Arch Capital (ACGL) 10-Q 8 Aug 14 2014 Q2 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	6 Months Ended
	Jun. 30, 2014	Jul. 31, 2014
Document and Entity Information [Abstract]	'	'
Entity Registrant Name	'ARCH CAPITAL GROUP LTD.	'
Entity Central Index Key	'0000947484	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Accelerated Filer	'
Document Type	'10-Q	'
Document Period End Date	30-Jun-14	'
Document Fiscal Year Focus	'2014	'
Document Fiscal Period Focus	'Q2	'
Amendment Flag	'false	'
Entity Common Stock, Shares Outstanding	'	135,112,921

Unaudited_Consolidated_Balance

Unaudited Consolidated Balance Sheets (USD $)	Jun. 30, 2014		Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Investments:	'		'	'	'
Fixed maturities available for sale, at fair value (amortized cost: $10,599,911 and $9,564,634)	$10,714,532		'	$9,571,776	'
Short-term investments available for sale, at fair value (amortized cost: $977,347 and $1,477,584)	977,058		'	1,478,367	'
Investment of funds received under securities lending, at fair value (amortized cost: $79,242 and $97,943)	82,603		'	100,584	'
Equity securities available for sale, at fair value (cost: $513,094 and $433,275)	608,820		'	496,824	'
Other investments available for sale, at fair value (cost: $428,958 and $488,687)	457,567		'	498,310	'
Investments accounted for using the fair value option	2,041,091		'	1,221,534	'
Investments accounted for using the equity method	281,464		'	244,339	'
Total investments	15,163,135		'	13,611,734	'
Cash	926,443		'	434,057	371,041
Accrued investment income	64,869		'	66,848	'
Investment in joint venture (cost: $100,000)	103,934		'	104,856	'
Fixed maturities and short-term investments pledged under securities lending, at fair value	87,031		'	105,081	'
Premiums receivable	1,098,692		'	753,924	'
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	1,796,403		'	1,804,330	'
Contractholder receivables	1,234,392		'	1,064,246	'
Prepaid reinsurance premiums	430,214		'	328,343	'
Deferred acquisition costs, net	399,385		'	342,314	'
Receivable for securities sold	261,669		'	50,555	'
Goodwill and intangible assets	118,721		'	27,319	'
Other assets	888,627		'	872,487	'
Total assets	22,573,515		'	19,566,094	'
Liabilities	'		'	'	'
Reserve for losses and loss adjustment expenses	9,018,989		'	8,824,696	'
Unearned premiums	2,299,692		'	1,896,365	'
Reinsurance balances payable	263,347		'	196,167	'
Contractholder payables	1,234,392		'	1,064,246	'
Deposit accounting liabilities	397,337		'	421,297	'
Senior notes	800,000		'	800,000	'
Revolving credit agreement borrowings	100,000		'	100,000	'
Securities lending payable	89,298		'	107,999	'
Payable for securities purchased	552,075		'	51,318	'
Other liabilities	577,320		'	456,510	'
Total liabilities	15,332,450		'	13,918,598	'
Commitments and Contingencies	'		'	'	'
Redeemable noncontrolling interests	219,326		'	0	'
Shareholders' Equity	'		'	'	'
Non-cumulative preferred shares	325,000		'	325,000	'
Common shares ($0.0033 par, shares issued: 171,096,771 and 169,560,591)	570		'	565	'
Additional paid-in capital	353,208		'	299,517	'
Retained earnings	6,421,701		'	6,042,154	'
Accumulated other comprehensive income, net of deferred income tax	233,883		'	74,964	'
Common shares held in treasury, at cost (shares: 36,065,885 and 35,885,707)	-1,104,963		'	-1,094,704	'
Total shareholders' equity available to Arch	6,229,399		'	5,647,496	'
Non-redeemable noncontrolling interests (1)	792,340	[1]	'	0	'
Total shareholders' equity	7,021,739		'	5,647,496	'
Total liabilities, noncontrolling interests and shareholders' equity	$22,573,515		'	$19,566,094	'
[1]	See Note 4.

Unaudited_Consolidated_Balance1

Unaudited Consolidated Balance Sheets (Parentheticals) (USD $)	Jun. 30, 2014	Dec. 31, 2013
In Thousands, except Share data, unless otherwise specified
Fixed maturities available for sale, at amortized cost	$10,599,911	$9,564,634
Short-term investments available for sale, at amortized cost	977,347	1,477,584
Investment of funds received under securities lending agreements, at cost	79,242	97,943
Equity securities, at cost	513,094	433,275
Other investments, at cost	428,958	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)	171,096,771	169,560,591
Common shares held in treasury (shares)	36,065,885	35,885,707

Unaudited_Consolidated_Stateme

Unaudited Consolidated Statements of Income (USD $)	3 Months Ended				6 Months Ended
In Thousands, except Share data, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Revenues	'		'		'		'
Net premiums written	$971,928		$810,535		$2,036,918		$1,763,311
Change in unearned premiums	-64,776		-51,719		-269,986		-251,725
Net premiums earned	907,152		758,816		1,766,932		1,511,586
Net investment income	72,990		68,369		139,984		134,041
Net realized gains	54,144		12,652		73,841		70,992
Other-than-temporary impairment losses	-14,749		-724		-17,720		-2,972
Less investment impairments recognized in other comprehensive income, before taxes	0		0		0		2
Net impairment losses recognized in earnings	-14,749		-724		-17,720		-2,970
Other underwriting income	2,033		902		3,615		1,440
Equity in net income of investment funds accounted for using the equity method	9,240		10,941		12,493		24,764
Other income (loss)	4,850		834		2,746		2,078
Total revenues	1,035,660		851,790		1,981,891		1,741,931
Expenses	'		'		'		'
Losses and loss adjustment expenses	485,518		418,653		921,758		818,056
Acquisition expenses	158,158		131,677		318,500		259,269
Other operating expenses	156,350		127,408		302,149		247,591
Interest expense	14,334		5,852		28,738		11,750
Net foreign exchange losses (gains)	2,294		-13,811		8,857		-38,075
Total expenses	816,654		669,779		1,580,002		1,298,591
Income before income taxes	219,006		182,011		401,889		443,340
Income tax expense	-7,289		-5,071		-11,027		-9,924
Net income	211,717		176,940		390,862		433,416
Amounts attributable to noncontrolling interests	-3,701	[1]	0	[1]	-346	[1]	0	[1]
Net income available to Arch	208,016		176,940		390,516		433,416
Preferred dividends	-5,485		-5,485		-10,969		-10,969
Net income available to common shareholders	$202,531		$171,455		$379,547		$422,447
Net income per common share	'		'		'		'
Basic (per share)	$1.53		$1.31		$2.87		$3.22
Diluted (per share)	$1.48		$1.26		$2.78		$3.11
Weighted average common shares and common share equivalents outstanding	'		'		'		'
Basic (shares)	132,650,634		131,377,274		132,256,462		131,143,885
Diluted (shares)	136,889,944		135,849,050		136,716,889		135,624,226
[1]	See Note 4.

