolling interests	796,904
Amounts attributable to noncontrolling interests	-27,823
Non-redeemable noncontrolling interest, end of year	$769,081
Common shares | Watford Re | Noncontrolling Interest [Member]
Noncontrolling Interest [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners	89.00%
[1]	See Note 4.

Variable_Interest_Entity_and_N4

Variable Interest Entity and Noncontrolling Interests - Redeemable noncontrolling interest (Details) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Redeemable Noncontrolling Interest [Line Items]
Preferred shares, par value per share	$0.01
Amounts attributable to noncontrolling interests	$13,095	[1]	$0	[1]	$0	[1]
Amounts attributable to noncontrolling interests	-27,823		0		0
Dividends attributable to redeemable noncontrolling interests	14,728		0		0
Watford Re
Redeemable Noncontrolling Interest [Line Items]
Preference shares, number of shares issued	9,065,200
Preferred shares, par value per share	$0.01
Liquidation preference per share	$25
Issue price per share	$24.50
Preferred shares, dividend rate (as a percent)	8.50%
Other
Redeemable Noncontrolling Interest [Line Items]
Amounts attributable to noncontrolling interests	-27,823		0		0
Dividends attributable to redeemable noncontrolling interests	14,728		0		0
3 month US dollar LIBOR | Watford Re
Redeemable Noncontrolling Interest [Line Items]
Preferred shares, dividend rate (as a percent)	1.00%
Redeemable preference shares | Watford Re
Redeemable Noncontrolling Interest [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners	100.00%
Noncontrolling Interest [Member] | Watford Re
Redeemable Noncontrolling Interest [Line Items]
Amounts attributable to noncontrolling interests	($27,823)
[1]	See Note 4.

Segment_Information_Analysis_o

Segment Information - Analysis of underwriting income by segment and reconciliation to net income available to common shareholders (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Gross premiums written									$4,840,616		$4,196,623		$3,869,161
Premiums ceded									-948,678		-845,256		-816,926
Net premiums written	895,481	959,539	971,928	1,064,990	748,921	839,135	810,535	952,776	3,891,938		3,351,367		3,052,235
Change in unearned premiums									-298,190		-205,415		-117,095
Net premiums earned	923,165	903,651	907,152	859,780	839,366	795,000	758,816	752,770	3,593,748		3,145,952		2,935,140
Other underwriting income									10,142		7,639		8,090
Losses and loss adjustment expenses									-1,919,250		-1,679,424		-1,861,277
Acquisition expenses, net									-657,262		-564,103		-508,884
Other operating expenses									-556,280		-458,327		-430,035
Underwriting income (loss)	113,464	101,087	124,091	132,456	128,318	110,992	96,029	116,398	471,098		451,737		143,034
Net investment income	82,496	80,105	72,990	66,994	67,095	66,083	68,369	65,672	302,585		267,219		294,895
Net realized gains	10,561	18,515	54,144	19,697	9,048	-6,022	12,652	58,340	102,917		74,018		194,228
Net impairment losses recognized in earnings	-3,837	-8,593	-14,749	-2,971	-88	-728	-724	-2,246	-30,150		-3,786		-11,388
Equity in net income (loss) of investment funds accounted for using the equity method									19,883		35,701		73,510
Other income (loss)									-10,252		-586		-12,094
Other expenses									-49,944		-42,403		-35,318
Interest expense									-45,634		-27,060		-28,525
Net foreign exchange (gains) losses									83,744		-12,335		-28,955
Income before income taxes									844,247		742,505		589,387
Income tax (expense) benefit									-22,987		-32,774		4,010
Net income	207,134	223,264	211,717	179,145	161,490	114,825	176,940	256,476	821,260		709,731		593,397
Dividends attributable to redeemable noncontrolling interests									-14,728		0		0
Amounts attributable to noncontrolling interests									27,823		0		0
Net income available to Arch									834,355		709,731		593,397
Preferred dividends	-5,485	-5,484	-5,485	-5,484	-5,485	-5,484	-5,485	-5,484	-21,938		-21,938		-25,079
Loss on repurchase of preferred shares									0		0		-10,612
Net income available to common shareholders	209,679	223,191	202,531	177,016	156,005	109,341	171,455	250,992	812,417		687,793		557,706
Underwriting Ratios
Loss ratio									53.40%		53.40%		63.40%
Acquisition expense ratio									18.30%		17.90%		17.30%
Other operating expense ratio									15.50%		14.60%		14.70%
Combined ratio									87.20%		85.90%		95.40%
Goodwill and intangible assets	109,539				27,319				109,539		27,319		38,259
Investable assets	15,773,209				14,049,525				15,773,209		14,049,525		13,045,134
Total assets	22,009,543				19,566,094				22,009,543		19,566,094		17,816,762
Total liabilities	14,890,897				13,918,598				14,890,897		13,918,598		12,647,884
Insurance
Segment Reporting Information [Line Items]
Premiums ceded									-862,015	[1]	-763,713	[1]	-768,625	[1]
Net premiums written									2,146,654		1,948,796		1,825,334
Reinsurance
Segment Reporting Information [Line Items]
Premiums ceded									-261,255	[1]	-86,620	[1]	-55,099	[1]
Net premiums written									1,265,991		1,313,001		1,158,790
Mortgage
Segment Reporting Information [Line Items]
Premiums ceded									-22,519	[1]	0	[1]	0	[1]
Net premiums written									204,837		89,570		68,111
Other
Segment Reporting Information [Line Items]
Number of segments									2
Gross premiums written									288,627	[2]	0	[2]	0	[2]
Premiums ceded									-14,171	[1]	0		0
Net premiums written									274,456		0		0
Change in unearned premiums									-170,979		0		0
Net premiums earned									103,477		0		0
Other underwriting income									0		0		0
Losses and loss adjustment expenses									-70,173		0		0
Acquisition expenses, net									-30,116		0		0
Other operating expenses									-6,268		0		0
Underwriting income (loss)									-3,080		0		0
Net investment income									18,249		0		0
Net realized gains									-30,467		0		0
Net impairment losses recognized in earnings									0		0		0
Equity in net income (loss) of investment funds accounted for using the equity method									0		0		0
Other income (loss)									0		0		0
Other expenses									-2,329		0		0
Interest expense									0		0		0
Net foreign exchange (gains) losses									1,086		0		0
Income before income taxes									-16,541		0		0
Income tax (expense) benefit									0		0		0
Net income									-16,541		0		0
Dividends attributable to redeemable noncontrolling interests									-14,728		0		0
Amounts attributable to noncontrolling interests									27,823		0		0
Net income available to Arch									-3,446		0		0
Preferred dividends									0		0		0
Loss on repurchase of preferred shares													0
Net income available to common shareholders									-3,446		0		0
Underwriting Ratios
Loss ratio									67.80%		0.00%		0.00%
Acquisition expense ratio									29.10%		0.00%		0.00%
Other operating expense ratio									6.10%		0.00%		0.00%
Combined ratio									103.00%		0.00%		0.00%
Goodwill and intangible assets	0				0				0		0		0
Investable assets	1,163,240				0				1,163,240		0		0
Total assets	1,482,516				0				1,482,516		0		0
Total liabilities	398,660				0				398,660		0		0
Underwriting segments
Segment Reporting Information [Line Items]
Number of segments									3
Gross premiums written									4,760,394	[2]	4,196,623	[2]	3,869,161	[2]
Premiums ceded									-1,142,912		-845,256		-816,926
Net premiums written									3,617,482		3,351,367		3,052,235
Change in unearned premiums									-127,211		-205,415		-117,095
Net premiums earned									3,490,271		3,145,952		2,935,140
Other underwriting income									10,142		7,639		8,090
Losses and loss adjustment expenses									-1,849,077		-1,679,424		-1,861,277
Acquisition expenses, net									-627,146		-564,103		-508,884
Other operating expenses									-550,012		-458,327		-430,035
Underwriting income (loss)									474,178		451,737		143,034
Net investment income									284,336		267,219		294,895
Net realized gains									133,384		74,018		194,228
Net impairment losses recognized in earnings									-30,150		-3,786		-11,388
Equity in net income (loss) of investment funds accounted for using the equity method									19,883		35,701		73,510
Other income (loss)									-10,252		-586		-12,094
Other expenses									-47,615		-42,403		-35,318
Interest expense									-45,634		-27,060		-28,525
Net foreign exchange (gains) losses									82,658		-12,335		-28,955
Income before income taxes									860,788		742,505		589,387
Income tax (expense) benefit									-22,987		-32,774		4,010
Net income									837,801		709,731		593,397
Dividends attributable to redeemable noncontrolling interests									0		0		0
Amounts attributable to noncontrolling interests									0		0		0
Net income available to Arch									837,801		709,731		593,397
Preferred dividends									-21,938		-21,938		-25,079
Loss on repurchase of preferred shares													-10,612
Net income available to common shareholders									815,863		687,793		557,706
Underwriting Ratios
Loss ratio									53.00%		53.40%		63.40%
Acquisition expense ratio									18.00%		17.90%		17.30%
Other operating expense ratio									15.80%		14.60%		14.70%
Combined ratio									86.80%		85.90%		95.40%
Goodwill and intangible assets	109,539				27,319				109,539		27,319		38,259
Investable assets	14,609,969				14,049,525				14,609,969		14,049,525		13,045,134
Total assets	20,527,027				19,566,094				20,527,027		19,566,094		17,816,762
Total liabilities	14,492,237				13,918,598				14,492,237		13,918,598		12,647,884
Underwriting segments | Insurance
Segment Reporting Information [Line Items]
Gross premiums written									3,008,669	[2]	2,712,509	[2]	2,593,959	[2]
Premiums ceded									-862,015		-763,713		-768,625
Net premiums written									2,146,654		1,948,796		1,825,334
Change in unearned premiums									-129,284		-72,782		-24,991
Net premiums earned									2,017,370		1,876,014		1,800,343
Other underwriting income									2,135		2,122		2,335
Losses and loss adjustment expenses									-1,260,953		-1,188,445		-1,283,841
Acquisition expenses, net									-316,308		-311,904		-298,983
Other operating expenses									-335,157		-315,387		-307,489
Underwriting income (loss)									107,087		62,400		-87,635
Underwriting Ratios
Loss ratio									62.50%		63.30%		71.30%
Acquisition expense ratio									15.70%		16.60%		16.60%
Other operating expense ratio									16.60%		16.80%		17.10%
Combined ratio									94.80%		96.70%		105.00%
Goodwill and intangible assets	28,331				20,419				28,331		20,419		21,357
Underwriting segments | Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written									1,527,245	[2]	1,399,621	[2]	1,213,889	[2]
Premiums ceded									-261,254		-86,620		-55,099
Net premiums written									1,265,991		1,313,001		1,158,790
Change in unearned premiums									13,337		-94,329		-40,663
Net premiums earned									1,279,328		1,218,672		1,118,127
Other underwriting income									3,167		5,258		5,755
Losses and loss adjustment expenses									-532,450		-486,236		-574,821
Acquisition expenses, net									-261,438		-234,373		-204,903
Other operating expenses									-147,964		-134,563		-118,245
Underwriting income (loss)									340,643		368,758		225,913
Underwriting Ratios
Loss ratio									41.60%		39.90%		51.40%
Acquisition expense ratio									20.40%		19.20%		18.30%
Other operating expense ratio									11.60%		11.00%		10.60%
Combined ratio									73.60%		70.10%		80.30%
Goodwill and intangible assets	3,333				6,900				3,333		6,900		16,902
Underwriting segments | Mortgage
Segment Reporting Information [Line Items]
Gross premiums written									227,356	[2]	89,570	[2]	68,111	[2]
Premiums ceded									-22,519		0		0
Net premiums written									204,837		89,570		68,111
Change in unearned premiums									-11,264		-38,304		-51,441
Net premiums earned									193,573		51,266		16,670
Other underwriting income									4,840		259		0
Losses and loss adjustment expenses									-55,674		-4,743		-2,615
Acquisition expenses, net									-49,400		-17,826		-4,998
Other operating expenses									-66,891		-8,377		-4,301
Underwriting income (loss)									26,448		20,579		4,756
Underwriting Ratios
Loss ratio									28.80%		9.30%		15.70%
Acquisition expense ratio									25.50%		34.80%		30.00%
Other operating expense ratio									34.60%		16.30%		25.80%
Combined ratio									88.90%		60.40%		71.50%
Goodwill and intangible assets	$77,875				$0				$77,875		$0		$0
[1]	Certain amounts included in the gross premiums written of each segment are related to intersegment transactions and are included in the gross premiums written of each segment. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.
[2]	Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.

Segment_Information_Summary_of

Segment Information - Summary of net premiums written and earned by major line of business and net premiums written by location (Details) (USD $)

3 Months Ended

12 Months Ended

In Thousands, unless otherwise specified

Dec. 31, 2014

Sep. 30, 2014

Jun. 30, 2014

Mar. 31, 2014

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Segment Reporting Information [Line Items]

Net premiums written

$895,481

$959,539

$971,928

$1,064,990

$748,921

$839,135

$810,535

$952,776

$3,891,938

$3,351,367

$3,052,235

Net premiums earned

923,165

903,651

907,152

859,780

839,366

795,000

758,816

752,770

3,593,748

3,145,952

2,935,140

Reinsurance

Segment Reporting Information [Line Items]

Net premiums written

1,265,991

1,313,001

1,158,790

Insurance

Segment Reporting Information [Line Items]

Net premiums written

2,146,654

1,948,796

1,825,334

Mortgage

Segment Reporting Information [Line Items]

Net premiums written

204,837

89,570

68,111

Other

Segment Reporting Information [Line Items]

Net premiums written

274,456

0

0

Net premiums earned

103,477

0

0

Other | Client Location [Member] | United States

Segment Reporting Information [Line Items]

Net premiums written

186,795

[1]

0

[1]

0

[1]

Other | Client Location [Member] | Europe

Segment Reporting Information [Line Items]

Net premiums written

54,131

[1]

0

[1]

0

[1]

Other | Client Location [Member] | Bermuda

Segment Reporting Information [Line Items]

Net premiums written

27,977

[1]

0

[1]

0

[1]

Other | Client Location [Member] | Other geographic location

Segment Reporting Information [Line Items]

Net premiums written

5,553

[1]

0

[1]

0

[1]

Other | Other specialty [Member]

Segment Reporting Information [Line Items]

Net premiums written

66,524

[1],[2]

0

[1],[2]

0

[1],[2]

Net premiums earned

24,157

[1],[2]

0

[1],[2]

0

[1],[2]

Other | Property excluding property catastrophe [Member]

Segment Reporting Information [Line Items]

Net premiums written

564

[1]

0

[1]

0

[1]

Net premiums earned

182

[1]

0

[1]

0

[1]

Other | Casualty [Member]

Segment Reporting Information [Line Items]

Net premiums written

179,054

[1],[3]

0

[1],[3]

0

[1],[3]

Net premiums earned

72,395

[1],[3]

0

[1],[3]

0

[1],[3]

Other | Property catastrophe [Member]

Segment Reporting Information [Line Items]

Net premiums written

9,280

[1]

0

[1]

0

[1]

Net premiums earned

5,115

[1]

0

[1]

0

[1]

Other | Marine and aviation [Member]

Segment Reporting Information [Line Items]

Net premiums written

1,251

[1]

0

[1]

0

[1]

Net premiums earned

789

[1]

0

[1]

0

[1]

Other | Other [Member]

Segment Reporting Information [Line Items]

Net premiums written

17,783

[1],[4]

0

[1],[4]

0

[1],[4]

Net premiums earned

839

[1],[4]

0

[1],[4]

0

[1],[4]

Underwriting segments

Segment Reporting Information [Line Items]

Net premiums written

3,617,482

3,351,367

3,052,235

Net premiums earned

3,490,271

3,145,952

2,935,140

Underwriting segments | Reinsurance

Segment Reporting Information [Line Items]