Unaudited_Consolidated_Stateme1

Unaudited Consolidated Statements of Comprehensive Income (USD $)	3 Months Ended				6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Comprehensive Income (Loss)	'		'		'		'
Net income	$211,717		$176,940		$390,862		$433,416
Unrealized appreciation (decline) in value of available-for-sale investments:	'		'		'		'
Unrealized holding gains (losses) arising during period	108,428		-259,562		179,781		-250,091
Portion of other-than-temporary impairment losses recognized in other comprehensive income, net of deferred income tax	0		0		0		-2
Reclassification of net realized gains, net of income taxes, included in net income	-8,285		-13,916		-29,534		-52,617
Foreign currency translation adjustments	10,021		-5,407		8,672		-33,629
Comprehensive income (loss)	321,881		-101,945		549,781		97,077
Amounts attributable to noncontrolling interests	-3,701	[1]	0	[1]	-346	[1]	0	[1]
Comprehensive income (loss) available to Arch	$318,180		($101,945)		$549,435		$97,077
[1]	See Note 4.

Unaudited_Consolidated_Stateme2

Unaudited 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 (Loss)	Unrealized gains on available-for-sale securities	Foreign currency translation adjustments	Common Shares Held in Treasury, at Cost
In Thousands
Balance at beginning of year at Dec. 31, 2012		'	$325,000	$561	$227,778	$5,354,361	$287,017	$289,956	($2,939)	($1,025,839)
Common shares issued, net		'	'	3	5,362	'	'	'	'	'
Exercise of stock options		'	'	'	6,022	'	'	'	'	'
Amortization of share-based compensation		'	'	'	31,466	'	'	'	'	'
Other		'	'	'	2,327	'	'	'	'	'
Net income		433,416	'	'	'	433,416	'	'	'	'
Amounts attributable to noncontrolling interests	[1]	0	'	'	'	0	'	'	'	'
Preferred share dividends		-10,969	'	'	'	-10,969	'	'	'	'
Unrealized holding gains (losses) arising during period, net of reclassification adjustment		'	'	'	'	'	'	-302,708	'	'
Portion of other-than-temporary impairment losses recognized in other comprehensive income, net of deferred income tax		-2	'	'	'	'	'	-2	'	'
Foreign currency translation adjustments		-33,629	'	'	'	'	'	'	-33,629	'
Shares repurchased for treasury		'	'	'	'	'	'	'	'	-65,848
Noncontrolling interest	[1]	0	'	'	'	'	'	'	'	'
Total shareholders' equity at Jun. 30, 2013		5,234,318	'	'	'	'	'	'	'	'
Balance at end of period at Jun. 30, 2013		5,234,318	325,000	564	272,955	5,776,808	-49,322	-12,754	-36,568	-1,091,687
Balance at beginning 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
Common shares issued, net		'	'	5	6,360	'	'	'	'	'
Exercise of stock options		'	'	'	11,233	'	'	'	'	'
Amortization of share-based compensation		'	'	'	35,627	'	'	'	'	'
Other		'	'	'	471	'	'	'	'	'
Net income		390,862	'	'	'	390,862	'	'	'	'
Amounts attributable to noncontrolling interests	[1]	-346	'	'	'	-346	'	'	'	'
Preferred share dividends		-10,969	'	'	'	-10,969	'	'	'	'
Unrealized holding gains (losses) arising during period, net of reclassification adjustment		'	'	'	'	'	'	150,247	'	'
Portion of other-than-temporary impairment losses recognized in other comprehensive income, net of deferred income tax		0	'	'	'	'	'	0	'	'
Foreign currency translation adjustments		8,672	'	'	'	'	'	'	8,672	'
Shares repurchased for treasury		'	'	'	'	'	'	'	'	-10,259
Noncontrolling interest	[1]	792,340	'	'	'	'	'	'	'	'
Total shareholders' equity at Jun. 30, 2014		7,021,739	'	'	'	'	'	'	'	'
Balance at end of period at Jun. 30, 2014		$6,229,399	$325,000	$570	$353,208	$6,421,701	$233,883	$230,939	$2,944	($1,104,963)
[1]	See Note 4.

Unaudited_Consolidated_Stateme3

Unaudited Consolidated Statements of Cash Flows (USD $)	6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013
Operating Activities	'		'
Net income	$390,862		$433,416
Adjustments to reconcile net income to net cash provided by operating activities:	'		'
Net realized gains	-87,520		-73,611
Net impairment losses recognized in earnings	17,720		2,970
Equity in net income or loss of investment funds accounted for using the equity method and other income or loss	-135		37,493
Share-based compensation	35,627		31,466
Changes in:	'		'
Reserve for losses and loss adjustment expenses, net of unpaid losses and loss adjustment expenses recoverable	60,474		-33,163
Unearned premiums, net of prepaid reinsurance premiums	269,986		251,725
Premiums receivable	-325,953		-205,044
Deferred acquisition costs, net	-55,822		-51,971
Reinsurance balances payable	65,803		24,267
Other liabilities	43,133		-26,981
Other items	38,888		-2,213
Net Cash Provided By Operating Activities	453,063		388,354
Investing Activities	'		'
Purchases of fixed maturity investments	-14,311,748		-8,599,697
Purchases of equity securities	-174,687		-272,323
Purchases of other investments	-1,022,987		-648,915
Proceeds from the sale of fixed maturity investments	13,204,854		8,468,641
Proceeds from the sale of equity securities	98,687		194,212
Proceeds from the sale of other investments	618,707		506,434
Proceeds from redemptions and maturities of fixed maturity investments	432,040		424,953
Net sales (purchases) of short-term investments	430,304		-375,146
Change in investment of securities lending collateral	18,701		3,221
Purchase of business, net of cash acquired	-235,578	[1]	0	[1]
Purchases of furniture, equipment and other assets	-10,360		-7,092
Net Cash Used For Investing Activities	-952,067		-305,712
Financing Activities	'		'
Purchases of common shares under share repurchase program	0		-56,463
Proceeds from common shares issued, net	2,521		-517
Change in securities lending collateral	-18,701		-3,221
Third party investment in non-redeemable noncontrolling interests	796,903	[2]	0	[2]
Third party investment in redeemable noncontrolling interests	219,233	[2]	0	[2]
Dividends paid to redeemable noncontrolling interests	-4,816	[2]	0	[2]
Other	4,706		5,042
Preferred dividends paid	-10,969		-10,969
Net Cash Provided By (Used For) Financing Activities	988,877		-66,128
Effects of exchange rate changes on foreign currency cash	2,513		-12,436
Increase in cash	492,386		4,078
Cash beginning of year	434,057		371,041
Cash end of period	$926,443		$375,119
[1]	See Note 2.
[2]	See Note 4.

General

General	6 Months Ended
	Jun. 30, 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 subsidiaries (together with ACGL, the “Company”).
	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. (“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”). Watford is considered a variable interest entity (“VIE”) and the Company concluded that it is the primary beneficiary of Watford. As such, the results of Watford are included in the Company’s consolidated financial statements (see Note 4).
	The interim 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 the Company and Watford. 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. In the opinion of management, the accompanying unaudited interim consolidated financial statements reflect all adjustments (consisting of normally recurring accruals) necessary for a fair statement of results on an interim basis. The results of any interim period are not necessarily indicative of the results for a full year or any future periods.
	Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted; however, management believes that the disclosures are adequate to make the information presented not misleading. This report should be read in conjunction with the Company’s Annual Report on Form 10-K for the year ended December 31, 2013 (“2013 Form 10-K”), including the Company’s audited consolidated financial statements and related notes.
	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, comprehensive 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	6 Months Ended
	Jun. 30, 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 Fannie Mae and Freddie Mac (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. 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 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 to PMI, 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 ($53.1 million at June 30, 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 June 30, 2014, the Company recorded amortization expense of $9.3 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 6).

Significant_Accounting_Policie

Significant Accounting Policies	6 Months Ended
	Jun. 30, 2014
Accounting Policies [Abstract]	'
Significant Accounting Policies	'
	Significant Accounting Policies
	As discussed in Note 2, the Company completed its acquisition of the CMG Entities on January 30, 2014. As such, the Company has added relevant sections pertaining to mortgage insurance to its significant accounting policies as described in Note 2, “Significant Accounting Policies,” of its audited consolidated financial statements and related notes of the 2013 Form 10-K.
	(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 expected life 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 operations.
	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 charged against revenue 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 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 and anticipated investment income. A premium deficiency 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.
	(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 due from insureds 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
	(p) 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 investment income. 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.
	(q) 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 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 realizable values with the related expense recorded in the Company’s results of operations.

Variable_Interest_Entities_and

Variable Interest Entities and Noncontrolling Interests	6 Months Ended
	Jun. 30, 2014
Noncontrolling Interest [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 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’s reinsurance manager, and Highbridge Principal Strategies, LLC (“Highbridge”), a subsidiary of JPMorgan Chase & Co., manages Watford’s investment assets, each under separate long term services agreements. John Rathgeber, previously Vice Chairman of Arch Worldwide Reinsurance Group, was named CEO of Watford. 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.
	The Company concluded that Watford 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. In addition, these agreements allow for both AUL and Highbridge to participate in the favorable results of Watford in the form of performance fees. To determine if the Company is the primary beneficiary of Watford, the Company concluded that the most significant activity of Watford 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 the relationship with Watford 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’s CEO resulted in the Company concluding that it is the primary beneficiary of Watford. As such, the results of Watford are included in the Company’s consolidated financial statements.
	The Company concluded that Watford represents a separate operating segment and provides the income statement and total investable assets, total assets and total liabilities of Watford within Note 6. Because Watford is an independent company, the assets of Watford can be used only to settle obligations of Watford and Watford is solely responsible for its own liabilities and commitments. The Company’s financial exposure to Watford is limited to its investment in Watford’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’s common equity attributable to third party investors in the shareholders’ equity section of its consolidated balance sheets. The noncontrolling ownership in Watford’s common shares was approximately 89% at June 30, 2014. The portion of Watford’s income or loss attributable to third party investors is recorded in the consolidated statements of income in ‘amounts attributable to noncontrolling interests.’ The following table sets forth activity in the non-redeemable noncontrolling interests:
		Three Months Ended June 30, 2014			Six Months Ended June 30, 2014
	Balance, beginning of period	$	793,496		$	—
	Sale of shares to noncontrolling interests	—			796,903
	Amounts attributable to noncontrolling interests	(1,156		)	(4,563		)
	Balance, end of period	$	792,340		$	792,340
	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’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 IRSB18. The Watford Preference Shares may be redeemed by Watford 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, 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 June 30, 2014. Preferred dividends, including the accretion of the discount and issuance costs, are included in ‘amounts attributable to noncontrolling interests’ in the Company’s consolidated statements of income.

Earnings_Per_Common_Share

Earnings Per Common Share	6 Months Ended
	Jun. 30, 2014
Earnings Per Share [Abstract]	'
Earnings Per Common Share	'
	Earnings Per Common Share
	The following table sets forth the computation of basic and diluted earnings per common share:
		Three Months Ended						Six Months Ended
		June 30,						June 30,
		2014			2013			2014			2013
	Numerator:
	Net income	$	211,717		$	176,940		$	390,862		$	433,416
	Amounts attributable to noncontrolling interests	(3,701		)	—			(346		)	—
	Net income available to Arch	208,016			176,940			390,516			433,416
	Preferred dividends	(5,485		)	(5,485		)	(10,969		)	(10,969		)
	Net income available to Arch common shareholders	$	202,531		$	171,455		$	379,547		$	422,447
	Denominator:
	Weighted average common shares outstanding — basic	132,650,634			131,377,274			132,256,462			131,143,885
	Effect of dilutive common share equivalents:
	Nonvested restricted shares	1,144,621			1,088,030			1,236,408			1,181,947
	Stock options (1)	3,094,689			3,383,746			3,224,019			3,298,394
	Weighted average common shares and common share equivalents outstanding — diluted	136,889,944			135,849,050			136,716,889			135,624,226
	Earnings per common share:
	Basic	$	1.53		$	1.31		$	2.87		$	3.22
	Diluted	$	1.48		$	1.26		$	2.78		$	3.11
	_________________________________________________
	-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 the 2014 second quarter and 2013 second quarter, the number of stock options excluded were 978,237 and 1,428,616, respectively. For the six months ended June 30, 2014 and 2013, the number of stock options excluded were 1,318,662 and 1,730,313, respectively.