Net premiums written

1,265,991

1,313,001

1,158,790

Net premiums earned

1,279,328

1,218,672

1,118,127

Underwriting segments | Reinsurance | Pro Rata [Member]

Segment Reporting Information [Line Items]

Net premiums written

663,135

[5]

692,024

[5]

530,763

[5]

Net premiums earned

686,201

[5]

608,586

[5]

499,094

[5]

Underwriting segments | Reinsurance | Excess Of Loss [Member]

Segment Reporting Information [Line Items]

Net premiums written

602,856

[5]

620,977

[5]

628,027

[5]

Net premiums earned

593,127

[5]

610,086

[5]

619,033

[5]

Underwriting segments | Reinsurance | Client Location [Member] | United States

Segment Reporting Information [Line Items]

Net premiums written

589,255

[5]

706,388

[5]

568,043

[5]

Underwriting segments | Reinsurance | Client Location [Member] | Europe

Segment Reporting Information [Line Items]

Net premiums written

355,735

[5]

327,059

[5]

341,653

[5]

Underwriting segments | Reinsurance | Client Location [Member] | Asia and Pacific

Segment Reporting Information [Line Items]

Net premiums written

142,626

[5]

94,252

[5]

97,879

[5]

Underwriting segments | Reinsurance | Client Location [Member] | Bermuda

Segment Reporting Information [Line Items]

Net premiums written

77,620

[5]

87,047

[5]

72,864

[5]

Underwriting segments | Reinsurance | Client Location [Member] | Other geographic location

Segment Reporting Information [Line Items]

Net premiums written

100,755

[5]

98,255

[5]

78,351

[5]

Underwriting segments | Reinsurance | Underwriting Location [Member] | United States

Segment Reporting Information [Line Items]

Net premiums written

492,891

[5]

507,183

[5]

379,239

[5]

Underwriting segments | Reinsurance | Underwriting Location [Member] | Europe

Segment Reporting Information [Line Items]

Net premiums written

394,351

[5]

459,467

[5]

527,909

[5]

Underwriting segments | Reinsurance | Underwriting Location [Member] | Bermuda

Segment Reporting Information [Line Items]

Net premiums written

343,823

[5]

309,129

[5]

225,470

[5]

Underwriting segments | Reinsurance | Underwriting Location [Member] | Other geographic location

Segment Reporting Information [Line Items]

Net premiums written

34,926

[5]

37,222

[5]

26,172

[5]

Underwriting segments | Reinsurance | Other specialty [Member]

Segment Reporting Information [Line Items]

Net premiums written

405,126

[5],[6]

417,865

[5],[6]

308,104

[5],[6]

Net premiums earned

424,725

[5],[6]

387,630

[5],[6]

309,101

[5],[6]

Underwriting segments | Reinsurance | Property excluding property catastrophe [Member]

Segment Reporting Information [Line Items]

Net premiums written

343,043

[5],[7]

292,536

[5],[7]

265,783

[5],[7]

Net premiums earned

303,496

[5],[7]

274,719

[5],[7]

254,338

[5],[7]

Underwriting segments | Reinsurance | Casualty [Member]

Segment Reporting Information [Line Items]

Net premiums written

317,996

[5],[8]

306,304

[5],[8]

205,925

[5],[8]

Net premiums earned

327,518

[5],[8]

241,774

[5],[8]

188,963

[5],[8]

Underwriting segments | Reinsurance | Property catastrophe [Member]

Segment Reporting Information [Line Items]

Net premiums written

137,471

[5]

220,749

[5]

283,677

[5]

Net premiums earned

150,761

[5]

232,423

[5]

280,185

[5]

Underwriting segments | Reinsurance | Marine and aviation [Member]

Segment Reporting Information [Line Items]

Net premiums written

50,444

[5]

64,380

[5]

84,649

[5]

Net premiums earned

61,118

[5]

70,105

[5]

76,145

[5]

Underwriting segments | Reinsurance | Other [Member]

Segment Reporting Information [Line Items]

Net premiums written

11,911

[5],[9]

11,167

[5],[9]

10,652

[5],[9]

Net premiums earned

11,710

[5],[9]

12,021

[5],[9]

9,395

[5],[9]

Underwriting segments | Insurance

Segment Reporting Information [Line Items]

Net premiums written

2,146,654

1,948,796

1,825,334

Net premiums earned

2,017,370

1,876,014

1,800,343

Underwriting segments | Insurance | Client Location [Member] | United States

Segment Reporting Information [Line Items]

Net premiums written

1,726,181

[10]

1,526,156

[10]

1,314,577

[10]

Underwriting segments | Insurance | Client Location [Member] | Europe

Segment Reporting Information [Line Items]

Net premiums written

240,136

[10]

226,254

[10]

271,278

[10]

Underwriting segments | Insurance | Client Location [Member] | Asia and Pacific

Segment Reporting Information [Line Items]

Net premiums written

79,564

[10]

95,970

[10]

120,492

[10]

Underwriting segments | Insurance | Client Location [Member] | Other geographic location

Segment Reporting Information [Line Items]

Net premiums written

100,773

[10]

100,416

[10]

118,987

[10]

Underwriting segments | Insurance | Underwriting Location [Member] | United States

Segment Reporting Information [Line Items]

Net premiums written

1,688,887

[10]

1,478,930

[10]

1,254,623

[10]

Underwriting segments | Insurance | Underwriting Location [Member] | Europe

Segment Reporting Information [Line Items]

Net premiums written

394,430

[10]

389,763

[10]

472,132

[10]

Underwriting segments | Insurance | Underwriting Location [Member] | Other geographic location

Segment Reporting Information [Line Items]

Net premiums written

63,337

[10]

80,103

[10]

98,579

[10]

Underwriting segments | Insurance | Programs [Member]

Segment Reporting Information [Line Items]

Net premiums written

480,580

[10]

419,673

[10]

340,130

[10]

Net premiums earned

460,392

[10]

386,840

[10]

318,740

[10]

Underwriting segments | Insurance | Professional lines [Member]

Segment Reporting Information [Line Items]

Net premiums written

476,604

[10],[11]

476,193

[10],[11]

548,423

[10],[11]

Net premiums earned

456,508

[10],[11]

491,791

[10],[11]

536,971

[10],[11]

Underwriting segments | Insurance | Construction and national accounts [Member]

Segment Reporting Information [Line Items]

Net premiums written

286,994

[10]

271,110

[10]

211,130

[10]

Net premiums earned

277,811

[10]

250,729

[10]

209,217

[10]

Underwriting segments | Insurance | Property, energy, marine and aviation [Member]

Segment Reporting Information [Line Items]

Net premiums written

244,640

[10]

280,551

[10]

294,690

[10]

Net premiums earned

244,974

[10]

304,294

[10]

313,081

[10]

Underwriting segments | Insurance | Excess and surplus casualty [Member]

Segment Reporting Information [Line Items]

Net premiums written

212,519

[10],[12]

149,286

[10],[12]

112,307

[10],[12]

Net premiums earned

182,024

[10],[12]

118,395

[10],[12]

113,597

[10],[12]

Underwriting segments | Insurance | Travel, accident and health [Member]

Segment Reporting Information [Line Items]

Net premiums written

145,732

[10]

104,903

[10]

106,871

[10]

Net premiums earned

127,691

[10]

97,135

[10]

98,686

[10]

Underwriting segments | Insurance | Lenders products [Member]

Segment Reporting Information [Line Items]

Net premiums written

100,407

[10]

101,576

[10]

99,724

[10]

Net premiums earned

94,438

[10]

99,847

[10]

103,478

[10]

Underwriting segments | Insurance | Other [Member]

Segment Reporting Information [Line Items]

Net premiums written

199,178

[10],[13]

145,504

[10],[13]

112,059

[10],[13]

Net premiums earned

173,532

[10],[13]

126,983

[10],[13]

106,573

[10],[13]

Underwriting segments | Mortgage

Segment Reporting Information [Line Items]

Net premiums written

204,837

89,570

68,111

Net premiums earned

193,573

51,266

16,670

Underwriting segments | Mortgage | Client Location [Member] | United States

Segment Reporting Information [Line Items]

Net premiums written

184,333

63,692

61,571

Underwriting segments | Mortgage | Client Location [Member] | Other geographic location

Segment Reporting Information [Line Items]

Net premiums written

20,504

25,878

6,540

Underwriting segments | Mortgage | Underwriting Location [Member] | United States

Segment Reporting Information [Line Items]

Net premiums written

98,809

0

0

Underwriting segments | Mortgage | Underwriting Location [Member] | Other geographic location

Segment Reporting Information [Line Items]

Net premiums written

$106,028

$89,570

$68,111

[1]

Other segment results include premiums written and earned assumed through intersegment transactions and exclude premiums written and earned ceded through intersegment transactions.

[2]

Includes non-excess motor and other.

[3]

Includes professional liability, excess motor, programs and other.

[4]

Includes mortgage and other.

[5]

Reinsurance segment results include premiums written and earned assumed through intersegment transactions and exclude premiums written and earned ceded through intersegment transactions.

[6]

Includes non-excess motor, surety, accident and health, workers’ compensation catastrophe, agriculture, trade credit and other.

[7]

Includes facultative business.

[8]

Includes executive assurance, professional liability, workers’ compensation, excess motor, healthcare and other.

[9]

Includes life, casualty clash and other.

[10]

Insurance segment results include premiums written and earned assumed through intersegment transactions and exclude premiums written and earned ceded through intersegment transactions.

[11]

Includes professional liability, executive assurance and healthcare business.

[12]

Includes casualty and contract binding business.

[13]

Includes alternative markets, excess workers' compensation and surety business.

Reserve_for_Losses_and_Loss_Ad2

Reserve for Losses and Loss Adjustment Expenses - Reconciliation of beginning and ending balances of losses and loss adjustment reserves (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Reserve for losses and loss adjustment expenses at beginning of year	$8,824,696		$8,933,292		$8,456,210
Unpaid losses and loss adjustment expenses recoverable	1,748,250		1,829,070		1,818,047
Net reserve for losses and loss adjustment expenses at beginning of year	7,076,446		7,104,222		6,638,163
Net incurred losses and loss adjustment expenses relating to losses occurring in:
Current year	2,246,152		1,943,466		2,082,805
Prior years	-326,902		-264,042		-221,528
Total net incurred losses and loss adjustment expenses	1,919,250		1,679,424		1,861,277
Net losses and loss adjustment expense reserves of acquired business (1)	120,671	[1]	0	[1]	31,977	[1]
Foreign exchange (gains) losses	-160,486		1,617		38,184
Net paid losses and loss adjustment expenses relating to losses occurring in:
Current year	-347,270		-288,114		-295,984
Prior years	-1,350,466		-1,420,703		-1,169,395
Total net paid losses and loss adjustment expenses	-1,697,736		-1,708,817		-1,465,379
Net reserve for losses and loss adjustment expenses at end of year	7,258,145		7,076,446		7,104,222
Unpaid losses and loss adjustment expenses recoverable	1,778,303		1,748,250		1,829,070
Reserve for losses and loss adjustment expenses at end of year	$9,036,448		$8,824,696		$8,933,292
[1]	2014 amount relates to the Company’s acquisition of Arch MI U.S. while the 2012 amount relates to the Company’s acquisition of the trade credit and surety operations of Ariel Reinsurance Company.

Reserve_for_Losses_and_Loss_Ad3

Reserve for Losses and Loss Adjustment Expenses - Prior years reserve development (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	($326,902)	($264,042)	($221,528)
Underwriting segments | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	-267,300	-217,900	-190,300
Prior year reserve development (points)	20.90%	17.90%	17.00%
Underwriting segments | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	-58,700	-45,100	-31,200
Prior year reserve development (points)	2.90%	2.40%	1.70%
Underwriting segments | Mortgage
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	-900	-1,000
Underwriting segments | Short tailed lines [Member] | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	-146,700	-111,100	-117,600
Underwriting segments | Short tailed lines [Member] | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	-73,500	-67,000	-79,300
Underwriting segments | Medium tailed and long tailed lines [Member] | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	-120,600	-106,800	-72,700
Underwriting segments | Medium tailed and long tailed lines [Member] | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	14,800	21,900	48,100
Underwriting segments | Casualty [Member] | Medium tailed and long tailed lines [Member] | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	-101,600
Underwriting segments | Property [Member] | Short tailed lines [Member] | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	-60,500
Underwriting segments | Property [Member] | Short tailed lines [Member] | Insurance | Named catastrophic events [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	-7,900
Underwriting segments | Specialty Casualty [Member] | Medium tailed and long tailed lines [Member] | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development		28,000
Underwriting segments | Programs [Member] | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development		26,200
Underwriting segments | Professional liability [Member] | Medium tailed and long tailed lines [Member] | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development			19,800
Underwriting segments | 2011-2013 | Property catastrophe and property other than property catastrophe [Member] | Short tailed lines [Member] | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	-107,600
Underwriting segments | 2011-2013 | Surety [Member] | Medium tailed and long tailed lines [Member] | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	7,800
Underwriting segments | 2005-2013 | Property catastrophe and property other than property catastrophe [Member] | Short tailed lines [Member] | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	-23,300
Underwriting segments | 2008-2012 | Marine and aviation [Member] | Medium tailed and long tailed lines [Member] | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	-14,700
Underwriting segments | 2009-2013 | Construction [Member] | Medium tailed and long tailed lines [Member] | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	41,300
Underwriting segments | 2011 and 2012 | Programs [Member] | Medium tailed and long tailed lines [Member] | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	26,200
Underwriting segments | 2003-2011 | Professional liability [Member] | Medium tailed and long tailed lines [Member] | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development	39,000
Underwriting segments | 2009-2012 | Property catastrophe and property other than property catastrophe [Member] | Short tailed lines [Member] | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development		-110,100
Underwriting segments | 2005-2012 | Property catastrophe [Member] | Short tailed lines [Member] | Reinsurance | Named catastrophic events [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development		-28,600
Underwriting segments | 2005-2012 | Property [Member] | Short tailed lines [Member] | Insurance | Named catastrophic events [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development		-15,900
Underwriting segments | 2003-2009 | Casualty [Member] | Medium tailed and long tailed lines [Member] | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development		-98,800	-55,900
Underwriting segments | 2007 and 2009 | Marine and aviation [Member] | Medium tailed and long tailed lines [Member] | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development		-8,100
Underwriting segments | 2008-2011 | Property catastrophe and property other than property catastrophe [Member] | Short tailed lines [Member] | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development			-92,100
Underwriting segments | 2008-2011 | Property [Member] | Short tailed lines [Member] | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development		-62,400
Underwriting segments | 2008-2010 | Marine and aviation [Member] | Medium tailed and long tailed lines [Member] | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development			-16,800
Underwriting segments | 2007-2011 | Property [Member] | Short tailed lines [Member] | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development			-69,000
Underwriting segments | 2005-2011 | Property catastrophe [Member] | Short tailed lines [Member] | Reinsurance | Named catastrophic events [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development			-16,800
Underwriting segments | 2005-2011 | Property [Member] | Short tailed lines [Member] | Insurance | Named catastrophic events [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development			-19,100
Underwriting segments | 2003-2005 | Specialty Casualty [Member] | Medium tailed and long tailed lines [Member] | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development			23,000
Underwriting segments | 2011 | Programs [Member] | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development, percentage attributable to a singe cause		0.9
Underwriting segments | United Kingdom | 2007-2010 | Medium tailed and long tailed lines [Member] | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year reserve development			$38,900