Segment_Information

Segment Information	6 Months Ended
	Jun. 30, 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 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 ‘other’ segment includes the results of Watford (see Note 4). Watford 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 following tables summarize the Company’s underwriting income or loss by segment, together with a reconciliation of underwriting income or loss to net income available to common shareholders:
		Three Months Ended
		30-Jun-14
		Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
	Gross premiums written (1)	$	852,231		$	349,841		$	55,476		$	1,256,934		$	54,562		$	1,271,761
	Premiums ceded	(273,349		)	(58,994		)	(5,079		)	(336,808		)	(2,760		)	(299,833		)
	Net premiums written	578,882			290,847			50,397			920,126			51,802			971,928
	Change in unearned premiums	(71,170		)	44,780			436			(25,954		)	(38,822		)	(64,776		)
	Net premiums earned	507,712			335,627			50,833			894,172			12,980			907,152
	Other underwriting income	514			303			1,216			2,033			—			2,033
	Losses and loss adjustment expenses	(311,526		)	(150,325		)	(15,473		)	(477,324		)	(8,194		)	(485,518		)
	Acquisition expenses, net	(76,449		)	(66,035		)	(11,481		)	(153,965		)	(4,193		)	(158,158		)
	Other operating expenses	(85,829		)	(37,666		)	(16,288		)	(139,783		)	(1,635		)	(141,418		)
	Underwriting income (loss)	$	34,422		$	81,904		$	8,807		125,133			(1,042		)	124,091
	Net investment income										72,458			532			72,990
	Net realized gains										50,966			3,178			54,144
	Net impairment losses recognized in earnings										(14,749		)	—			(14,749		)
	Equity in net income of investment funds accounted for using the equity method										9,240			—			9,240
	Other income (loss)										4,850			—			4,850
	Other expenses										(15,279		)	347			(14,932		)
	Interest expense										(14,334		)	—			(14,334		)
	Net foreign exchange gains (losses)										(2,764		)	470			(2,294		)
	Income before income taxes										215,521			3,485			219,006
	Income tax expense										(7,289		)	—			(7,289		)
	Net income										208,232			3,485			211,717
	Dividends attributable to redeemable noncontrolling interests										—			(4,857		)	(4,857		)
	Amounts attributable to noncontrolling interests										—			1,156			1,156
	Net income available to Arch										208,232			(216		)	208,016
	Preferred dividends										(5,485		)	—			(5,485		)
	Net income available to Arch common shareholders										$	202,747		$	(216	)	$	202,531
	Underwriting Ratios
	Loss ratio	61.4		%	44.8		%	30.4		%	53.4		%	63.1		%	53.5		%
	Acquisition expense ratio (2)	15		%	19.7		%	22.6		%	17.2		%	32.3		%	17.4		%
	Other operating expense ratio	16.9		%	11.2		%	32		%	15.6		%	12.6		%	15.6		%
	Combined ratio	93.3		%	75.7		%	85		%	86.2		%	108		%	86.5		%
	Goodwill and intangible assets	$	24,498		$	4,942		$	89,281		$	118,721		$	—		$	118,721
	Total investable assets										$	14,688,881		$	1,114,719		$	15,803,600
	Total assets										21,210,197			1,363,318			22,573,515
	Total liabilities										15,078,943			253,507			15,332,450
	_________________________________________________
	-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.
	-2	The acquisition expense ratio is adjusted to include certain other underwriting income.
		Three Months Ended
		30-Jun-13
		Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
	Gross premiums written (1)	$	703,904		$	318,898		$	18,744		$	1,040,738		$	—		$	1,040,738
	Premiums ceded	(202,336		)	(28,675		)	—			(230,203		)	—			(230,203		)
	Net premiums written	501,568			290,223			18,744			810,535			—			810,535
	Change in unearned premiums	(42,912		)	(2,731		)	(6,076		)	(51,719		)	—			(51,719		)
	Net premiums earned	458,656			287,492			12,668			758,816			—			758,816
	Other underwriting income	529			373			—			902			—			902
	Losses and loss adjustment expenses	(291,192		)	(125,283		)	(2,178		)	(418,653		)	—			(418,653		)
	Acquisition expenses, net	(74,249		)	(53,291		)	(4,137		)	(131,677		)	—			(131,677		)
	Other operating expenses	(80,167		)	(31,902		)	(1,290		)	(113,359		)	—			(113,359		)
	Underwriting income	$	13,577		$	77,389		$	5,063		96,029			—			96,029
	Net investment income										68,369			—			68,369
	Net realized gains										12,652			—			12,652
	Net impairment losses recognized in earnings										(724		)	—			(724		)
Equity in net income of investment funds accounted for using the equity method										10,941			—			10,941
Other income (loss)										834			—			834
Other expenses										(14,049		)	—			(14,049		)
Interest expense										(5,852		)	—			(5,852		)
Net foreign exchange gains										13,811			—			13,811
Income before income taxes										182,011			—			182,011
Income tax expense										(5,071		)	—			(5,071		)
Net income										176,940			—			176,940
Dividends attributable to redeemable noncontrolling interests										—			—			—
Amounts attributable to noncontrolling interests										—			—			—
Net income available to Arch										176,940			—			176,940
Preferred dividends										(5,485		)	—			(5,485		)
Net income available to Arch common shareholders										$	171,455		$	—		$	171,455
Underwriting Ratios
Loss ratio	63.5		%	43.6		%	17.2		%	55.2		%	—		%	55.2		%
Acquisition expense ratio (2)	16.1		%	18.5		%	32.7		%	17.3		%	—		%	17.3		%
Other operating expense ratio	17.5		%	11.1		%	10.2		%	14.9		%	—		%	14.9		%
Combined ratio	97.1		%	73.2		%	60.1		%	87.4		%	—		%	87.4		%
Goodwill and intangible assets	$	20,888		$	10,072		$	—		$	30,960		$	—		$	30,960
Total investable assets										$	12,960,811		$	—		$	12,960,811
Total assets										18,917,683			—			18,917,683
Total liabilities										13,683,365			—			13,683,365
_________________________________________________
-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.
-2	The acquisition expense ratio is adjusted to include certain other underwriting income.
	Six Months Ended
	30-Jun-14
	Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
Gross premiums written (1)	$	1,582,877		$	866,894		$	103,383		$	2,552,070		$	86,756		$	2,566,897
Premiums ceded	(458,393		)	(132,121		)	(9,718		)	(599,148		)	(2,760		)	(529,979		)
Net premiums written	1,124,484			734,773			93,665			1,952,922			83,996			2,036,918
Change in unearned premiums	(139,271		)	(57,798		)	(4,067		)	(201,136		)	(68,850		)	(269,986		)
Net premiums earned	985,213			676,975			89,598			1,751,786			15,146			1,766,932
Other underwriting income	1,014			619			1,982			3,615			—			3,615
Losses and loss adjustment expenses	(598,296		)	(289,961		)	(23,951		)	(912,208		)	(9,550		)	(921,758		)
Acquisition expenses, net	(153,381		)	(139,468		)	(20,635		)	(313,484		)	(5,016		)	(318,500		)
Other operating expenses	(166,973		)	(73,861		)	(30,164		)	(270,998		)	(2,744		)	(273,742		)
Underwriting income (loss)	$	67,577		$	174,304		$	16,830		258,711			(2,164		)	256,547
Net investment income										139,451			533			139,984
Net realized gains										70,663			3,178			73,841
Net impairment losses recognized in earnings										(17,720		)	—			(17,720		)
Equity in net income of investment funds accounted for using the equity method										12,493			—			12,493
Other income (loss)										2,746			—			2,746
Other expenses										(26,078		)	(2,329		)	(28,407		)
Interest expense										(28,738		)	—			(28,738		)
Net foreign exchange gains (losses)										(9,420		)	563			(8,857		)
Income before income taxes										402,108			(219		)	401,889
Income tax expense										(11,027		)	—			(11,027		)
Net income (loss)										391,081			(219		)	390,862
Dividends attributable to redeemable noncontrolling interests										—			(4,909		)	(4,909		)
Amounts attributable to noncontrolling interests										—			4,563			4,563
Net income available to Arch										391,081			(565		)	390,516
Preferred dividends										(10,969		)	—			(10,969		)
Net income available to Arch common shareholders										$	380,112		$	(565	)	$	379,547
Underwriting Ratios
Loss ratio	60.7		%	42.8		%	26.7		%	52.1		%	63.1		%	52.2		%
Acquisition expense ratio (2)	15.5		%	20.6		%	23		%	17.8		%	33.1		%	18		%
Other operating expense ratio	16.9		%	10.9		%	33.7		%	15.5		%	18.1		%	15.5		%
Combined ratio	93.1		%	74.3		%	83.4		%	85.4		%	114.3		%	85.7		%
_________________________________________________
-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.
-2	The acquisition expense ratio is adjusted to include certain other underwriting income.
	Six Months Ended
	30-Jun-13
	Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
Gross premiums written (1)	$	1,392,721		$	769,345		$	44,502		$	2,204,437		$	—		$	2,204,437
Premiums ceded	(386,603		)	(56,654		)	—			(441,126		)	—			(441,126		)
Net premiums written	1,006,118			712,691			44,502			1,763,311			—			1,763,311
Change in unearned premiums	(102,497		)	(129,046		)	(20,182		)	(251,725		)	—			(251,725		)
Net premiums earned	903,621			583,645			24,320			1,511,586			—			1,511,586
Other underwriting income	1,054			386			—			1,440			—			1,440
Losses and loss adjustment expenses	(574,659		)	(239,140		)	(4,257		)	(818,056		)	—			(818,056		)
Acquisition expenses, net	(145,007		)	(106,434		)	(7,828		)	(259,269		)	—			(259,269		)
Other operating expenses	(156,482		)	(64,099		)	(2,693		)	(223,274		)	—			(223,274		)
Underwriting income	$	28,527		$	174,358		$	9,542		212,427			—			212,427
Net investment income										134,041			—			134,041
Net realized gains										70,992			—			70,992
Net impairment losses recognized in earnings										(2,970		)	—			(2,970		)
Equity in net income of investment funds accounted for using the equity method										24,764			—			24,764
Other income (loss)										2,078			—			2,078
Other expenses										(24,317		)	—			(24,317		)
Interest expense										(11,750		)	—			(11,750		)
Net foreign exchange gains										38,075			—			38,075
Income before income taxes										443,340			—			443,340
Income tax expense										(9,924		)	—			(9,924		)
Net income										433,416			—			433,416
Dividends attributable to redeemable noncontrolling interests										—			—			—
Amounts attributable to noncontrolling interests										—			—			—
Net income available to Arch										433,416			—			433,416
Preferred dividends										(10,969		)	—			(10,969		)
Net income available to Arch common shareholders										$	422,447		$	—		$	422,447
Underwriting Ratios
Loss ratio	63.6		%	41		%	17.5		%	54.1		%	—		%	54.1		%
Acquisition expense ratio (2)	15.9		%	18.2		%	32.2		%	17.1		%	—		%	17.1		%
Other operating expense ratio	17.3		%	11		%	11.1		%	14.8		%	—		%	14.8		%
Combined ratio	96.8		%	70.2		%	60.8		%	86		%	—		%	86		%
_________________________________________________
-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.
-2	The acquisition expense ratio is adjusted to include certain other underwriting income.

Investment_Information

Investment Information	6 Months Ended
	Jun. 30, 2014
Disclosure Investment Information [Abstract]	'
Investment Information	'
	Investment Information
	At June 30, 2014, total investable assets of $15.80 billion included $14.69 billion managed by the Company and $1.11 billion attributable to Watford.
	Available For Sale Investments
	The following table summarizes the fair value and cost or amortized cost of the Company’s investments classified as available for sale:
		Fair			Gross			Gross			Cost or			OTTI
		Value			Unrealized			Unrealized			Amortized			Unrealized
					Gains			Losses			Cost			Losses (2)
	30-Jun-14
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	2,906,484		$	48,736		$	(10,472	)	$	2,868,220		$	(16	)
	Mortgage backed securities	1,138,385			25,925			(8,572		)	1,121,032			(3,602		)
	Municipal bonds	1,413,131			33,549			(1,095		)	1,380,677			—
	Commercial mortgage backed securities	1,119,401			18,238			(6,051		)	1,107,214			—
	U.S. government and government agencies	1,435,974			5,137			(2,162		)	1,432,999			(19		)
	Non-U.S. government securities	1,073,256			14,936			(5,399		)	1,063,719			—
	Asset backed securities	1,710,631			8,988			(6,740		)	1,708,383			(47		)
	Total	10,797,262			155,509			(40,491		)	10,682,244			(3,684		)
	Equity securities	608,820			100,476			(4,750		)	513,094			—
	Other investments	457,567			28,609			—			428,958			—
	Short-term investments	981,359			902			(1,191		)	981,648			—
	Total	$	12,845,008		$	285,496		$	(46,432	)	$	12,605,944		$	(3,684	)
	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 June 30, 2014, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $2.0 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
		Fair			Gross			Fair			Gross			Fair			Gross
		Value			Unrealized			Value			Unrealized			Value			Unrealized
					Losses						Losses						Losses
	30-Jun-14
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	546,004		$	(3,596	)	$	238,526		$	(6,876	)	$	784,530		$	(10,472	)
	Mortgage backed securities	176,720			(1,893		)	142,523			(6,679		)	319,243			(8,572		)
	Municipal bonds	99,417			(290		)	59,847			(805		)	159,264			(1,095		)
	Commercial mortgage backed securities	163,092			(2,219		)	95,877			(3,832		)	258,969			(6,051		)
	U.S. government and government agencies	338,288			(1,783		)	13,616			(379		)	351,904			(2,162		)
	Non-U.S. government securities	361,383			(2,571		)	69,218			(2,828		)	430,601			(5,399		)
	Asset backed securities	362,407			(2,135		)	215,752			(4,605		)	578,159			(6,740		)
	Total	2,047,311			(14,487		)	835,359			(26,004		)	2,882,670			(40,491		)
	Equity securities	81,731			(4,750		)	—			—			81,731			(4,750		)
	Other investments	—			—			—			—			—			—
	Short-term investments	95,699			(1,191		)	—			—			95,699			(1,191		)
	Total	$	2,224,741		$	(20,428	)	$	835,359		$	(26,004	)	$	3,060,100		$	(46,432	)
	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 June 30, 2014, on a lot level basis, approximately 1,070 security lots out of a total of approximately 4,450 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 $3.0 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.
		30-Jun-14						31-Dec-13
Maturity		Fair Value			Amortized			Fair Value			Amortized
					Cost						Cost
Due in one year or less		$	209,116		$	203,646		$	235,330		$	232,652
Due after one year through five years		4,408,427			4,369,079			3,738,500			3,718,920
Due after five years through 10 years		1,958,624			1,924,301			1,966,536			1,979,510
Due after 10 years		252,678			248,589			196,305			198,286
		6,828,845			6,745,615			6,136,671			6,129,368
Mortgage backed securities		1,138,385			1,121,032			1,133,095			1,139,034
Commercial mortgage backed securities		1,119,401			1,107,214			1,074,497			1,075,749
Asset backed securities		1,710,631			1,708,383			1,332,594			1,326,356
Total		$	10,797,262		$	10,682,244		$	9,676,857		$	9,670,507
Securities Lending Agreements
The Company operates a securities lending program under which 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. The fair value and amortized cost of fixed maturities and short-term investments pledged under securities lending agreements were $87.0 million and $86.6 million, respectively, at June 30, 2014, compared to $105.1 million and $105.9 million, respectively, at December 31, 2013. The fair value of the portfolio of collateral backing the Company’s securities lending program was $82.6 million at June 30, 2014, compared to $100.6 million at December 31, 2013. Such amounts included approximately $6.2 million of sub-prime securities at June 30, 2014, compared to $6.3 million at December 31, 2013. 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. An indemnification agreement with the lending agent protects the Company in the event a borrower becomes insolvent or fails to return any of the securities on loan to the Company.
Other Investments
The following table summarizes the Company’s other investments, including available for sale and fair value option components:
	June 30,			December 31,
	2014			2013
Available for sale:
Asian and emerging markets	$	292,772		$	331,984
Investment grade fixed income	164,795			159,115
Other	—			7,211
Total available for sale	457,567			498,310
Fair value option:
Term loan investments (par value: $909,538 and $494,502)	953,649			512,076
Asian and emerging markets	25,321			14,054
Investment grade fixed income	81,737			75,062
Other (1)	310,829			172,088
Total fair value option	1,371,536			773,280
Total	$	1,829,103		$	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:
	June 30,			December 31,
	2014			2013
Fixed maturities	$	438,048		$	448,254
Other investments	1,371,536			773,280
Short-term investments	231,507			—
Investments accounted for using the fair value option	$	2,041,091		$	1,221,534
Net Investment Income
The components of net investment income were derived from the following sources:
	Three Months Ended						Six Months Ended
	June 30,						June 30,
	2014			2013			2014			2013
Fixed maturities	$	65,869		$	62,004		$	128,318		$	124,010
Term loan investments	6,908			6,026			12,577			10,243
Equity securities (dividends)	3,271			3,164			6,192			4,587
Short-term investments	107			364			512			756
Other (1)	9,099			4,734			13,819			11,033
Gross investment income	85,254			76,292			161,418			150,629
Investment expenses	(12,264		)	(7,923		)	(21,434		)	(16,588		)
Net investment income	$	72,990		$	68,369		$	139,984		$	134,041
_________________________________________________
-1	Includes dividends on investment funds and other items.
Net Realized Gains (Losses)
Net realized gains (losses) were as follows, excluding other-than-temporary impairment provisions:
	Three Months Ended						Six Months Ended
	June 30,						June 30,
	2014			2013			2014			2013
Available for sale securities:
Gross gains on investment sales	$	56,763		$	68,853		$	122,814		$	135,873
Gross losses on investment sales	(33,028		)	(54,783		)	(73,041		)	(79,839		)
Change in fair value of assets and liabilities accounted for using the fair value option:
Fixed maturities	10,984			(6,702		)	9,341			(1,644		)
Other investments	17,985			(9,502		)	28,761			449
Equity securities	—			5			—			704
Derivative instruments (1)	12,402			15,646			(366		)	15,268
Other (2)	(10,962		)	(865		)	(13,668		)	181
Net realized gains (losses)	$	54,144		$	12,652		$	73,841		$	70,992
_________________________________________________
-1	See Note 9 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:
	Three Months Ended						Six Months Ended
	June 30,						June 30,
	2014			2013			2014			2013
Fixed maturities:
Mortgage backed securities	$	—		$	—		$	—		$	(15	)
Corporate bonds	(664		)	—			(664		)	—
Asset backed securities	(5		)	—			(11		)	(20		)
Total	(669		)	—			(675		)	(35		)
Equity securities	(278		)	(724		)	(278		)	(2,935		)
Other investments	(13,802		)	—			(16,767		)	—
Net impairment losses recognized in earnings	$	(14,749	)	$	(724	)	$	(17,720	)	$	(2,970	)
A description of the methodology and significant inputs used to measure the amount of net impairment losses recognized in earnings in the 2014 periods is as follows:
•	Other investments — the Company utilized information received from fund managers and positive and negative evidence on the fund investment, including the business prospects, recent events, industry and market data and other factors. Net impairment losses for the 2014 second quarter primarily related to one 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 cost basis of such fund was adjusted down accordingly. The remainder of the net impairment losses for the 2014 periods related to a fund investment which, due to its convex investment strategy, operates at a loss in absence of certain market stress events. The Company determined that the fair value of the fund investment was unlikely to increase based on the underlying strategy and the cost basis was adjusted down accordingly;
•	Corporate bonds — the Company reviewed the business prospects, credit ratings, estimated loss given default factors 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;
•	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 $3.7 million of OTTI included in accumulated other comprehensive income at June 30, 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 June 30, 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:
	Three Months Ended						Six Months Ended
	June 30,						June 30,
	2014			2013			2014			2013
Balance at start of period	$	47,256		$	61,956		$	60,062		$	62,001
Credit loss impairments recognized on securities not previously impaired	—			—			—			33
Credit loss impairments recognized on securities previously impaired	5			—			11			2
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	(25,820		)	(507		)	(38,632		)	(587		)
Balance at end of period	$	21,441		$	61,449		$	21,441		$	61,449
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 10 for further details. The following table details the value of the Company’s restricted assets:
	June 30,			December 31,
	2014			2013
Assets used for collateral or guarantees:
Affiliated transactions	$	4,352,540		$	4,060,533
Third party agreements	943,551			856,890
Deposits with U.S. regulatory authorities	344,087			302,809
Deposits with non-U.S. regulatory authorities	—			6,546
Trust funds	81,942			75,264
Total restricted assets	$	5,722,120		$	5,302,042