Reinsurance_Effects_of_reinsur

Reinsurance - Effects of reinsurance (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Premiums Written
Direct									$3,100,946	$2,754,582	$2,673,864
Assumed									1,739,670	1,442,041	1,195,297
Ceded									-948,678	-845,256	-816,926
Net	895,481	959,539	971,928	1,064,990	748,921	839,135	810,535	952,776	3,891,938	3,351,367	3,052,235
Premiums Earned
Direct									2,914,755	2,666,104	2,572,078
Assumed									1,574,049	1,296,048	1,165,371
Ceded									-895,056	-816,200	-802,309
Net premiums earned	923,165	903,651	907,152	859,780	839,366	795,000	758,816	752,770	3,593,748	3,145,952	2,935,140
Losses and Loss Adjustment Expenses
Direct									1,763,492	1,603,369	1,725,707
Assumed									621,346	450,618	578,081
Ceded									-465,588	-374,563	-442,511
Net									$1,919,250	$1,679,424	$1,861,277

Reinsurance_Ceded_credit_risk_

Reinsurance - Ceded credit risk (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Concentration Risk [Line Items]
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	1,812,845	1,804,330
Reinsurer concentration risk | Stockholders' equity
Concentration Risk [Line Items]
Concentration risk percentage	3.70%	4.20%
AM Best Aminus Or Better Rating
Concentration Risk [Line Items]
Percentage of reinsurance recoverable	83.00%	86.50%

Investment_Information_Summary

Investment Information - Summary of Available for Sale Securities (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Available for sale securities:
Estimated Fair Value	$12,553,204		$12,150,358
Gross Unrealized Gains	282,262		232,673
Gross Unrealized Losses	-111,730		-152,368
Cost or Amortized Cost	12,382,672		12,070,053
OTTI Unrealized Losses	-3,646		-12,942
Fixed maturities and fixed maturities pledged under securities lending agreements
Available for sale securities:
Estimated Fair Value	10,801,572		9,676,857
Gross Unrealized Gains	140,673	[1]	133,450	[1]
Gross Unrealized Losses	-92,734	[1]	-127,100	[1]
Cost or Amortized Cost	10,753,633		9,670,507
OTTI Unrealized Losses	-3,646	[2]	-12,942	[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Available for sale securities:
Estimated Fair Value	3,108,513		2,267,263
Gross Unrealized Gains	37,928	[1]	35,289	[1]
Gross Unrealized Losses	-38,974	[1]	-35,537	[1]
Cost or Amortized Cost	3,109,559		2,267,511
OTTI Unrealized Losses	-317	[2]	-16	[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Available for sale securities:
Estimated Fair Value	943,343		1,133,095
Gross Unrealized Gains	18,843	[1]	16,270	[1]
Gross Unrealized Losses	-3,842	[1]	-22,209	[1]
Cost or Amortized Cost	928,342		1,139,034
OTTI Unrealized Losses	-3,307	[2]	-9,269	[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds
Available for sale securities:
Estimated Fair Value	1,494,122		1,481,738
Gross Unrealized Gains	31,227	[1]	29,378	[1]
Gross Unrealized Losses	-1,044	[1]	-9,730	[1]
Cost or Amortized Cost	1,463,939		1,462,090
OTTI Unrealized Losses	0	[2]	-17	[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Available for sale securities:
Estimated Fair Value	1,114,528		1,074,497
Gross Unrealized Gains	14,594	[1]	13,972	[1]
Gross Unrealized Losses	-3,822	[1]	-15,224	[1]
Cost or Amortized Cost	1,103,756		1,075,749
OTTI Unrealized Losses	0	[2]	-199	[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies
Available for sale securities:
Estimated Fair Value	1,447,972		1,301,809
Gross Unrealized Gains	8,345	[1]	3,779	[1]
Gross Unrealized Losses	-1,760	[1]	-11,242	[1]
Cost or Amortized Cost	1,441,387		1,309,272
OTTI Unrealized Losses	0	[2]	-19	[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities
Available for sale securities:
Estimated Fair Value	1,015,153		1,085,861
Gross Unrealized Gains	21,311	[1]	14,729	[1]
Gross Unrealized Losses	-37,203	[1]	-19,363	[1]
Cost or Amortized Cost	1,031,045		1,090,495
OTTI Unrealized Losses	0	[2]	0	[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Available for sale securities:
Estimated Fair Value	1,677,941		1,332,594
Gross Unrealized Gains	8,425	[1]	20,033	[1]
Gross Unrealized Losses	-6,089	[1]	-13,795	[1]
Cost or Amortized Cost	1,675,605		1,326,356
OTTI Unrealized Losses	-22	[2]	-3,422	[2]
Equity securities
Available for sale securities:
Estimated Fair Value	658,182		496,824
Gross Unrealized Gains	109,012	[1]	69,487	[1]
Gross Unrealized Losses	-13,364	[1]	-5,938	[1]
Cost or Amortized Cost	562,534		433,275
OTTI Unrealized Losses	0		0
Other investments
Available for sale securities:
Estimated Fair Value	296,224		498,310
Gross Unrealized Gains	31,839	[1]	28,082	[1]
Gross Unrealized Losses	-362	[1]	-18,459	[1]
Cost or Amortized Cost	264,747		488,687
OTTI Unrealized Losses	0		0
Short-term investments
Available for sale securities:
Estimated Fair Value	797,226		1,478,367
Gross Unrealized Gains	738	[1]	1,654	[1]
Gross Unrealized Losses	-5,270	[1]	-871	[1]
Cost or Amortized Cost	801,758		1,477,584
OTTI Unrealized Losses	$0		$0
[1]	In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged. For purposes of this table, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged. See “—Securities Lending Agreements.”
[2]	Represents the total other-than-temporary impairments (“OTTI”) recognized in accumulated other comprehensive income (“AOCI”). It does not include the change in fair value subsequent to the impairment measurement date. At December 31, 2014, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $0.9 million, compared to a net unrealized gain of $6.0 million at December 31, 2013.

Investment_Information_Aging_o

Investment Information - Aging of Available For Sale Securities in an Unrealized Loss Position (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Thousands, unless otherwise specified
Available for sale securities:
Estimated Fair Value - Less than 12 Months	$4,108,030	[1]	$6,158,245	[1]
Gross Unrealized Losses - Less than 12 Months	-90,017	[1]	-137,655	[1]
Estimated Fair Value - 12 Months or More	641,407	[1]	258,290	[1]
Gross Unrealized Losses - 12 Months or More	-21,713	[1]	-14,713	[1]
Estimated Fair Value - Total	4,749,437	[1]	6,416,535	[1]
Gross Unrealized Losses - Total	-111,730	[1]	-152,368	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements
Available for sale securities:
Estimated Fair Value - Less than 12 Months	3,788,119	[1]	5,887,621	[1]
Gross Unrealized Losses - Less than 12 Months	-71,021	[1]	-115,071	[1]
Estimated Fair Value - 12 Months or More	641,407	[1]	210,974	[1]
Gross Unrealized Losses - 12 Months or More	-21,713	[1]	-12,029	[1]
Estimated Fair Value - Total	4,429,526	[1]	6,098,595	[1]
Gross Unrealized Losses - Total	-92,734	[1]	-127,100	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Available for sale securities:
Estimated Fair Value - Less than 12 Months	1,309,637	[1]	1,183,625	[1]
Gross Unrealized Losses - Less than 12 Months	-32,903	[1]	-32,837	[1]
Estimated Fair Value - 12 Months or More	148,963	[1]	46,673	[1]
Gross Unrealized Losses - 12 Months or More	-6,071	[1]	-2,700	[1]
Estimated Fair Value - Total	1,458,600	[1]	1,230,298	[1]
Gross Unrealized Losses - Total	-38,974	[1]	-35,537	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	293,624	[1]	778,693	[1]
Gross Unrealized Losses - Less than 12 Months	-1,476	[1]	-20,253	[1]
Estimated Fair Value - 12 Months or More	59,107	[1]	43,634	[1]
Gross Unrealized Losses - 12 Months or More	-2,366	[1]	-1,956	[1]
Estimated Fair Value - Total	352,731	[1]	822,327	[1]
Gross Unrealized Losses - Total	-3,842	[1]	-22,209	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds
Available for sale securities:
Estimated Fair Value - Less than 12 Months	210,614	[1]	589,009	[1]
Gross Unrealized Losses - Less than 12 Months	-588	[1]	-9,422	[1]
Estimated Fair Value - 12 Months or More	13,643	[1]	6,092	[1]
Gross Unrealized Losses - 12 Months or More	-456	[1]	-308	[1]
Estimated Fair Value - Total	224,257	[1]	595,101	[1]
Gross Unrealized Losses - Total	-1,044	[1]	-9,730	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	232,147	[1]	677,617	[1]
Gross Unrealized Losses - Less than 12 Months	-770	[1]	-15,110	[1]
Estimated Fair Value - 12 Months or More	125,894	[1]	1,612	[1]
Gross Unrealized Losses - 12 Months or More	-3,052	[1]	-114	[1]
Estimated Fair Value - Total	358,041	[1]	679,229	[1]
Gross Unrealized Losses - Total	-3,822	[1]	-15,224	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies
Available for sale securities:
Estimated Fair Value - Less than 12 Months	618,381	[1]	1,144,809	[1]
Gross Unrealized Losses - Less than 12 Months	-1,626	[1]	-11,242	[1]
Estimated Fair Value - 12 Months or More	3,438	[1]	0	[1]
Gross Unrealized Losses - 12 Months or More	-134	[1]	0	[1]
Estimated Fair Value - Total	621,819	[1]	1,144,809	[1]
Gross Unrealized Losses - Total	-1,760	[1]	-11,242	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	510,766	[1]	821,506	[1]
Gross Unrealized Losses - Less than 12 Months	-31,172	[1]	-15,776	[1]
Estimated Fair Value - 12 Months or More	46,910	[1]	24,334	[1]
Gross Unrealized Losses - 12 Months or More	-6,031	[1]	-3,587	[1]
Estimated Fair Value - Total	557,676	[1]	845,840	[1]
Gross Unrealized Losses - Total	-37,203	[1]	-19,363	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	612,950	[1]	692,362	[1]
Gross Unrealized Losses - Less than 12 Months	-2,486	[1]	-10,431	[1]
Estimated Fair Value - 12 Months or More	243,452	[1]	88,629	[1]
Gross Unrealized Losses - 12 Months or More	-3,603	[1]	-3,364	[1]
Estimated Fair Value - Total	856,402	[1]	780,991	[1]
Gross Unrealized Losses - Total	-6,089	[1]	-13,795	[1]
Equity securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	181,002	[1]	76,563	[1]
Gross Unrealized Losses - Less than 12 Months	-13,364	[1]	-5,938	[1]
Estimated Fair Value - 12 Months or More	0	[1]	0	[1]
Gross Unrealized Losses - 12 Months or More	0	[1]	0	[1]
Estimated Fair Value - Total	181,002	[1]	76,563	[1]
Gross Unrealized Losses - Total	-13,364	[1]	-5,938	[1]
Other investments
Available for sale securities:
Estimated Fair Value - Less than 12 Months	59,638	[1]	165,891	[1]
Gross Unrealized Losses - Less than 12 Months	-362	[1]	-15,775	[1]
Estimated Fair Value - 12 Months or More	0	[1]	47,316	[1]
Gross Unrealized Losses - 12 Months or More	0	[1]	-2,684	[1]
Estimated Fair Value - Total	59,638	[1]	213,207	[1]
Gross Unrealized Losses - Total	-362	[1]	-18,459	[1]
Short-term investments
Available for sale securities:
Estimated Fair Value - Less than 12 Months	79,271	[1]	28,170	[1]
Gross Unrealized Losses - Less than 12 Months	-5,270	[1]	-871	[1]
Estimated Fair Value - 12 Months or More	0	[1]	0	[1]
Gross Unrealized Losses - 12 Months or More	0	[1]	0	[1]
Estimated Fair Value - Total	79,271	[1]	28,170	[1]
Gross Unrealized Losses - Total	($5,270)	[1]	($871)	[1]
[1]	In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged. For purposes of this table, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged. See “—Securities Lending Agreements.”

Investment_Information_Maturit

Investment Information - Maturity Profile of Available For Sale Securities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Estimated Fair Value:
Estimated fair value	$10,750,770	$9,571,776
Amortized Cost:
Amortized cost	10,701,557	9,564,634
Fixed maturities and fixed maturities pledged under securities lending agreements
Estimated Fair Value:
Due in one year or less	235,930	235,330
Due after one year through five years	4,074,562	3,738,500
Due after five years through 10 years	2,475,726	1,966,536
Due after 10 years	279,542	196,305
Single maturity date	7,065,760	6,136,671
Estimated fair value	10,801,572	9,676,857
Amortized Cost:
Due in one year or less	233,794	232,652
Due after one year through five years	4,077,408	3,718,920
Due after five years through 10 years	2,461,356	1,979,510
Due after 10 years	273,372	198,286
Single maturity date	7,045,930	6,129,368
Amortized cost	10,753,633	9,670,507
Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Estimated Fair Value:
Securities without single maturity date	943,343	1,133,095
Amortized Cost:
Securities without single maturity date	928,342	1,139,034
Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Estimated Fair Value:
Securities without single maturity date	1,114,528	1,074,497
Amortized Cost:
Securities without single maturity date	1,103,756	1,075,749
Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Estimated Fair Value:
Securities without single maturity date	1,677,941	1,332,594
Amortized Cost:
Securities without single maturity date	$1,675,605	$1,326,356

Investment_Information_Other_I

Investment Information - Other Investments (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Thousands, unless otherwise specified
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	$1,848,131		$1,271,590
Available for sale
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	296,224		498,310
Available for sale | Asian and emerging markets
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	236,586		331,984
Available for sale | Investment grade fixed income
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	59,638		159,115
Available for sale | Other
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	0		7,211
Fair value option
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	1,551,907		773,280
Fair value option | Asian and emerging markets
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	25,800		14,054
Fair value option | Investment grade fixed income
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	69,108		75,062
Fair value option | Other
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	147,573	[1]	119,130	[1]
Fair value option | Term loan investments
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	1,073,649		399,591
Other investments par	1,076,011		360,204
Fair value option | Mezzanine Debt Funds [Member]
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	121,341		102,172
Fair value option | Credit Related Funds [Member]
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	$114,436		$63,271
[1]	Includes fund investments with strategies in mortgage servicing rights, transportation and infrastructure assets and other.

Investment_Information_Fair_Va

Investment Information - Fair Value Option (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	$2,435,532	$1,221,534
Fixed Maturities
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	632,024	448,254
Other investments
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	1,551,907	773,280
Short-term investments
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	$251,601	$0

Investment_Information_Net_Inv

Investment Information - Net Investment Income (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Net investment income:
Gross investment income									$348,921		$300,329		$320,562
Investment expenses									-46,336		-33,110		-25,667
Net investment income	82,496	80,105	72,990	66,994	67,095	66,083	68,369	65,672	302,585		267,219		294,895
Fixed Maturities
Net investment income:
Gross investment income									265,219		249,833		281,140
Term loan investments
Net investment income:
Gross investment income									39,940		20,608		15,283
Short-term investments
Net investment income:
Gross investment income									1,888		1,259		1,980
Equity securities
Net investment income:
Gross investment income									13,005		8,919		7,963
Other
Net investment income:
Gross investment income									$28,869	[1]	$19,710	[1]	$14,196	[1]
[1]	Includes dividends on investment funds and other items.

Investment_Information_Net_Rea

Investment Information - Net Realized Gains and Losses (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Net realized gains and losses:
Available for sale securities, gross gains on investment sales									$248,031		$239,421		$247,291
Available for sale securities, gross losses on investment sales									-145,808		-203,077		-71,722
Change in fair value of assets and liabilities accounted for using the fair value option - derivatives									43,934	[1]	20,912	[1]	-1,326	[1]
Change in fair value of assets and liabilities accounted for using the fair value option - other investments									-24,774	[2]	-1,499	[2]	-520	[2]
Net realized gains	10,561	18,515	54,144	19,697	9,048	-6,022	12,652	58,340	102,917		74,018		194,228
Fixed Maturities
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option									-20,743		54		13,195
Equity securities
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option									0		704		-73
Other investments
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option									2,672		17,503		7,383
Short-term investments
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option									($395)		$0		$0
[1]	See Note 11 for information on the Company’s derivative instruments.
[2]	Includes accretion of contingent consideration liability amounts related to the acquisition of the CMG Entities (see Note 2 for further details).