Fair_Value

Fair Value	6 Months Ended
	Jun. 30, 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
	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 including a review of deep dive reports on selected securities which indicate the use of observable inputs in the pricing process; (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) 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. The Company did not adjust any of the prices obtained from the pricing services at June 30, 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 $14.94 billion of financial assets and liabilities measured at fair value at June 30, 2014, approximately $635.5 million, or 4.3%, were priced using non-binding broker-dealer quotes. Of the $13.37 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 advisers and others. No transfers were made in the periods presented. 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.
	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. Other investments also include term loan investments for which fair values are estimated by using quoted prices of term loan investments with similar characteristics, pricing models or matrix pricing. Such investments are generally classified as Level 2 securities.
	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 following table presents the Company’s financial assets and liabilities measured at fair value by level at June 30, 2014:
					Fair Value Measurement Using:
		Fair			Quoted Prices in			Significant			Significant
		Value			Active Markets			Other			Unobservable
					for Identical			Observable			Inputs
					Assets			Inputs			(Level 3)
					(Level 1)			(Level 2)
	Assets measured at fair value:
	Available for sale securities:
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	2,906,484		$	—		$	2,906,484		$	—
	Mortgage backed securities	1,138,385			—			1,138,385			—
	Municipal bonds	1,413,131			—			1,413,131			—
	Commercial mortgage backed securities	1,119,401			—			1,119,401			—
	U.S. government and government agencies	1,435,974			1,435,974			—			—
	Non-U.S. government securities	1,073,256			—			1,073,256			—
	Asset backed securities	1,710,631			—			1,710,631			—
	Total	10,797,262			1,435,974			9,361,288			—
	Equity securities	608,820			608,820			—			—
	Other investments	457,567			—			354,354			103,213
	Short-term investments	981,359			948,468			32,891			—
	Fair value option:
	Corporate bonds	343,607			—			343,607			—
	Non-U.S. government bonds	60,268			—			60,268			—
	Mortgage backed securities	16,807			—			16,807			—
	Asset backed securities	17,366			—			17,366			—
	Other investments	1,371,536			—			920,500			451,036
	Short-term investments	231,507			231,507			—			—
	Total	2,041,091			231,507			1,358,548			451,036
	Total assets measured at fair value	$	14,886,099		$	3,224,769		$	11,107,081		$	554,249
	Liabilities measured at fair value:
	Contingent consideration liability	$	53,099		$	—		$	—		$	53,099
	Total liabilities measured at fair value	$	53,099		$	—		$	—		$	53,099
	_________________________________________________
	-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:
		Fair			Quoted Prices in			Significant			Significant
		Value			Active Markets			Other			Unobservable
					for Identical			Observable			Inputs
					Assets			Inputs			(Level 3)
					(Level 1)			(Level 2)
	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:
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 tables present a reconciliation of the beginning and ending balances for all financial assets and liabilities measured at fair value on a recurring basis using Level 3 inputs:
	Fair Value Measurements Using:
	Significant Unobservable Inputs (Level 3)
	Assets												Liabilities
s	Available-For-Sale						Fair Value Option
	Corporate			Other			Other			Total			Contingent Consideration Liability
	Bonds			Investments			Investments
Three Months Ended June 30, 2014
Balance at beginning of period	$	—		$	104,335		$	419,146		$	523,481		$	43,156
Total gains or (losses) (realized/unrealized)
Included in earnings (1)	—			(1,722		)	16,136			14,414			9,943
Included in other comprehensive income	—			600			932			1,532			—
Purchases, issuances, sales and settlements
Purchases	—			—			66,615			66,615			—
Issuances	—			—			—			—			—
Sales	—			—			—			—			—
Settlements	—			—			(51,793		)	(51,793		)	—
Transfers in and/or out of Level 3	—			—			—			—			—
Balance at end of period	$	—		$	103,213		$	451,036		$	554,249		$	53,099
Three Months Ended June 30, 2013
Balance at beginning of period	$	97,298		$	185,934		$	179,370		$	462,602		$	—
Total gains or (losses) (realized/unrealized)
Included in earnings (1)	3,187			—			(2,075		)	1,112			—
Included in other comprehensive income	(1,363		)	3,959			—			2,596			—
Purchases, issuances, sales and settlements
Purchases	—			—			118,916			118,916			—
Issuances	—			—			—			—			—
Sales	(96,655		)	—			—			(96,655		)	—
Settlements	(107		)	—			(6,022		)	(6,129		)	—
Transfers in and/or out of Level 3	—			—			—			—			—
Balance at end of period	$	2,360		$	189,893		$	290,189		$	482,442		$	—
_________________________________________________
-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. Gains or losses on the contingent consideration liability were included in net realized gains (losses).
	Fair Value Measurements Using:
	Significant Unobservable Inputs (Level 3)
	Assets												Liabilities
s	Available-For-Sale						Fair Value Option
	Corporate			Other			Other			Total			Contingent Consideration Liability
	Bonds			Investments			Investments
Six Months Ended June 30, 2014
Balance at beginning of period	$	2,045		$	170,420		$	377,525		$	549,990		$	—
Total gains or (losses) (realized/unrealized)
Included in earnings (1)	—			553			22,446			22,999			11,337
Included in other comprehensive income	—			(1,596		)	932			(664		)	—
Purchases, issuances, sales and settlements
Purchases	—			—			106,725			106,725			—
Issuances	—			—			—			—			41,762
Sales	(2,045		)	(66,164		)	—			(68,209		)	—
Settlements	—			—			(56,592		)	(56,592		)	—
Transfers in and/or out of Level 3	—			—			—			—			—
Balance at end of period	$	—		$	103,213		$	451,036		$	554,249		$	53,099
Six Months Ended June 30, 2013
Balance at beginning of period	$	98,404		$	184,202		$	195,350		$	477,956		$	—
Total gains or (losses) (realized/unrealized)
Included in earnings (1)	4,679			4,762			(1,702		)	7,739			—
Included in other comprehensive income	(3,051		)	5,871			—			2,820			—
Purchases, issuances, sales and settlements
Purchases	—			5,000			160,570			165,570			—
Issuances	—			—			—			—			—
Sales	(96,655		)	—			—			(96,655		)	—
Settlements	(1,017		)	(9,942		)	(64,029		)	(74,988		)	—
Transfers in and/or out of Level 3	—			—			—			—			—
Balance at end of period	$	2,360		$	189,893		$	290,189		$	482,442		$	—
_________________________________________________
-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. Gains or losses on the contingent consideration liability were included in net realized gains (losses).
The amount of total gains for the 2014 second quarter included in earnings attributable to the change in unrealized gains or losses relating to assets still held at June 30, 2014 was $14.4 million, while the amount of total gains for the six months ended June 30, 2014 included in earnings attributable to the change in unrealized gains or losses relating to assets still held at June 30, 2014 was $17.8 million. The amount of total losses for the 2013 second quarter included in earnings attributable to the change in unrealized gains or losses relating to assets still held at June 30, 2013 was $2.2 million, while the amount of total gains for the six months ended June 30, 2013 included in earnings attributable to the change in unrealized gains or losses relating to assets still held at June 30, 2013 was $4.4 million.
Financial Instruments Disclosed, But Not Carried, At Fair Value
The Company uses various financial instruments in the normal course of its business. The carrying values of cash, accrued investment income, receivable for securities sold, certain other assets, payable for securities purchased and certain other liabilities approximated their fair values at June 30, 2014, due to their respective short maturities. As these financial instruments are not actively traded, their respective fair values are classified within Level 2.
At June 30, 2014, the senior notes of ACGL were carried at their cost of $300.0 million and had a fair value of $412.6 million while the senior notes of Arch-U.S. were carried at their cost of $500.0 million and had a fair value of $540.1 million. The fair values of the senior notes were obtained from a third party pricing service and are based on observable market inputs. As such, the fair value of the senior notes is classified within Level 2.