Investment_Information_Other_T

Investment Information - Other Than Temporary Impairments Recognized in Earnings (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	($3,837)	($8,593)	($14,749)	($2,971)	($88)	($728)	($724)	($2,246)	($30,150)	($3,786)	($11,388)
Fixed Maturities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings									-3,634	-511	-4,612
Fixed Maturities | Mortgage backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings									-1,525	-295	-2,491
Fixed Maturities | Corporate bonds
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings									-2,062	-88	-1,512
Fixed Maturities | Non-US government securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings									0	0	-261
Fixed Maturities | Commercial mortgage backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings									-7	0	-211
Fixed Maturities | Asset backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings									-40	-128	-127
Fixed Maturities | US government and government agencies
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings									0	0	-10
Other investments
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings									-25,410	0	0
Investment of funds received under securities lending arrangements
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings									0	0	-87
Equity securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings									($1,106)	($3,275)	($6,689)

Investment_Information_Other_T1

Investment Information - Other Than Temporary Impairments Rollforward (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Rollforward:
Balance at start of year	$60,062	$62,001	$66,545
Credit loss impairments recognized on securities not previously impaired	691	423	1,962
Credit loss impairments recognized on securities previously impaired	162	88	2,735
Reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security	0	0	0
Reductions for securities sold during the period	-40,719	-2,450	-9,241
Balance at end of year	$20,196	$60,062	$62,001

Investment_Information_Restric

Investment Information - Restricted Assets (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Restricted Assets [Line Items]
Restricted assets	$5,519,089	$5,302,042
Collateral or guarantees - affiliated transactions
Restricted Assets [Line Items]
Restricted assets	4,138,527	4,060,533
Collateral or guarantees - third party agreements
Restricted Assets [Line Items]
Restricted assets	970,120	856,890
Deposits with US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	337,981	302,809
Deposits with non-US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	0	6,546
Trust funds
Restricted Assets [Line Items]
Restricted assets	$72,461	$75,264

Investment_Information_Narrati

Investment Information - Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	lots	lots
Segment Reporting Information [Line Items]
Investable assets	$15,773,209,000	$14,049,525,000	$13,045,134,000
OTTI unrealized losses at current fair value	900,000	6,000,000
Continuous unrealized loss, qualitative disclosures:
Number of positions in an unrealized loss position (lots)	1,900	2,080
Total number of positions (lots)	4,790	4,400
Largest single loss	2,900,000	3,500,000
OTTI Unrealized Losses	3,646,000	12,942,000
Securities lending, qualitative disclosures:
Fair value of fixed maturities and short-term investments pledged under securities lending agreements	50,800,000	105,100,000
Amortized cost of fixed maturities and short-term investments pledged under securities lending agreements	52,100,000	105,900,000
Approximate fair value of sub-prime securities backing the securities lending program	5,800,000	6,300,000
Equity Method Investment, Financial Statement, Reported Amounts [Abstract]
Equity in net income (loss) of investment funds accounted for using the equity method	19,883,000	35,701,000	73,510,000
Equity method investments, time lag for reporting - low end of range	1 month
Equity method investments, time lag for reporting - high end of range	3 months
Investments accounted for using the equity method	349,014,000	244,339,000
ACGL
Segment Reporting Information [Line Items]
Investable assets	14,610,000,000
Watford Re
Segment Reporting Information [Line Items]
Investable assets	$1,160,000,000

Fair_Value_Fair_Value_Hierarch

Fair Value - Fair Value Hierarchy (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Thousands, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	($61,845)
Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	-61,845
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	0
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	0
Significant Other Observable Inputs (Level 2) | Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	0
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	-61,845
Significant Unobservable Inputs (Level 3) | Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	-61,845
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	3,116,355		3,226,291
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,447,972		1,301,809
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,447,972	[1]	1,301,809	[1]
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	658,182		496,738
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	759,621		1,427,744
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	250,580		0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Other investments fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	250,580
Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	11,244,994		9,595,611
Recurring | Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	9,296,100		8,373,003
Recurring | Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	3,108,513	[1]	2,265,218	[1]
Recurring | Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	943,343	[1]	1,133,095	[1]
Recurring | Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,494,122	[1]	1,481,738	[1]
Recurring | Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,114,528	[1]	1,074,497	[1]
Recurring | Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Recurring | Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,015,153	[1]	1,085,861	[1]
Recurring | Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,620,441	[1]	1,332,594	[1]
Recurring | Significant Other Observable Inputs (Level 2) | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		86
Recurring | Significant Other Observable Inputs (Level 2) | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	192,687		327,890
Recurring | Significant Other Observable Inputs (Level 2) | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	37,605		50,623
Recurring | Significant Other Observable Inputs (Level 2) | Fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,718,602		844,009
Recurring | Significant Other Observable Inputs (Level 2) | Fair value option | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	497,101		334,065
Recurring | Significant Other Observable Inputs (Level 2) | Fair value option | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	22,190		41,033
Recurring | Significant Other Observable Inputs (Level 2) | Fair value option | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	88,411		73,156
Recurring | Significant Other Observable Inputs (Level 2) | Fair value option | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	24,322
Recurring | Significant Other Observable Inputs (Level 2) | Fair value option | Other investments fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,085,557		395,755
Recurring | Significant Other Observable Inputs (Level 2) | Fair value option | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,021
Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	627,387		549,990
Recurring | Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	57,500		2,045
Recurring | Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	2,045	[1]
Recurring | Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Recurring | Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Recurring | Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Recurring | Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Recurring | Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Recurring | Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	57,500	[1]	0	[1]
Recurring | Significant Unobservable Inputs (Level 3) | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Recurring | Significant Unobservable Inputs (Level 3) | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	103,537		170,420
Recurring | Significant Unobservable Inputs (Level 3) | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Recurring | Significant Unobservable Inputs (Level 3) | Fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	466,350		377,525
Recurring | Significant Unobservable Inputs (Level 3) | Fair value option | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Recurring | Significant Unobservable Inputs (Level 3) | Fair value option | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Recurring | Significant Unobservable Inputs (Level 3) | Fair value option | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Recurring | Significant Unobservable Inputs (Level 3) | Fair value option | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0
Recurring | Significant Unobservable Inputs (Level 3) | Fair value option | Other investments fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	466,350		377,525
Recurring | Significant Unobservable Inputs (Level 3) | Fair value option | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0
Recurring | Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	14,988,736		13,371,892
Recurring | Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	10,801,572		9,676,857
Recurring | Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	3,108,513		2,267,263
Recurring | Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	943,343		1,133,095
Recurring | Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,494,122		1,481,738
Recurring | Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,114,528		1,074,497
Recurring | Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,447,972		1,301,809
Recurring | Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,015,153		1,085,861
Recurring | Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,677,941		1,332,594
Recurring | Estimated Fair Value | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	658,182		496,824
Recurring | Estimated Fair Value | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	296,224		498,310
Recurring | Estimated Fair Value | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	797,226		1,478,367
Recurring | Estimated Fair Value | Fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	2,435,532		1,221,534
Recurring | Estimated Fair Value | Fair value option | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	497,101		334,065
Recurring | Estimated Fair Value | Fair value option | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	22,190		41,033
Recurring | Estimated Fair Value | Fair value option | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	88,411		73,156
Recurring | Estimated Fair Value | Fair value option | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	24,322
Recurring | Estimated Fair Value | Fair value option | Other investments fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,551,907		773,280
Recurring | Estimated Fair Value | Fair value option | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	$251,601
[1]	In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged. For purposes of this table, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged.

Fair_Value_Rollforward_of_Leve

Fair Value - Rollforward of Level 3 assets and liabilities (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Level 3 assets rollforward
Balance at beginning of year	$549,990		$477,956
Total gains or (losses) (realized/unrealized) - included in earnings	16,779	[1]	25,337	[1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	252		-2,578
Purchases, issuances, sales and settlements
Purchases	185,224		300,067
Issuances	0		0
Sales	-68,209		-116,811
Settlements	-114,149		-133,981
Transfers in and/or out of Level 3	57,500		0
Balance at end of year	627,387		549,990
Contingent consideration liability
Level 3 liabilities rollforward
Balance at beginning of year	0		0
Total gains or (losses) (realized/unrealized) - included in earnings	-20,083	[1]	0	[1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	0		0
Purchases, issuances, sales and settlements
Purchases	0		0
Issuances	-41,762		0
Sales	0		0
Settlements	0		0
Transfers in and/or out of Level 3	0		0
Balance at end of year	-61,845		0
Available for sale | Asset backed securities
Level 3 assets rollforward
Balance at beginning of year	0		0
Total gains or (losses) (realized/unrealized) - included in earnings	0	[1]	0	[1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	0		0
Purchases, issuances, sales and settlements
Purchases	0		0
Issuances	0		0
Sales	0		0
Settlements	0		0
Transfers in and/or out of Level 3	57,500		0
Balance at end of year	57,500		0
Available for sale | Corporate bonds
Level 3 assets rollforward
Balance at beginning of year	2,045		98,404
Total gains or (losses) (realized/unrealized) - included in earnings	0	[1]	4,679	[1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	0		-3,051
Purchases, issuances, sales and settlements
Purchases	0		0
Issuances	0		0
Sales	-2,045		-96,655
Settlements	0		-1,332
Transfers in and/or out of Level 3	0		0
Balance at end of year	0		2,045
Available for sale | Other investments
Level 3 assets rollforward
Balance at beginning of year	170,420		184,202
Total gains or (losses) (realized/unrealized) - included in earnings	-39	[1]	8,915	[1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	-680		473
Purchases, issuances, sales and settlements
Purchases	0		25,000
Issuances	0		0
Sales	-66,164		0
Settlements	0		-48,170
Transfers in and/or out of Level 3	0		0
Balance at end of year	103,537		170,420
Fair value option | Other investments
Level 3 assets rollforward
Balance at beginning of year	377,525		195,350
Total gains or (losses) (realized/unrealized) - included in earnings	16,818	[1]	11,743	[1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	932		0
Purchases, issuances, sales and settlements
Purchases	185,224		275,067
Issuances	0		0
Sales	0		-20,156
Settlements	-114,149		-84,479
Transfers in and/or out of Level 3	0		0
Balance at end of year	$466,350		$377,525
[1]	Gains or losses on corporate bonds were included in net realized gains (losses) while gains or losses on other investments were included in net realized gains (losses) or net investment income. Amounts related to the contingent consideration liability were included in net realized gains (losses).

Fair_Value_Narrative_Details

Fair Value - Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Fair Value Disclosures [Abstract]
Total assets and liabilities measured at fair value	$15,100,000,000	$13,400,000,000
Total assets and liabilities measured at fair value priced using non-binding broker quotes	260,800,000	601,900,000
Total assets and liabilities measured at fair value priced using non-binding broker quotes (percentage)	1.70%	4.50%
Transfers in and/or out of Level 3	57,500,000	0
Gains (losses) including in earnings attributable to the change in unrealized gains or losses related to assets still held at balance sheet date	$11,500,000	$22,000,000

Investment_in_Joint_Venture_De

Investment in Joint Venture (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	31-May-08	Oct. 01, 2014
Schedule of Equity Method Investments [Line Items]
Investment in joint venture, at cost	$100,000,000	$100,000,000
Income (loss) from joint venture investme	19,883,000	35,701,000	73,510,000
Gulf Re
Schedule of Equity Method Investments [Line Items]
Investment in joint venture, at cost				100,000,000
Joint venture, ownership percentage				50.00%
Initial capital amount				200,000,000
Income (loss) from joint venture investme	14,100,000	400,000	1,100,000
Quota share agreement, percentage	90.00%
Gulf Re | Unearned premium reserves
Schedule of Equity Method Investments [Line Items]
Portfolio transfer amount					50,000,000
Gulf Re | Reserves for losses and loss expenses
Schedule of Equity Method Investments [Line Items]
Portfolio transfer amount					$65,000,000

Derivative_Instruments_Details

Derivative Instruments (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Derivative offsetting [Abstract]
Derivative assets subject to master netting agreements	$25,300,000		$28,000,000
Derivative liabilities subject to master netting agreements	5,400,000		14,600,000
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	43,934,000	[1]	20,912,000	[1]	-1,326,000	[1]
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Asset derivatives - fair value	26,468,000		39,859,000
Liability derivatives - fair value	-5,397,000		-26,472,000
Net derivatives - fair value	21,071,000		13,387,000
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	43,934,000		20,912,000		-1,326,000
Not Designated as Hedging Instrument [Member] | Futures contracts
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	30,444,000		10,742,000		-3,307,000
Not Designated as Hedging Instrument [Member] | Futures contracts | Other investments available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	2,156,000		461,000
Liability derivatives - fair value	-1,907,000		-110,000
Net derivatives - fair value	249,000		351,000
Net derivatives - notional value	2,549,027,000	[2]	475,967,000	[2]
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	13,430,000		9,762,000		-214,000
Not Designated as Hedging Instrument [Member] | Foreign currency forward contracts | Other investments available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	10,511,000		5,023,000
Liability derivatives - fair value	-1,145,000		-3,090,000
Net derivatives - fair value	9,366,000		1,933,000
Net derivatives - notional value	397,106,000	[2]	330,746,000	[2]
Not Designated as Hedging Instrument [Member] | TBAs
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	-342,000		-1,623,000		4,413,000
Not Designated as Hedging Instrument [Member] | TBAs | Fixed maturities available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	10,592,000		33,455,000
Liability derivatives - fair value	0		-21,731,000
Net derivatives - fair value	10,592,000		11,724,000
Net derivatives - notional value	10,056,000	[2]	56,160,000	[2]
Not Designated as Hedging Instrument [Member] | Other
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	402,000		2,031,000		-2,218,000
Not Designated as Hedging Instrument [Member] | Other | Other investments available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	3,209,000		920,000
Liability derivatives - fair value	-2,345,000		-1,541,000
Net derivatives - fair value	864,000		-621,000
Net derivatives - notional value	$735,684,000	[2]	$347,916,000	[2]
[1]	See Note 11 for information on the Company’s derivative instruments.
[2]	Represents the absolute notional value of all outstanding contracts, consisting of long and short positions.

Other_Comprehensive_Income_Com

Other Comprehensive Income - Components of accumulated other comprehensive income (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$74,964	$287,017	$153,923
Other comprehensive income (loss) before reclassifications	118,996	-179,367	290,918
Amounts reclassified from accumulated other comprehensive income	-65,104	-32,686	-157,824
Net current period other comprehensive income (loss)	53,892	-212,053	133,094
Ending balance	128,856	74,964	287,017
Unrealized Appreciation on Available-For-Sale Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	80,692	289,956	174,636
Other comprehensive income (loss) before reclassifications	146,010	-176,578	273,144
Amounts reclassified from accumulated other comprehensive income	-65,104	-32,686	-157,824
Net current period other comprehensive income (loss)	80,906	-209,264	115,320
Ending balance	161,598	80,692	289,956
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	-5,728	-2,939	-20,713
Other comprehensive income (loss) before reclassifications	-27,014	-2,789	17,774
Amounts reclassified from accumulated other comprehensive income	0	0	0
Net current period other comprehensive income (loss)	-27,014	-2,789	17,774
Ending balance	($32,742)	($5,728)	($2,939)

Other_Comprehensive_Income_Amo

Other Comprehensive Income - Amounts reclassified from accumulated other comprehensive income (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other-than-temporary impairment losses	($3,837)	($8,593)	($14,749)	($2,971)	($88)	($728)	($724)	($2,246)	($30,150)	($3,786)	($11,388)
Income before income taxes									844,247	742,505	589,387
Income tax expense									-22,987	-32,774	4,010
Net of tax									834,355	709,731	593,397
Reclassification out of accumulated other comprehensive income | Unrealized Appreciation on Available-For-Sale Investments
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains									102,223	39,308	180,144
Other-than-temporary impairment losses									-30,470	-3,961	-12,175
Income before income taxes									71,753	35,347	167,969
Income tax expense									-6,649	-2,661	-10,145
Net of tax									$65,104	$32,686	$157,824

Other_Comprehensive_Income_Com1

Other Comprehensive Income - Components of other comprehensive income (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Before tax amount:
Unrealized holding gains (losses) arising during period, before tax	$161,685	($201,386)	$278,917
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), before tax	-320	-175	-787
Less reclassification of net realized gains included in net income, before tax	71,753	35,347	167,969
Foreign currency translation adjustments, before tax	-27,014	-2,789	18,748
Other comprehensive income (loss), before tax	62,598	-239,697	128,909
Tax expense (benefit):
Unrealized holding gains (losses) arising during period, tax	15,355	-24,983	4,986
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), tax	0	0	0
Less reclassification of net realized gains included in net income, tax	6,649	2,661	10,145
Foreign currency translation adjustments, tax	0	0	974
Other comprehensive income (loss), tax	8,706	-27,644	-4,185
Net of tax amount:
Unrealized holding gains (losses) arising during period, net of tax	146,330	-176,403	273,931
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), net of tax	-320	-175	-787
Less reclassification of net realized gains included in net income, net of tax	65,104	32,686	157,824
Foreign currency translation adjustments, net of tax	-27,014	-2,789	17,774
Net current period other comprehensive income (loss)	$53,892	($212,053)	$133,094

Earnings_Per_Common_Share_Deta

Earnings Per Common Share (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Numerator:
Net income	$207,134	$223,264	$211,717	$179,145	$161,490	$114,825	$176,940	$256,476	$821,260		$709,731		$593,397
Amounts attributable to noncontrolling interests									13,095	[1]	0	[1]	0	[1]
Net income available to Arch									834,355		709,731		593,397
Preferred dividends	-5,485	-5,484	-5,485	-5,484	-5,485	-5,484	-5,485	-5,484	-21,938		-21,938		-25,079
Loss on repurchase of preferred shares									0		0		-10,612
Net income available to common shareholders	$209,679	$223,191	$202,531	$177,016	$156,005	$109,341	$171,455	$250,992	$812,417		$687,793		$557,706
Denominator:
Weighted average common shares outstanding — basic									130,817,610		131,355,392		134,446,158
Effect of dilutive common share equivalents:
Nonvested restricted shares									1,128,540		1,090,100		857,174
Stock options									2,976,172	[2]	3,331,691	[2]	2,955,515	[2]
Weighted average common shares and common share equivalents outstanding — diluted									134,922,322		135,777,183		138,258,847
Earnings per common share:
Basic (per share)	$1.65	$1.69	$1.53	$1.34	$1.19	$0.83	$1.31	$1.92	$6.21		$5.24		$4.15
Diluted (per share)	$1.60	$1.64	$1.48	$1.30	$1.14	$0.80	$1.26	$1.85	$6.02		$5.07		$4.03
Stock Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per common share (shares)									1,435,955		892,439		839,414
[1]	See Note 4.
[2]	Certain stock options were not included in the computation of diluted earnings per share where the exercise price of the stock options exceeded the average market price and would have been anti-dilutive or where, when applying the treasury stock method to in-the-money options, the sum of the proceeds, including unrecognized compensation, exceeded the average market price and would have been anti-dilutive. For 2014, 2013 and 2012, the number of stock options excluded were 1,435,955, 892,439 and 839,414, respectively.

Income_Taxes_Components_of_inc

Income Taxes - Components of income tax (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current expense (benefit):
United States	$29,749	$27,537	$8,267
Non-U.S.	801	5,159	737
Current Income Tax Expense (Benefit)	30,550	32,696	9,004
Deferred expense (benefit):
United States	-8,168	1,937	-9,779
Non-U.S.	605	-1,859	-3,235
Deferred Income Tax Expense (Benefit)	-7,563	78	-13,014
Income tax expense (benefit)	$22,987	$32,774	($4,010)

Income_Taxes_Income_before_tax

Income Taxes - Income before taxes by jurisdiction (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Line Items]
Income (loss) before income taxes	$844,247	$742,505	$589,387
Bermuda
Income Tax Disclosure [Line Items]
Income (loss) before income taxes	794,926	717,661	609,710
United States
Income Tax Disclosure [Line Items]
Income (loss) before income taxes	53,055	87,032	9,600
Other
Income Tax Disclosure [Line Items]
Income (loss) before income taxes	($3,734)	($62,188)	($29,923)

Income_Taxes_Statutory_tax_rat

Income Taxes - Statutory tax rate by jurisdiction (Details)	12 Months Ended
	Dec. 31, 2014
Bermuda
Income Tax Disclosure [Line Items]
Statutory tax rate (percentage)	0.00%
United States
Income Tax Disclosure [Line Items]
Statutory tax rate (percentage)	35.00%
United Kingdom
Income Tax Disclosure [Line Items]
Statutory tax rate (percentage)	21.50%
Ireland
Income Tax Disclosure [Line Items]
Statutory tax rate (percentage)	12.50%
Denmark
Income Tax Disclosure [Line Items]
Statutory tax rate (percentage)	24.50%
Canada
Income Tax Disclosure [Line Items]
Statutory tax rate (percentage)	24.00%
Gibraltar
Income Tax Disclosure [Line Items]
Statutory tax rate (percentage)	10.00%
Netherlands
Income Tax Disclosure [Line Items]
Statutory tax rate (percentage)	23.90%

Income_Taxes_Reconciliation_of

Income Taxes - Reconciliation of effective tax rate (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Expected income tax expense (benefit) computed on pre-tax income at weighted average income tax rate	$18,178	$17,506	($3,426)
Addition (reduction) in income tax expense (benefit) resulting from:
Tax-exempt investment income	-8,048	-8,255	-9,257
Meals and entertainment	711	599	688
State taxes, net of U.S. federal tax benefit	1,281	431	270
Foreign branch taxes	464	703	544
Prior year adjustment	320	2,810	-1,581
Foreign exchange gains & losses	746	-1,254	-436
Changes in applicable tax rate	-51	2,007	1,193
Dividend withholding taxes	3,276	4,619	2,511
Change in valuation allowance	-736	11,795	4,281
Contingent consideration	6,763	0	0
Other	83	1,813	1,203
Income tax expense (benefit)	$22,987	$32,774	($4,010)

Income_Taxes_Components_of_def

Income Taxes - Components of deferred tax assets and liabilities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Deferred income tax assets:
Net operating loss	$12,017	$15,294
AMT credit carryforward	5,309	14,456
Discounting of net loss reserves	63,880	53,293
Net unearned premium reserve	34,087	30,698
Compensation liabilities	27,765	26,481
Foreign tax credit carryforward	4,628	4,343
Interest expense	5,208	6,049
Goodwill	885	0
Tax and loss bonds	21,247	0
Other, net	13,408	13,347
Deferred tax assets before valuation allowance	188,434	163,961
Valuation allowance	-12,624	-15,548
Deferred tax assets net of valuation allowance	175,810	148,413
Deferred income tax liabilities:
Depreciation and amortization	-7,579	-3,810
Deferred acquisition costs, net	-2,744	-3,369
Deposit accounting liability	-4,170	-4,581
Contingency reserve	-18,480	0
Net unrealized appreciation of investments	-11,695	-6,354
Other, net	-150	-1,738
Total deferred tax liabilities	-44,818	-19,852
Net deferred income tax assets	$130,992	$128,561

Income_Taxes_Summary_of_open_t

Income Taxes - Summary of open tax years by major jurisdiction (Details)	12 Months Ended
	Dec. 31, 2014
United States | Minimum
Income Tax Contingency [Line Items]
Open tax year	2009
United States | Maximum
Income Tax Contingency [Line Items]
Open tax year	2014
United Kingdom | Minimum
Income Tax Contingency [Line Items]
Open tax year	2009
United Kingdom | Maximum
Income Tax Contingency [Line Items]
Open tax year	2014
Ireland | Minimum
Income Tax Contingency [Line Items]
Open tax year	2010
Ireland | Maximum
Income Tax Contingency [Line Items]
Open tax year	2014
Canada | Minimum
Income Tax Contingency [Line Items]
Open tax year	2010
Canada | Maximum
Income Tax Contingency [Line Items]
Open tax year	2014
Switzerland | Minimum
Income Tax Contingency [Line Items]
Open tax year	2011
Switzerland | Maximum
Income Tax Contingency [Line Items]
Open tax year	2014
Denmark | Minimum
Income Tax Contingency [Line Items]
Open tax year	2011
Denmark | Maximum
Income Tax Contingency [Line Items]
Open tax year	2014

Income_Taxes_Narrative_Details

Income Taxes - Narrative (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Operating Loss Carryforwards [Line Items]
Net operating loss	$12,017,000	$15,294,000
Valuation allowance	12,624,000	15,548,000
Foreign tax credit carryforward	4,628,000	4,343,000
AMT credit carryforward	5,309,000	14,456,000
Tax and loss bonds	21,247,000	0
Current income tax payable	3,600,000
United Kingdom
Operating Loss Carryforwards [Line Items]
Net operating loss	1,200,000
Operating loss carryforwards	20,700,000
Foreign tax credit carryforward	4,600,000
Operating loss carryforward, valuation allowance	200,000
Tax credit carryforward, valuation allowance	4,600,000
Ireland
Operating Loss Carryforwards [Line Items]
Net operating loss	2,500,000
Operating loss carryforwards	10,000,000
Operating loss carryforward, valuation allowance	300,000
United States
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	18,900,000
Operating loss carryforward, portion subject to utilization limits	12,100,000
Operating loss carryforward, annual utilization limit	$600,000

Transactions_with_Related_Part1

Transactions with Related Parties (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2006
Related Party Transaction [Line Items]
Investment commitments	$968,900,000
Purchases of other investments	2,257,481,000	1,326,729,000	1,000,049,000
Aeolus LP | Investee
Related Party Transaction [Line Items]
Initial contribution amount				50,000,000
Ownership percentage	4.00%
Dividends received	600,000	2,100,000	19,800,000
The Carlyle Group | Common director
Related Party Transaction [Line Items]
Investment commitments	395,900,000
Unfunded commitments	260,200,000
Purchases of other investments	51,700,000	60,800,000	43,000,000
Aggregate distributions received	$33,600,000	$15,000,000	$4,500,000

Commitments_and_Contingencies_1

Commitments and Contingencies - Concentrations of credit risk (Details) (USD $)	12 Months Ended
In Billions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Concentration Risk [Line Items]
Net reinsurance recoverables	1.97	1.94
Aon Corporation [Member] | Customer Concentration Risk [Member] | Gross Written Premiums [Member]
Concentration Risk [Line Items]
Concentration risk percentage	13.40%	16.00%	20.30%
Marsh And McLennan Companies [Member] | Customer Concentration Risk [Member] | Gross Written Premiums [Member]
Concentration Risk [Line Items]
Concentration risk percentage	15.00%	17.00%	14.10%

Commitments_and_Contingencies_2

Commitments and Contingencies - Future minimum rental payments (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Rent expense
Rental expense net of income from subleases	$23,100,000	$18,700,000	$17,100,000
Office Space [Member]
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2015	23,343,000
2016	23,670,000
2017	22,539,000
2018	20,803,000
2019	17,705,000
Thereafter	46,672,000
Total	$154,732,000

Commitments_and_Contingencies_3

Commitments and Contingencies - Credit facilities and other commitments (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Line of Credit Facility [Line Items]
Investment commitments	$968,900,000
Estimated purchase commitments, primarily related to software and computerized systems	22,200,000	35,900,000
Line of Credit Facility [Abstract]
Amount outstanding	100,000,000	100,000,000
LOC Facilities
Line of Credit Facility [Abstract]
Amount outstanding	423,200,000
Investments pledged as collateral	483,500,000
Aggregate limit on unsecured letters of credit
Line of Credit Facility [Abstract]
Maximum borrowing capacity	100,000,000
Credit Agreement
Line of Credit Facility [Abstract]
Debt to total capital ratio (expressed as a percent)	0.35
Minimum tangible net worth	3,950,000,000
Percentage of aggregate future net income for each quarterly period	25.00%
Percentage of future aggregate proceeds from issuance of common or preferred equity	25.00%
Unsecured revolving loan and letter of credit facility
Line of Credit Facility [Abstract]
Maximum borrowing capacity	300,000,000
Secured letter of credit facility
Line of Credit Facility [Abstract]
Maximum borrowing capacity	500,000,000
Other secured letter of credit facilities
Line of Credit Facility [Abstract]
Maximum borrowing capacity	190,900,000
Watford Re
Line of Credit Facility [Abstract]
Maximum borrowing capacity	200,000,000
Amount outstanding	$26,000,000

Senior_Notes_Details

Senior Notes (Details) (Unsecured debt, USD $)	Dec. 31, 2014	Dec. 31, 2013	4-May-04	Dec. 13, 2013
In Millions, unless otherwise specified
7.35% senior notes due 2034
Debt Instrument [Line Items]
Face amount of issued debt			$300
Stated interest rate (percentage)			7.35%
Estimated fair value of senior notes	462.4	381
5.144% senior notes due 2043
Debt Instrument [Line Items]
Face amount of issued debt				500
Stated interest rate (percentage)				5.14%
Estimated fair value of senior notes	$609.90	$514.10

Goodwill_and_Intangible_Assets2

Goodwill and Intangible Assets - Analysis of goodwill and intangible assets (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Goodwill And Intangible Assets [Line Items]
Goodwill, beginning of year	$0	$0
Goodwill, acquisitions	14,965	0
Goodwill, foreign currency translation adjustment	-19	0
Goodwill, end of year	14,946	0
Intangible assets with an indefinite life, beginning of year	16,666	16,666
Intangible assets with an indefinite life, acquisitions	16,858	0
Intangible assets with an indefinite life, foreign currency translation adjustment	0	0
Intangible assets with an indefinite life, end of year	33,524	16,666
Intangible assets with a finite life, beginning of year	10,653	21,593
Intangible assets with a finite life, acquisitions	76,435	0
Intangible assets with a finite life, amortization	-25,520	-11,170
Intangible assets with a finite life, foreign currency translation adjustment	-499	230
Intangible assets with a finite life, end of year	61,069	10,653
Goodwill and intangible assets, beginning of year	27,319	38,259
Goodwill and intangible assets, acquisitions	108,258	0
Goodwill and intangible assets, foreign currency translation adjustment	-518	230
Goodwill and intangible assets, end of year	109,539	27,319
Goodwill and intangible assets, gross balance	173,397
Intangible assets with a finite life, accumulated amortization	-63,439	-37,919
Goodwill and intangible assets, accumulated foreign currency translation adjustment	-419	99
Goodwill and intangible assets, net balance	109,539	27,319
Goodwill
Schedule of Goodwill And Intangible Assets [Line Items]
Goodwill, end of year	14,946
Goodwill and intangible assets, end of year	14,946
Goodwill, gross balance	14,965
Goodwill and intangible assets, accumulated foreign currency translation adjustment	-19
Goodwill and intangible assets, net balance	14,946
Intangible assets with an indefinite life
Schedule of Goodwill And Intangible Assets [Line Items]
Intangible assets with an indefinite life, end of year	33,524
Goodwill and intangible assets, accumulated foreign currency translation adjustment	0
Intangible assets with a finite life
Schedule of Goodwill And Intangible Assets [Line Items]
Intangible assets with a finite life, end of year	61,069
Intangible assets with a finite life, gross balance	124,908
Intangible assets with a finite life, accumulated amortization	-63,439
Goodwill and intangible assets, accumulated foreign currency translation adjustment	($400)