Derivative_Instruments

Derivative Instruments	6 Months Ended
	Jun. 30, 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 June 30, 2014 or December 31, 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)
	30-Jun-14
	Futures contracts	$	986			$	(198	)		$	788			$	783,914
	Foreign currency forward contracts	856				(1,949		)		(1,093		)		279,559
	TBAs	160,074				(149,782		)		10,292				296,380
	Other	1,100				(722		)		378				504,709
	Total	$	163,016			$	(152,651	)		$	10,365
	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 June 30, 2014, asset derivatives and liability derivatives of $77.8 million and $36.5 million, respectively, were subject to a master netting agreement, 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:
			Three Months Ended								Six Months Ended
	Derivatives not designated as		June 30,								June 30,
	hedging instruments		2014				2013				2014				2013
	Futures contracts		$	15,185			$	10,623			$	5,323			$	9,794
	Foreign currency forward contracts		(1,811		)		7,605				(4,068		)		6,880
	TBAs		(270		)		(3,751		)		(307		)		(3,210		)
	Other		(702		)		1,169				(1,314		)		1,804
	Total		$	12,402			$	15,646			$	(366	)		$	15,268

Commitments_and_Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2014
Commitments and Contingencies Disclosure [Abstract]	'
Commitments and Contingencies	'
	Commitments and Contingencies
	Letter of Credit and Revolving Credit Facilities
	As of June 30, 2014, the Company had a $300 million unsecured revolving loan and letter of credit facility and a $500 million secured letter of credit facility (the “Credit Agreement”). The Credit Agreement expires on June 30, 2019. In addition, the Company had access to secured letter of credit facilities of approximately $194.8 million as of June 30, 2014, 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”). At June 30, 2014, the Company had $424.2 million in outstanding letters of credit under the LOC Facilities, which were secured by investments with a fair value of $484.1 million, and had $100.0 million of borrowings outstanding under the Credit Agreement which are due on June 30, 2019. The Company was in compliance with all covenants contained in the LOC Facilities at June 30, 2014. As of June 30, 2014, Watford had a $200 million line of credit facility that expires on May 19, 2015. At June 30, 2014, Watford had $1.4 million in outstanding letters of credit under that credit facility. Watford was in compliance with all covenants contained in its credit facility at June 30, 2014.
	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 $966.1 million at June 30, 2014.

Share_Transactions

Share Transactions	6 Months Ended
	Jun. 30, 2014
Stockholders' Equity Note [Abstract]	'
Share Transactions	'
	Share Transactions
	Share Repurchases
	The board of directors of ACGL has authorized the investment in ACGL’s common shares through a share repurchase program. Repurchases under the program may be effected from time to time in open market or privately negotiated transactions through December 2014. Since the inception of the share repurchase program, ACGL has repurchased approximately 109.9 million common shares for an aggregate purchase price of $2.79 billion. During the 2014 second quarter and six months ended June 30, 2014, ACGL did not repurchase any common shares. During the 2013 second quarter and six months ended June 30, 2013, ACGL repurchased 307,659 and 1,238,418 common shares, respectively, for an aggregate purchase price of $15.5 million and $56.5 million, respectively. At June 30, 2014, $712.1 million of share repurchases were available under the program. 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.
	Share-Based Compensation
	During the 2014 second quarter, the Company made a stock grant of 551,836 stock appreciation rights and stock options and 571,108 restricted shares and units to certain employees and directors with weighted average grant-date fair values of $15.23 and $57.25, respectively. During the 2013 second quarter, the Company made a stock grant of 516,859 stock appreciation rights and stock options and 544,075 restricted shares and units to certain employees and directors with weighted average grant-date fair values of $13.35 and $53.50, respectively. The stock appreciation rights and stock options were valued at the grant date using the Black-Scholes option pricing model. Such values are being amortized over the respective substantive vesting period. 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.