Goodwill_and_Intangible_Assets3

Goodwill and Intangible Assets - Summary of intangible assets by major class (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Schedule of Goodwill And Intangible Assets [Line Items]
Goodwill and intangible assets, gross balance	$173,397	$65,139
Intangible assets with a finite life, accumulated amortization	-63,439	-37,919
Goodwill and intangible assets, accumulated foreign currency translation adjustment	-419	99
Goodwill and intangible assets, net balance	109,539	27,319	38,259
Intangible liability - unfavorable service contract
Schedule of Goodwill And Intangible Assets [Line Items]
Goodwill and intangible assets, gross balance	-9,533
Intangible assets with a finite life, accumulated amortization	2,229
Goodwill and intangible assets, accumulated foreign currency translation adjustment	0
Goodwill and intangible assets, net balance	-7,304
Intangible assets with a finite life
Schedule of Goodwill And Intangible Assets [Line Items]
Intangible assets with a finite life, accumulated amortization	-63,439
Goodwill and intangible assets, accumulated foreign currency translation adjustment	-400
Intangible assets with a finite life | Other
Schedule of Goodwill And Intangible Assets [Line Items]
Goodwill and intangible assets, gross balance	1,057
Intangible assets with a finite life, accumulated amortization	-193
Goodwill and intangible assets, accumulated foreign currency translation adjustment	0
Goodwill and intangible assets, net balance	864
Intangible assets with a finite life | Acquired insurance contracts
Schedule of Goodwill And Intangible Assets [Line Items]
Goodwill and intangible assets, gross balance	77,574	31,102
Intangible assets with a finite life, accumulated amortization	-47,710	-28,208
Goodwill and intangible assets, accumulated foreign currency translation adjustment	-107	0
Goodwill and intangible assets, net balance	29,757	2,894
Intangible assets with a finite life | Operating platform
Schedule of Goodwill And Intangible Assets [Line Items]
Goodwill and intangible assets, gross balance	29,900
Intangible assets with a finite life, accumulated amortization	-5,482
Goodwill and intangible assets, net balance	24,418
Intangible assets with a finite life | Customer relationships
Schedule of Goodwill And Intangible Assets [Line Items]
Goodwill and intangible assets, gross balance	25,910	17,371
Intangible assets with a finite life, accumulated amortization	-12,283	-9,711
Goodwill and intangible assets, accumulated foreign currency translation adjustment	-293	99
Goodwill and intangible assets, net balance	13,334	7,759
Intangible assets with an indefinite life
Schedule of Goodwill And Intangible Assets [Line Items]
Goodwill and intangible assets, accumulated foreign currency translation adjustment	0
Intangible assets with an indefinite life | Insurance licenses
Schedule of Goodwill And Intangible Assets [Line Items]
Goodwill and intangible assets, gross balance	33,524	16,666
Goodwill and intangible assets, accumulated foreign currency translation adjustment	0	0
Goodwill and intangible assets, net balance	33,524	16,666
Goodwill
Schedule of Goodwill And Intangible Assets [Line Items]
Goodwill and intangible assets, gross balance	14,965
Goodwill and intangible assets, accumulated foreign currency translation adjustment	-19
Goodwill and intangible assets, net balance	$14,946

Goodwill_and_Intangible_Assets4

Goodwill and Intangible Assets - Narrative (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Future amortization expense, 2015	21.8
Future amortization expense, 2016	17
Future amortization expense, 2017	13.3
Future amortization expense, 2018	9.4
Future amortization expense, 2019	0.8
Minimum
Finite-Lived Intangible Assets [Line Items]
Remaining useful lives of intangible assets	2 years
Maximum
Finite-Lived Intangible Assets [Line Items]
Remaining useful lives of intangible assets	8 years

Shareholders_Equity_Rollforwar

Shareholders' Equity - Roll-forward of changes in issued and outstanding common shares (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Authorized and Issued:
Common shares authorized	600,000,000
Common shares, par value per share	$0.00		$0.00
Preferred shares authorized	50,000,000
Preferred shares, par value per share	$0.01
Common Shares:
Shares issued, beginning of year	169,560,591		168,255,572		164,636,338
Shares issued	1,493,524	[1]	811,732	[1]	2,066,065	[1]
Restricted shares issued, net of cancellations	618,293		493,287		1,553,169
Shares issued, end of year	171,672,408		169,560,591		168,255,572
Common shares held in treasury, end of year	-44,304,474		-35,885,707		-34,412,959
Common shares outstanding, end of year	127,367,934		133,674,884		133,842,613
[1]	Includes shares issued from the exercise of stock options and stock appreciation rights, and shares issued from the employee share purchase plan.

Shareholders_Equity_Share_repu

Shareholders' Equity - Share repurchases and treasury shares (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Class of Stock [Line Items]
Common shares held in treasury (shares)	44,304,474	35,885,707	34,412,959
Common shares held in treasury, at cost	$1,562,019,000	$1,094,704,000
Common shares
Class of Stock [Line Items]
Cumulative number of shares acquired since inception of share repurchase program	118,100,000
Aggregate purchase price of shares acquired since inception of share repurchase program	3,240,000,000
Treasury stock, shares acquired (shares)	8,200,000	1,300,000	3,900,000
Treasury stock, value of shares acquired	454,100,000	57,800,000	172,100,000
Reduction to weighted average shares outstanding as a result of share repurchases	111,900,000	109,600,000	105,000,000
Remaining authorized repurchase amount	887,100,000
February 2007 | Common shares
Class of Stock [Line Items]
Share repurchase authorizations, amount	1,000,000,000
May 2008 | Common shares
Class of Stock [Line Items]
Share repurchase authorizations, amount	500,000,000
November 2009 | Common shares
Class of Stock [Line Items]
Share repurchase authorizations, amount	1,000,000,000
February 2011 | Common shares
Class of Stock [Line Items]
Share repurchase authorizations, amount	1,000,000,000
November 2014 | Common shares
Class of Stock [Line Items]
Share repurchase authorizations, amount	$629,200,000

Shareholders_Equity_Preferred_

Shareholders' Equity - Preferred shares (Details) (USD $)	12 Months Ended			0 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	2-May-12	Apr. 02, 2012
Class of Stock [Line Items]
Shares issued - Series C	$0	$0	$315,763
Preferred dividends paid	21,938	21,938	28,381
Series A and B Preferred Shares
Class of Stock [Line Items]
Preferred shares, redemption price per share				$25
Series A Preferred Stock
Class of Stock [Line Items]
Shares redeemed during period, value				200,000
Preferred shares, dividend rate (as a percent)				8.00%
Series B Preferred Stock
Class of Stock [Line Items]
Shares redeemed during period, value				125,000
Preferred shares, dividend rate (as a percent)				7.88%
Series C Preferred Stock
Class of Stock [Line Items]
Shares issued - Series C					$325,000
Preferred shares, dividend rate (as a percent)	6.75%				6.75%
Preferred shares, redemption price per share	$25

ShareBased_Compensation_Long_T

Share-Based Compensation - Long Term Incentive Plan (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
2002 Long Term Incentive and Share Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares reserved for issuance	9,497,490
2007 Long Term Incentive and Share Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for issuance transferred	3,153,924
2007 Employee Share Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares reserved for issuance	2,250,000
Shares available for issuance	762,293
Length of offering period, in months	27 months
Maximum employee subscription rate	20.00%
Purchase price (percentage of market value)	85.00%
Maximum number of shares per employee	3,000
Maximum accumulation per calendar year	$25,000
Share based compensation expense	1,600,000	1,400,000	1,100,000
Tax benefit	$100,000	$100,000	$100,000
2012 Long Term Incentive and Share Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares reserved for issuance	4,280,000
Total number of shares reserved under plan	7,433,924
Maximum percentage of awards to be issued in connection with full value awarrds	50.00%
Maximum number of incentive stock options	2,000,000
Shares available for issuance	980,571

ShareBased_Compensation_Valuat

Share-Based Compensation - Valuation assumptions (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Dividend yield	0.00%	0.00%	0.00%
Expected volatility	22.80%	23.60%	24.30%
Risk free interest rate	1.90%	1.00%	1.00%
Expected option life	6 years	6 years	6 years 3 months

ShareBased_Compensation_Stock_

Share-Based Compensation - Stock option activity (Details) (Stock Options, USD $)	12 Months Ended
	Dec. 31, 2014
Stock Options
Number of Options / SARs
Outstanding, beginning of year (in shares)	8,338,480
Granted (in shares)	1,078,036
Exercised (in shares)	-1,542,828
Forfeited or expired (in shares)	-69,655
Outstanding, end of year (in shares)	7,804,033
Exercisable, end of year (in shares)	5,558,439
Weighted Average Exercise Price
Outstanding, beginning of year, weighted average exercise price	$28.82
Granted, weighted average exercise price	$56.71
Exercised, weighted average exercise price	$18.62
Forfeited or expired, weighted average exercise price	$46.91
Outstanding, end of year, weighted average exercise price	$34.52
Exercisable, end of year, weighted average exercise price	$28.91

ShareBased_Compensation_Restri

Share-Based Compensation - Restricted common shares and restricted stock units activity (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
Restricted Common Shares
Unvested Shares:
Unvested balance, beginning of year (in shares)	1,943,125
Granted (in shares)	660,043
Vested (in shares)	-591,576
Forfeited (in shares)	-41,750
Unvested balance, end of year (in shares)	1,969,842
Weighted Average Grant Date Fair Value:
Unvested balance, beginning of year, weighted average grant date fair value	$43.55
Granted, weighted average grant date fair value	$56.94
Vested, weighted average grant date fair value	$41.89
Forfeited, weighted average grant date fair value	$46.54
Unvested balance, end of year, weighted average grant date fair value	$48.47
Restricted Unit Awards
Unvested Shares:
Unvested balance, beginning of year (in shares)	308,322
Granted (in shares)	107,426
Vested (in shares)	-88,210
Forfeited (in shares)	-22,405
Unvested balance, end of year (in shares)	305,133
Weighted Average Grant Date Fair Value:
Unvested balance, beginning of year, weighted average grant date fair value	$43.32
Granted, weighted average grant date fair value	$57.18
Vested, weighted average grant date fair value	$41.86
Forfeited, weighted average grant date fair value	$45.07
Unvested balance, end of year, weighted average grant date fair value	$48.49

ShareBased_Compensation_Additi

Share-Based Compensation - Additional information (Details) (USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Life of award	10 years
Share based compensation expense	$12.50	$10.60	$8.50
Tax benefit	2.1	2	1.8
Unrecognized compensation cost related to unvested awards	14.8
Expected period for recognition (years)	1 year 9 months 18 days
Weighted average grant date fair value	$14.77	$12.96	$10.61
Aggregate intrinsic value of options exercised	58.4	21.7	64
Aggregate intrinsic value of options outstanding	191.8
Aggregate intrinsic value of options exercisable	167.8
Options outstanding, weighted average remaining contractual term (in years)	5 years 8 months 12 days
Options exercisable, weighted average remaining contractual term (in years)	4 years 7 months 6 days
Proceeds from stock options exercised	11
Tax benefit from stock options exercised	8.9
Restricted Common Shares And Restricted Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Share based compensation expense	30	27.4	24.6
Tax benefit	8.4	7.8	6.3
Granted (in shares)	767,469	590,118	1,819,657
Granted, weighted average grant date fair value	$56.97	$53.21	$40.75
Aggregate fair value of awards that vested in the period	38.6	41	33.2
Restricted Common Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost related to unvested awards	48.5
Expected period for recognition (years)	1 year 8 months 12 days
Granted (in shares)	660,043
Granted, weighted average grant date fair value	$56.94
Restricted Unit Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost related to unvested awards	8.1
Expected period for recognition (years)	1 year 9 months 18 days
Granted (in shares)	107,426
Granted, weighted average grant date fair value	$57.18
Aggregate intrinsic value of awards outstanding	26.4
Aggregate intrinsic value of awards exercisable	$8.40

Retirement_Plans_Details

Retirement Plans (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Compensation and Retirement Disclosure [Abstract]
Defined contribution plan, expense	$26.80	$21.50	$19.30

Statutory_Information_Summary_

Statutory Information - Summary of statutory capital, surplus and net income (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Bermuda
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	$6,447,803	[1]	$5,416,948	[1]
Required statutory capital and surplus	2,209,757	[1]	2,077,442	[1]
Statutory net income (loss)	692,676		719,267		631,483
Ireland
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	553,561	[1]	533,283	[1]
Required statutory capital and surplus	463,877	[1]	458,666	[1]
Statutory net income (loss)	43,197		-24,410		-1,940
United States
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	1,454,282	[1]	1,013,228	[1]
Required statutory capital and surplus	467,797	[1]	335,442	[1]
Statutory net income (loss)	46,110		62,605		-21,517
United Kingdom
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	384,525	[1]	392,734	[1]
Required statutory capital and surplus	307,596	[1]	353,330	[1]
Statutory net income (loss)	13,016		-11,353		-4,449
Canada
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	79,123	[1]	77,877	[1]
Required statutory capital and surplus	58,837	[1]	74,951	[1]
Statutory net income (loss)	$1,517		($36,203)		$0
[1]	Such amounts include ownership interests in affiliated insurance and reinsurance subsidiaries.

Statutory_Information_Narrativ

Statutory Information - Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Bermuda
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus	$2,209,757,000	[1]	$2,077,442,000	[1]
United States
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus	467,797,000	[1]	335,442,000	[1]
Arch Re Bermuda | Bermuda
Statutory Accounting Practices [Line Items]
Dividends, percentage permitted	25.00%
Dividend required notice period	7 days
Capital reduction, percentage permitted	15.00%
Dividends available for payment in 2014 without prior regulatory approval	1,360,000,000
Arch Re US | United States
Statutory Accounting Practices [Line Items]
Dividends available for payment in 2014, total	110,000,000
Arch MI US | United States
Statutory Accounting Practices [Line Items]
Statutory minimum capital funding required	400,000,000
Required statutory capital and surplus	$260,000,000
Maximum allowable risk to capital ratio	18
Dividend restriction period	3 years
[1]	Such amounts include ownership interests in affiliated insurance and reinsurance subsidiaries.

Unaudited_Condensed_Quarterly_2

Unaudited Condensed Quarterly Financial Information (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Net premiums written	$895,481	$959,539	$971,928	$1,064,990	$748,921	$839,135	$810,535	$952,776	$3,891,938	$3,351,367	$3,052,235
Net premiums earned	923,165	903,651	907,152	859,780	839,366	795,000	758,816	752,770	3,593,748	3,145,952	2,935,140
Net investment income	82,496	80,105	72,990	66,994	67,095	66,083	68,369	65,672	302,585	267,219	294,895
Net realized gains	10,561	18,515	54,144	19,697	9,048	-6,022	12,652	58,340	102,917	74,018	194,228
Net impairment losses recognized in earnings	-3,837	-8,593	-14,749	-2,971	-88	-728	-724	-2,246	-30,150	-3,786	-11,388
Underwriting Income Loss Segment	113,464	101,087	124,091	132,456	128,318	110,992	96,029	116,398	471,098	451,737	143,034
Net income	207,134	223,264	211,717	179,145	161,490	114,825	176,940	256,476	821,260	709,731	593,397
Dividends declared on preferred shares	-5,485	-5,484	-5,485	-5,484	-5,485	-5,484	-5,485	-5,484	-21,938	-21,938	-25,079
Net income available to common shareholders	209,679	223,191	202,531	177,016	156,005	109,341	171,455	250,992	812,417	687,793	557,706
Loss on repurchase of preferred shares									$0	$0	$10,612
Basic (per share)	$1.65	$1.69	$1.53	$1.34	$1.19	$0.83	$1.31	$1.92	$6.21	$5.24	$4.15
Diluted (per share)	$1.60	$1.64	$1.48	$1.30	$1.14	$0.80	$1.26	$1.85	$6.02	$5.07	$4.03

Guarantor_Financial_Informatio2

Guarantor Financial Information - Condensed consolidating balance sheet (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
In Thousands, unless otherwise specified
Assets
Total investments	$15,331,249		$13,611,734
Cash	485,702		434,057		371,041		351,699
Investment in subsidiaries	0		0
Due from subsidiaries and affiliates	0		0
Premiums receivable	948,695		753,924
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	1,812,845		1,804,330
Contractholder receivables	1,309,192		1,064,246
Prepaid reinsurance premiums	377,078		328,343
Deferred acquisition costs, net	414,525		342,314
Other assets	1,330,257		1,227,146
Total assets	22,009,543		19,566,094		17,816,762
Liabilities
Reserve for losses and loss adjustment expenses	9,036,448		8,824,696		8,933,292		8,456,210
Unearned premiums	2,231,578		1,896,365
Reinsurance balances payable	219,312		196,167
Contractholder payables	1,309,192		1,064,246
Deposit accounting liabilities	327,384		421,297
Senior notes	800,000		800,000
Revolving credit agreement borrowings	100,000		100,000
Due to subsidiaries and affiliates	0		0
Other liabilities	866,983		615,827
Total liabilities	14,890,897		13,918,598		12,647,884
Redeemable noncontrolling interests	219,512	[1]	0	[1]
Shareholders’ Equity
Total shareholders' equity available to Arch	6,130,053		5,647,496		5,168,878
Non-redeemable noncontrolling interests	769,081	[1]	0	[1]	0	[1]
Total shareholders' equity	6,899,134		5,647,496		5,168,878
Total liabilities, noncontrolling interests and shareholders' equity	22,009,543		19,566,094
Reportable legal entities | ACGL (Parent Guarantor)
Assets
Total investments	107		2,530
Cash	3,218		3,223		6,417		4,717
Investment in subsidiaries	6,536,644		6,046,060
Due from subsidiaries and affiliates	48		2,251
Premiums receivable	0		0
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	0		0
Contractholder receivables	0		0
Prepaid reinsurance premiums	0		0
Deferred acquisition costs, net	0		0
Other assets	7,590		6,598
Total assets	6,547,607		6,060,662
Liabilities
Reserve for losses and loss adjustment expenses	0		0
Unearned premiums	0		0
Reinsurance balances payable	0		0
Contractholder payables	0		0
Deposit accounting liabilities	0		0
Senior notes	300,000		300,000
Revolving credit agreement borrowings	100,000		100,000
Due to subsidiaries and affiliates	417		18
Other liabilities	17,137		13,148
Total liabilities	417,554		413,166
Redeemable noncontrolling interests	0		0
Shareholders’ Equity
Total shareholders' equity available to Arch	6,130,053		5,647,496
Non-redeemable noncontrolling interests	0	[1]	0
Total shareholders' equity	6,130,053		5,647,496
Total liabilities, noncontrolling interests and shareholders' equity	6,547,607		6,060,662
Reportable legal entities | Arch-U.S. (Subsidiary Issuer)
Assets
Total investments	62,867		408,957
Cash	2,787		509		612		6,712
Investment in subsidiaries	1,685,185		1,258,889
Due from subsidiaries and affiliates	7,517		0
Premiums receivable	0		0
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	0		0
Contractholder receivables	0		0
Prepaid reinsurance premiums	0		0
Deferred acquisition costs, net	0		0
Other assets	49,576		60,342
Total assets	1,807,932		1,728,697
Liabilities
Reserve for losses and loss adjustment expenses	0		0
Unearned premiums	0		0
Reinsurance balances payable	0		0
Contractholder payables	0		0
Deposit accounting liabilities	0		0
Senior notes	500,000		500,000
Revolving credit agreement borrowings	0		0
Due to subsidiaries and affiliates	7,505		10,250
Other liabilities	49,403		33,206
Total liabilities	556,908		543,456
Redeemable noncontrolling interests	0		0
Shareholders’ Equity
Total shareholders' equity available to Arch	1,251,024		1,185,241
Non-redeemable noncontrolling interests	0	[1]	0
Total shareholders' equity	1,251,024		1,185,241
Total liabilities, noncontrolling interests and shareholders' equity	1,807,932		1,728,697
Reportable legal entities | Other ACGL Subsidiaries
Assets
Total investments	15,268,275		13,200,247
Cash	479,697		430,325		364,012		340,270
Investment in subsidiaries	0		0
Due from subsidiaries and affiliates	370,429		405,110
Premiums receivable	1,331,511		1,085,369
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	5,584,973		5,645,156
Contractholder receivables	1,309,192		1,064,246
Prepaid reinsurance premiums	1,373,008		1,109,312
Deferred acquisition costs, net	414,525		342,314
Other assets	1,689,670		1,714,651
Total assets	27,821,280		24,996,730
Liabilities
Reserve for losses and loss adjustment expenses	12,784,030		12,625,766
Unearned premiums	3,227,508		2,677,334
Reinsurance balances payable	589,289		662,394
Contractholder payables	1,309,192		1,064,246
Deposit accounting liabilities	587,050		758,490
Senior notes	0		0
Revolving credit agreement borrowings	0		0
Due to subsidiaries and affiliates	370,072		397,093
Other liabilities	994,741		691,699
Total liabilities	19,861,882		18,877,022
Redeemable noncontrolling interests	219,512		0
Shareholders’ Equity
Total shareholders' equity available to Arch	6,970,805		6,119,708
Non-redeemable noncontrolling interests	769,081	[1]	0
Total shareholders' equity	7,739,886		6,119,708
Total liabilities, noncontrolling interests and shareholders' equity	27,821,280		24,996,730
Consolidating adjustments and eliminations
Assets
Total investments	0		0
Cash	0		0		0		0
Investment in subsidiaries	-8,221,829		-7,304,949
Due from subsidiaries and affiliates	-377,994		-407,361
Premiums receivable	-382,816		-331,445
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	-3,772,128		-3,840,826
Contractholder receivables	0		0
Prepaid reinsurance premiums	-995,930		-780,969
Deferred acquisition costs, net	0		0
Other assets	-416,579		-554,445
Total assets	-14,167,276		-13,219,995
Liabilities
Reserve for losses and loss adjustment expenses	-3,747,582		-3,801,070
Unearned premiums	-995,930		-780,969
Reinsurance balances payable	-369,977		-466,227
Contractholder payables	0		0
Deposit accounting liabilities	-259,666		-337,193
Senior notes	0		0
Revolving credit agreement borrowings	0		0
Due to subsidiaries and affiliates	-377,994		-407,361
Other liabilities	-194,298		-122,226
Total liabilities	-5,945,447		-5,915,046
Redeemable noncontrolling interests	0		0
Shareholders’ Equity
Total shareholders' equity available to Arch	-8,221,829		-7,304,949
Non-redeemable noncontrolling interests	0	[1]	0
Total shareholders' equity	-8,221,829		-7,304,949
Total liabilities, noncontrolling interests and shareholders' equity	($14,167,276)		($13,219,995)
[1]	See Note 4.

Guarantor_Financial_Informatio3

Guarantor Financial Information - Condensed consolidating statement of income and comprehensive income (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Revenues
Net premiums earned	$923,165	$903,651	$907,152	$859,780	$839,366	$795,000	$758,816	$752,770	$3,593,748		$3,145,952		$2,935,140
Net investment income	82,496	80,105	72,990	66,994	67,095	66,083	68,369	65,672	302,585		267,219		294,895
Net realized gains	10,561	18,515	54,144	19,697	9,048	-6,022	12,652	58,340	102,917		74,018		194,228
Net impairment losses recognized in earnings	-3,837	-8,593	-14,749	-2,971	-88	-728	-724	-2,246	-30,150		-3,786		-11,388
Other underwriting income									10,142		7,639		8,090
Equity in net income of investment funds accounted for using the equity method									19,883		35,701		73,510
Other income (loss)									-10,252		-586		-12,094
Total revenues									3,988,873		3,526,157		3,482,381
Expenses
Losses and loss adjustment expenses									1,919,250		1,679,424		1,861,277
Acquisition expenses									657,262		564,103		508,884
Other operating expenses									606,224		500,730		465,353
Interest expense									45,634		27,060		28,525
Net foreign exchange (gains) losses									-83,744		12,335		28,955
Total expenses									3,144,626		2,783,652		2,892,994
Income (loss) before income taxes									844,247		742,505		589,387
Income tax (expense) benefit									-22,987		-32,774		4,010
Income (loss) before equity in net income of subsidiaries									821,260		709,731		593,397
Equity in net income of subsidiaries									0		0		0
Net income	207,134	223,264	211,717	179,145	161,490	114,825	176,940	256,476	821,260		709,731		593,397
Amounts attributable to noncontrolling interests									13,095	[1]	0	[1]	0	[1]
Net income available to Arch									834,355		709,731		593,397
Preferred dividends	-5,485	-5,484	-5,485	-5,484	-5,485	-5,484	-5,485	-5,484	-21,938		-21,938		-25,079
Loss on repurchase of preferred shares									0		0		-10,612
Net income available to common shareholders	209,679	223,191	202,531	177,016	156,005	109,341	171,455	250,992	812,417		687,793		557,706
Comprehensive Income									888,247		497,678		726,491
Reportable legal entities | ACGL (Parent Guarantor)
Revenues
Net premiums earned									0		0		0
Net investment income									0		0		4
Net realized gains									0		0		0
Net impairment losses recognized in earnings									0		0		0
Other underwriting income									0		0		0
Equity in net income of investment funds accounted for using the equity method									0		0		0
Other income (loss)									0		0		0
Total revenues									0		0		4
Expenses
Losses and loss adjustment expenses									0		0		0
Acquisition expenses									0		0		0
Other operating expenses									46,074		38,702		35,570
Interest expense									23,423		23,272		23,496
Net foreign exchange (gains) losses									0		0		0
Total expenses									69,497		61,974		59,066
Income (loss) before income taxes									-69,497		-61,974		-59,062
Income tax (expense) benefit									0		0		0
Income (loss) before equity in net income of subsidiaries									-69,497		-61,974		-59,062
Equity in net income of subsidiaries									903,852		771,705		652,459
Net income									834,355		709,731		593,397
Amounts attributable to noncontrolling interests									0	[1]	0		0
Net income available to Arch									834,355		709,731		593,397
Preferred dividends									-21,938		-21,938		-25,079
Loss on repurchase of preferred shares													-10,612
Net income available to common shareholders									812,417		687,793		557,706
Comprehensive Income									888,247		497,678		726,491
Reportable legal entities | Arch-U.S. (Subsidiary Issuer)
Revenues
Net premiums earned									0		0		0
Net investment income									0		31		8
Net realized gains									5		0		0
Net impairment losses recognized in earnings									0		0		0
Other underwriting income									0		0		0
Equity in net income of investment funds accounted for using the equity method									0		0		0
Other income (loss)									0		0		0
Total revenues									5		31		8
Expenses
Losses and loss adjustment expenses									0		0		0
Acquisition expenses									0		0		0
Other operating expenses									3,387		2,691		1,218
Interest expense									25,817		1,316		0
Net foreign exchange (gains) losses									0		0		0
Total expenses									29,204		4,007		1,218
Income (loss) before income taxes									-29,199		-3,976		-1,210
Income tax (expense) benefit									10,125		1,383		424
Income (loss) before equity in net income of subsidiaries									-19,074		-2,593		-786
Equity in net income of subsidiaries									53,584		25,644		9,738
Net income									34,510		23,051		8,952
Amounts attributable to noncontrolling interests									0	[1]	0		0
Net income available to Arch									34,510		23,051		8,952
Preferred dividends									0		0		0
Loss on repurchase of preferred shares													0
Net income available to common shareholders									34,510		23,051		8,952
Comprehensive Income									33,671		-28,330		2,796
Reportable legal entities | Other ACGL Subsidiaries
Revenues
Net premiums earned									3,593,748		3,145,952		2,935,140
Net investment income									326,831		307,449		321,805
Net realized gains									102,912		74,018		194,228
Net impairment losses recognized in earnings									-30,150		-3,786		-11,388
Other underwriting income									10,142		7,639		8,090
Equity in net income of investment funds accounted for using the equity method									19,883		35,701		73,510
Other income (loss)									-10,252		-586		-12,094
Total revenues									4,013,114		3,566,387		3,509,291
Expenses
Losses and loss adjustment expenses									1,919,250		1,679,424		1,861,277
Acquisition expenses									657,262		564,103		508,884
Other operating expenses									556,763		459,337		428,565
Interest expense									20,640		42,733		31,951
Net foreign exchange (gains) losses									-53,222		2,145		21,787
Total expenses									3,100,693		2,747,742		2,852,464
Income (loss) before income taxes									912,421		818,645		656,827
Income tax (expense) benefit									-33,112		-34,157		3,586
Income (loss) before equity in net income of subsidiaries									879,309		784,488		660,413
Equity in net income of subsidiaries									0		0		0
Net income									879,309		784,488		660,413
Amounts attributable to noncontrolling interests									13,095	[1]	0		0
Net income available to Arch									892,404		784,488		660,413
Preferred dividends									0		0		0
Loss on repurchase of preferred shares													0
Net income available to common shareholders									892,404		784,488		660,413
Comprehensive Income									976,821		562,245		786,338
Consolidating adjustments and eliminations
Revenues
Net premiums earned									0		0		0
Net investment income									-24,246		-40,261		-26,922
Net realized gains									0		0		0
Net impairment losses recognized in earnings									0		0		0
Other underwriting income									0		0		0
Equity in net income of investment funds accounted for using the equity method									0		0		0
Other income (loss)									0		0		0
Total revenues									-24,246		-40,261		-26,922
Expenses
Losses and loss adjustment expenses									0		0		0
Acquisition expenses									0		0		0
Other operating expenses									0		0		0
Interest expense									-24,246		-40,261		-26,922
Net foreign exchange (gains) losses									-30,522		10,190		7,168
Total expenses									-54,768		-30,071		-19,754
Income (loss) before income taxes									30,522		-10,190		-7,168
Income tax (expense) benefit									0		0		0
Income (loss) before equity in net income of subsidiaries									30,522		-10,190		-7,168
Equity in net income of subsidiaries									-957,436		-797,349		-662,197
Net income									-926,914		-807,539		-669,365
Amounts attributable to noncontrolling interests									0	[1]	0		0
Net income available to Arch									-926,914		-807,539		-669,365
Preferred dividends									0		0		0
Loss on repurchase of preferred shares													0
Net income available to common shareholders									-926,914		-807,539		-669,365
Comprehensive Income									($1,010,492)		($533,915)		($789,134)
[1]	See Note 4.

Guarantor_Financial_Informatio4

Guarantor Financial Information - Condensed consolidating statement of cash flows (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Operating Activities
Net Cash Provided by (Used in) Operating Activities	$1,037,130		$850,868		$921,603
Investing Activities
Purchases of fixed maturity investments	-28,745,279		-18,174,988		-17,568,592
Purchases of equity securities	-520,817		-535,857		-268,999
Purchases of other investments	-2,257,481		-1,326,729		-1,000,049
Proceeds from the sale of fixed maturity investments	26,823,192		17,196,614		16,366,306
Proceeds from the sale of equity securities	411,362		462,787		313,617
Proceeds from sales, redemptions and maturities of other investments	1,643,000		1,162,707		443,630
Proceeds from redemptions and maturities of fixed maturity investments	762,995		731,708		1,115,594
Net (purchases) sales of short-term investments	577,126		-750,613		185,919
Change in investment of securities lending collateral	57,470		-55,643		6,190
Contributions to subsidiaries	0		0		0
Intercompany loans issued	0		0
Repayments of intercompany loans	0
Purchase of business, net of cash acquired	-237,106		0		28,948
Purchases of furniture, equipment and other assets	-19,883		-17,499		-18,532
Net Cash Used For Investing Activities	-1,505,421		-1,307,513		-395,968
Financing Activities
Proceeds from issuance of Series C preferred shares, net	0		0		315,763
Repurchase of Series A and B preferred shares	0		0		-325,000
Purchases of common shares under share repurchase program	-454,137		-57,796		-172,056
Proceeds from common shares issued, net	6,827		3,051		7,033
Proceeds from intercompany loans	0		0
Repayments of intercompany loans	0
Proceeds from borrowings	0		494,228		0
Repayments of borrowings	0		0		-310,868
Change in securities lending collateral	-57,470		55,643		-6,190
Third party investment in non-redeemable noncontrolling interests	796,903		0		0
Third party investment in redeemable noncontrolling interests	219,233		0		0
Dividends paid to redeemable noncontrolling interests	-14,448		0		0
Dividends paid to parent	0	[1]	0	[1]	0	[1]
Other	64,973		50,830		6,664
Preferred dividends paid	-21,938		-21,938		-28,381
Net Cash Used For Financing Activities	539,943		524,018		-513,035
Effects of exchange rate changes on foreign currency cash	-20,007		-4,357		6,742
Increase (decrease) in cash	51,645		63,016		19,342
Cash beginning of year	434,057		371,041		351,699
Cash end of period	485,702		434,057		371,041
Reportable legal entities | ACGL (Parent Guarantor)
Operating Activities
Net Cash Provided by (Used in) Operating Activities	467,091		68,562		209,500
Investing Activities
Purchases of fixed maturity investments	0		0		0
Purchases of equity securities	0		0		0
Purchases of other investments	0		0		0
Proceeds from the sale of fixed maturity investments	0		0		0
Proceeds from the sale of equity securities	0		0		0
Proceeds from sales, redemptions and maturities of other investments	0		0		0
Proceeds from redemptions and maturities of fixed maturity investments	0		0		0
Net (purchases) sales of short-term investments	2,423		5,799		-5,094
Change in investment of securities lending collateral	0		0		0
Contributions to subsidiaries	0		-160		0
Intercompany loans issued	0		0
Repayments of intercompany loans	0
Purchase of business, net of cash acquired	0				0
Purchases of furniture, equipment and other assets	-271		-712		-65
Net Cash Used For Investing Activities	2,152		4,927		-5,159
Financing Activities
Proceeds from issuance of Series C preferred shares, net					315,763
Repurchase of Series A and B preferred shares					-325,000
Purchases of common shares under share repurchase program	-454,137		-57,796		-172,056
Proceeds from common shares issued, net	6,827		3,051		7,033
Proceeds from intercompany loans	0		0
Repayments of intercompany loans	0
Proceeds from borrowings			0
Repayments of borrowings					0
Change in securities lending collateral	0		0		0
Third party investment in non-redeemable noncontrolling interests	0	[2]
Third party investment in redeemable noncontrolling interests	0	[2]
Dividends paid to redeemable noncontrolling interests	0	[2]
Dividends paid to parent	0	[1]	0	[1]	0	[1]
Other	0		0		0
Preferred dividends paid	-21,938		-21,938		-28,381
Net Cash Used For Financing Activities	-469,248		-76,683		-202,641
Effects of exchange rate changes on foreign currency cash	0		0		0
Increase (decrease) in cash	-5		-3,194		1,700
Cash beginning of year	3,223		6,417		4,717
Cash end of period	3,218		3,223		6,417
Reportable legal entities | Arch-U.S. (Subsidiary Issuer)
Operating Activities
Net Cash Provided by (Used in) Operating Activities	8,142		-6,569		-8,086
Investing Activities
Purchases of fixed maturity investments	-78,509		0		0
Purchases of equity securities	0		0		0
Purchases of other investments	0		0		0
Proceeds from the sale of fixed maturity investments	16,011		0		0
Proceeds from the sale of equity securities	0		0		0
Proceeds from sales, redemptions and maturities of other investments	0		0		0
Proceeds from redemptions and maturities of fixed maturity investments	0		0		0
Net (purchases) sales of short-term investments	408,591		-400,162		1,986
Change in investment of securities lending collateral	0		0		0
Contributions to subsidiaries	-341,707		-97,850		0
Intercompany loans issued	-7,500		0
Repayments of intercompany loans	0
Purchase of business, net of cash acquired	0				0
Purchases of furniture, equipment and other assets	0		0		0
Net Cash Used For Investing Activities	-3,114		-498,012		1,986
Financing Activities
Proceeds from issuance of Series C preferred shares, net					0
Repurchase of Series A and B preferred shares					0
Purchases of common shares under share repurchase program	0		0		0
Proceeds from common shares issued, net	0		0		0
Proceeds from intercompany loans	7,500		10,250
Repayments of intercompany loans	-10,250
Proceeds from borrowings			494,228
Repayments of borrowings					0
Change in securities lending collateral	0		0		0
Third party investment in non-redeemable noncontrolling interests	0	[2]
Third party investment in redeemable noncontrolling interests	0	[2]
Dividends paid to redeemable noncontrolling interests	0	[2]
Dividends paid to parent	0	[1]	0	[1]	0	[1]
Other	0		0		0
Preferred dividends paid	0		0		0
Net Cash Used For Financing Activities	-2,750		504,478		0
Effects of exchange rate changes on foreign currency cash	0		0		0
Increase (decrease) in cash	2,278		-103		-6,100
Cash beginning of year	509		612		6,712
Cash end of period	2,787		509		612
Reportable legal entities | Other ACGL Subsidiaries
Operating Activities
Net Cash Provided by (Used in) Operating Activities	1,077,597		908,375		978,679
Investing Activities
Purchases of fixed maturity investments	-28,666,770		-18,174,988		-17,568,592
Purchases of equity securities	-520,817		-535,857		-268,999
Purchases of other investments	-2,257,481		-1,326,729		-1,000,049
Proceeds from the sale of fixed maturity investments	26,807,181		17,196,614		16,366,306
Proceeds from the sale of equity securities	411,362		462,787		313,617
Proceeds from sales, redemptions and maturities of other investments	1,643,000		1,162,707		443,630
Proceeds from redemptions and maturities of fixed maturity investments	762,995		731,708		1,115,594
Net (purchases) sales of short-term investments	166,112		-356,250		189,027
Change in investment of securities lending collateral	57,470		-55,643		6,190
Contributions to subsidiaries	-128,825		-20,250		-38,576
Intercompany loans issued	-7,500		-10,250
Repayments of intercompany loans	38,964
Purchase of business, net of cash acquired	-237,106				28,948
Purchases of furniture, equipment and other assets	-19,612		-16,787		-18,467
Net Cash Used For Investing Activities	-1,951,027		-942,938		-431,371
Financing Activities
Proceeds from issuance of Series C preferred shares, net					0
Repurchase of Series A and B preferred shares					0
Purchases of common shares under share repurchase program	0		0		0
Proceeds from common shares issued, net	470,532		118,260		38,576
Proceeds from intercompany loans	7,500		0
Repayments of intercompany loans	-28,714
Proceeds from borrowings			0
Repayments of borrowings					-310,868
Change in securities lending collateral	-57,470		55,643		-6,190
Third party investment in non-redeemable noncontrolling interests	796,903	[2]
Third party investment in redeemable noncontrolling interests	219,233	[2]
Dividends paid to redeemable noncontrolling interests	-14,448	[2]
Dividends paid to parent	-515,700	[1]	-119,500	[1]	-258,490	[1]
Other	64,973		50,830		6,664
Preferred dividends paid	0		0		0
Net Cash Used For Financing Activities	942,809		105,233		-530,308
Effects of exchange rate changes on foreign currency cash	-20,007		-4,357		6,742
Increase (decrease) in cash	49,372		66,313		23,742
Cash beginning of year	430,325		364,012		340,270
Cash end of period	479,697		430,325		364,012
Consolidating adjustments and eliminations
Operating Activities
Net Cash Provided by (Used in) Operating Activities	-515,700		-119,500		-258,490
Investing Activities
Purchases of fixed maturity investments	0		0		0
Purchases of equity securities	0		0		0
Purchases of other investments	0		0		0
Proceeds from the sale of fixed maturity investments	0		0		0
Proceeds from the sale of equity securities	0		0		0
Proceeds from sales, redemptions and maturities of other investments	0		0		0
Proceeds from redemptions and maturities of fixed maturity investments	0		0		0
Net (purchases) sales of short-term investments	0		0		0
Change in investment of securities lending collateral	0		0		0
Contributions to subsidiaries	470,532		118,260		38,576
Intercompany loans issued	15,000		10,250
Repayments of intercompany loans	-38,964
Purchase of business, net of cash acquired	0				0
Purchases of furniture, equipment and other assets	0		0		0
Net Cash Used For Investing Activities	446,568		128,510		38,576
Financing Activities
Proceeds from issuance of Series C preferred shares, net					0
Repurchase of Series A and B preferred shares					0
Purchases of common shares under share repurchase program	0		0		0
Proceeds from common shares issued, net	-470,532		-118,260		-38,576
Proceeds from intercompany loans	-15,000		-10,250
Repayments of intercompany loans	38,964
Proceeds from borrowings			0
Repayments of borrowings					0
Change in securities lending collateral	0		0		0
Third party investment in non-redeemable noncontrolling interests	0	[2]
Third party investment in redeemable noncontrolling interests	0	[2]
Dividends paid to redeemable noncontrolling interests	0	[2]
Dividends paid to parent	515,700	[1]	119,500	[1]	258,490	[1]
Other	0		0		0
Preferred dividends paid	0		0		0
Net Cash Used For Financing Activities	69,132		-9,010		219,914
Effects of exchange rate changes on foreign currency cash	0		0		0
Increase (decrease) in cash	0		0		0
Cash beginning of year	0		0		0
Cash end of period	$0		$0		$0
[1]	Included in net cash provided by (used for) operating activities in the ACGL (Parent Guarantor) column.
[2]	See Note 4.

Schedule_III_Supplementary_Ins1

Schedule III - Supplementary Insurance Information (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs, Net	$414,525		$342,314		$262,822
Reserves for Losses and Loss Adjustment Expenses	9,036,448		8,824,696		8,933,292
Unearned Premiums	2,231,578		1,896,365		1,647,978
Net Premiums Earned	3,593,748		3,145,952		2,935,140
Net Losses and Loss Adjustment Expenses Incurred	1,919,250		1,679,424		1,861,277
Amortization of Deferred Acquisition Costs	657,262		564,103		508,884
Other Operating Expenses	556,280	[1]	458,327	[1]	430,035	[1]
Net Premiums Written	3,891,938		3,351,367		3,052,235
Insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs, Net	178,545		158,121		141,962
Reserves for Losses and Loss Adjustment Expenses	6,161,500		6,137,121		6,149,247
Unearned Premiums	1,237,099		1,192,188		1,077,211
Net Premiums Earned	2,017,370		1,876,014		1,800,343
Net Losses and Loss Adjustment Expenses Incurred	1,260,953		1,188,445		1,283,841
Amortization of Deferred Acquisition Costs	316,308		311,904		298,983
Other Operating Expenses	335,157	[1]	315,387	[1]	307,489	[1]
Net Premiums Written	2,146,654		1,948,796		1,825,334
Reinsurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs, Net	148,130		167,642		106,677
Reserves for Losses and Loss Adjustment Expenses	2,690,070		2,680,288		2,781,424
Unearned Premiums	666,233		612,725		517,619
Net Premiums Earned	1,279,328		1,218,672		1,118,127
Net Losses and Loss Adjustment Expenses Incurred	532,450		486,236		574,821
Amortization of Deferred Acquisition Costs	261,438		234,373		204,903
Other Operating Expenses	147,964	[1]	134,563	[1]	118,245	[1]
Net Premiums Written	1,265,991		1,313,001		1,158,790
Mortgage
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs, Net	38,321		16,551		14,183
Reserves for Losses and Loss Adjustment Expenses	118,550		7,287		2,621
Unearned Premiums	148,232		91,452		53,148
Net Premiums Earned	193,573		51,266		16,670
Net Losses and Loss Adjustment Expenses Incurred	55,674		4,743		2,615
Amortization of Deferred Acquisition Costs	49,400		17,826		4,998
Other Operating Expenses	66,891	[1]	8,377	[1]	4,301	[1]
Net Premiums Written	204,837		89,570		68,111
Other
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs, Net	49,529
Reserves for Losses and Loss Adjustment Expenses	66,328
Unearned Premiums	180,014
Net Premiums Earned	103,477
Net Losses and Loss Adjustment Expenses Incurred	70,173
Amortization of Deferred Acquisition Costs	30,116
Other Operating Expenses	6,268	[1]
Net Premiums Written	$274,456
[1]	Certain other operating expenses relate to the Company’s corporate segment (non-underwriting). Such amounts are not reflected in the table above. See Note 5.

Schedule_IV_Reinsurance_Detail

Schedule IV - Reinsurance (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount									$3,100,946		$2,754,582		$2,673,864
Ceded to other Companies									-948,678		-845,256		-816,926
Assumed From Other Companies									1,739,670		1,442,041		1,195,297
Net Amount	895,481	959,539	971,928	1,064,990	748,921	839,135	810,535	952,776	3,891,938		3,351,367		3,052,235
Percentage of Amount Assumed to Net									44.70%		43.00%		39.20%
Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount									2,974,996		2,682,446		2,562,788
Ceded to other Companies									-862,015	[1]	-763,713	[1]	-768,625	[1]
Assumed From Other Companies									33,673	[1]	30,063	[1]	31,171	[1]
Net Amount									2,146,654		1,948,796		1,825,334
Percentage of Amount Assumed to Net									1.60%		1.50%		1.70%
Reinsurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount									22,405		72,136		111,076
Ceded to other Companies									-261,255	[1]	-86,620	[1]	-55,099	[1]
Assumed From Other Companies									1,504,841	[1]	1,327,485	[1]	1,102,813	[1]
Net Amount									1,265,991		1,313,001		1,158,790
Percentage of Amount Assumed to Net									118.90%		101.10%		95.20%
Mortgage
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount									103,545		0		0
Ceded to other Companies									-22,519	[1]	0	[1]	0	[1]
Assumed From Other Companies									123,811	[1]	89,570	[1]	68,111	[1]
Net Amount									204,837		89,570		68,111
Percentage of Amount Assumed to Net									60.40%		100.00%		100.00%
Other
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount									0
Ceded to other Companies									-14,171	[1]	0		0
Assumed From Other Companies									288,627	[1]
Net Amount									$274,456		$0		$0
Percentage of Amount Assumed to Net									105.20%
[1]	Certain amounts included in the gross premiums written of each segment are related to intersegment transactions and are included in the gross premiums written of each segment. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.

Schedule_VI_Supplementary_Info1

Schedule VI - Supplementary Information For Property and Casualty Insurance Underwriters (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Net investment income	$82,496	$80,105	$72,990	$66,994	$67,095	$66,083	$68,369	$65,672	$302,585	$267,219	$294,895
Net Losees and Loss Adjustment Expenses Incurred Related to Current Year									2,246,152	1,943,466	2,082,805
Net Losees and Loss Adjustment Expenses Incurred Related to Prior Years									-326,902	-264,042	-221,528
Consolidated Subsidiaries
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Acquisition Costs, Net	414,525				342,314				414,525	342,314	262,822
Reserves for Losses and Loss Adjustment Expenses	9,036,448				8,824,696				9,036,448	8,824,696	8,933,292
Discount, if any, deducted in Column C	14,732				12,539				14,732	12,539	10,485
Unearned Premiums	2,231,578				1,896,365				2,231,578	1,896,365	1,647,978
Net Premiums Earned									3,593,748	3,145,952	2,935,140
Net Losees and Loss Adjustment Expenses Incurred Related to Current Year									2,246,152	1,943,466	2,082,805
Net Losees and Loss Adjustment Expenses Incurred Related to Prior Years									-326,902	-264,042	-221,528
Amortization of Deferred Acquisition Costs									657,262	564,103	508,884
Net Paid Losses and Loss Adjustment Expenses									1,697,736	1,708,817	1,465,379
Net Premiums Written									$3,891,938	$3,351,367	$3,052,235