Income_Taxes

Income Taxes	6 Months Ended
	Jun. 30, 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 are the United States, United Kingdom, Ireland, Canada, Switzerland and Denmark.
	The Company’s income tax provision on income before income taxes resulted in an expense of 2.7% for the six months ended June 30, 2014, compared to an expense of 2.2% for the 2013 period. These rates include the tax effect of unusual or infrequent items representing 0.1% and 0.3% of the Company’s effective tax rate for the six months ended June 30, 2014 and the 2013 period, respectively. The Company’s effective tax rate, which is based upon the expected annual effective tax rate, may fluctuate from period to period based on the relative mix of income or loss reported by jurisdiction and the varying tax rates in each jurisdiction. The Company had a net deferred tax asset of $151.5 million at June 30, 2014, compared to $128.6 million at December 31, 2013. In addition, the Company paid $8.3 million in income taxes for the six months ended June 30, 2014, while the Company paid $4.8 million for the 2013 period.

Other_Comprehensive_Income_Los

Other Comprehensive Income (Loss)	6 Months Ended
	Jun. 30, 2014
Comprehensive Income Note Disclosure [Abstract]	'
Other Comprehensive Income (Loss)	'
	Other Comprehensive Income (Loss)
	The following table presents details about amounts reclassified from accumulated other comprehensive income:
					Amounts Reclassed from AOCI
			Consolidated Statement of Income		Three Months Ended								Six Months Ended
	Details About		Line Item That Includes		June 30,								June 30,
	AOCI Components		Reclassification		2014				2013				2014			2013
	Unrealized appreciation on available-for-sale investments
			Net realized gains		$	23,735			$	14,634			$	49,773		$	59,001
			Other-than-temporary impairment losses		(14,749		)		(724		)		(17,720		)	(2,972		)
			Total before tax		8,986				13,910				32,053			56,029
			Income tax (expense) benefit		(701		)		6				(2,519		)	(3,412		)
			Net of tax		$	8,285			$	13,916			$	29,534		$	52,617
	The following table presents the tax effects allocated to each component of other comprehensive income (loss):
		Before Tax Amount				Tax Expense (Benefit)				Net of Tax Amount
	Three Months Ended June 30, 2014
	Unrealized appreciation (decline) in value of investments:
	Unrealized holding gains (losses) arising during period	$	115,150			$	6,722			$	108,428
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	—				—				—
	Less reclassification of net realized gains included in net income	8,986				701				8,285
	Foreign currency translation adjustments	10,021				—				10,021
	Other comprehensive income (loss)	$	116,185			$	6,021			$	110,164
	Three Months Ended June 30, 2013
	Unrealized appreciation (decline) in value of investments:
	Unrealized holding gains (losses) arising during period	$	(281,388	)		$	(21,826	)		$	(259,562	)
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	—				—				—
	Less reclassification of net realized gains included in net income	13,910				(6		)		13,916
	Foreign currency translation adjustments	(5,407		)		—				(5,407		)
	Other comprehensive income (loss)	$	(300,705	)		$	(21,820	)		$	(278,885	)
	Six Months Ended June 30, 2014
	Unrealized appreciation (decline) in value of investments:
	Unrealized holding gains (losses) arising during period	$	190,550			$	10,769			$	179,781
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	—				—				—
	Less reclassification of net realized gains included in net income	32,053				2,519				29,534
	Foreign currency translation adjustments	8,672				—				8,672
	Other comprehensive income (loss)	$	167,169			$	8,250			$	158,919
	Six Months Ended June 30, 2013
	Unrealized appreciation (decline) in value of investments:
	Unrealized holding gains (losses) arising during period	$	(271,041	)		$	(20,950	)		$	(250,091	)
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	(2		)		—				(2		)
	Less reclassification of net realized gains included in net income	56,029				3,412				52,617
	Foreign currency translation adjustments	(33,629		)		—				(33,629		)
	Other comprehensive income (loss)	$	(360,701	)		$	(24,362	)		$	(336,339	)

Guarantor_Financial_Informatio

Guarantor Financial Information	6 Months Ended
	Jun. 30, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]	'
Guarantor Financial Information	'
	Guarantor Financial Information
	The following tables present condensed financial information for ACGL, Arch Capital Group (U.S.) Inc. (“Arch-U.S.”), a 100% owned subsidiary of ACGL, and ACGL’s other subsidiaries.
			30-Jun-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		$	2,561			$	78,702			$	15,081,872			$	—			$	15,163,135
	Cash		4,715				7,746				913,982				—				926,443
	Investments in subsidiaries		6,629,043				1,639,914				—				(8,268,957		)		—
	Due from subsidiaries and affiliates		636				—				411,687				(412,323		)		—
	Premiums receivable		—				—				1,477,354				(378,662		)		1,098,692
	Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses		—				—				5,634,614				(3,838,211		)		1,796,403
	Contractholder receivables		—				—				1,234,392				—				1,234,392
	Prepaid reinsurance premiums		—				—				1,407,938				(977,724		)		430,214
	Deferred acquisition costs, net		—				—				399,385				—				399,385
	Other assets		8,025				64,629				2,022,687				(570,490		)		1,524,851
		Total assets	$	6,644,980			$	1,790,991			$	28,583,911			$	(14,446,367	)		$	22,573,515
	Liabilities
	Reserve for losses and loss adjustment expenses		$	—			$	—			$	12,848,119			$	(3,829,130	)		$	9,018,989
	Unearned premiums		—				—				3,277,416				(977,724		)		2,299,692
	Reinsurance balances payable		—				—				804,239				(540,892		)		263,347
	Contractholder payables		—				—				1,234,392				—				1,234,392
	Deposit accounting liabilities		—				—				715,369				(318,032		)		397,337
	Senior notes		300,000				500,000				—				—				800,000
	Revolving credit agreement borrowings		100,000				—				—				—				100,000
	Due to subsidiaries and affiliates		4,954				1,189				406,180				(412,323		)		—
	Other liabilities		10,627				45,057				1,262,318				(99,309		)		1,218,693
		Total liabilities	415,581				546,246				20,548,033				(6,177,410		)		15,332,450
	Redeemable noncontrolling interests (1)		—				—				219,326				—				219,326
	Shareholders’ Equity
	Total shareholders’ equity available to Arch		6,229,399				1,244,745				7,024,212				(8,268,957		)		6,229,399
	Non-redeemable noncontrolling interests (1)		—				—				792,340				—				792,340
		Total shareholders’ equity	6,229,399				1,244,745				7,816,552				(8,268,957		)		7,021,739
		Total liabilities, noncontrolling interests and shareholders’ equity	$	6,644,980			$	1,790,991			$	28,583,911			$	(14,446,367	)		$	22,573,515
	(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
		Three Months Ended June 30, 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		$	—			$	—			$	907,152			$	—			$	907,152
Net investment income		—				—				80,380				(7,390		)		72,990
Net realized gains		—				—				54,144				—				54,144
Net impairment losses recognized in earnings		—				—				(14,749		)		—				(14,749		)
Other underwriting income		—				—				2,033				—				2,033
Equity in net income of investment funds accounted for using the equity method		—				—				9,240				—				9,240
Other income (loss)		—				—				4,850				—				4,850
	Total revenues	—				—				1,043,050				(7,390		)		1,035,660
Expenses
Losses and loss adjustment expenses		—				—				485,518				—				485,518
Acquisition expenses		—				—				158,158				—				158,158
Other operating expenses		15,250				494				140,606				—				156,350
Interest expense		5,854				6,448				9,422				(7,390		)		14,334
Net foreign exchange losses		—				—				1,054				1,240				2,294
	Total expenses	21,104				6,942				794,758				(6,150		)		816,654
Income (loss) before income taxes		(21,104		)		(6,942		)		248,292				(1,240		)		219,006
Income tax benefit (expense)		—				3,294				(10,583		)		—				(7,289		)
Income (loss) before equity in net income of subsidiaries		(21,104		)		(3,648		)		237,709				(1,240		)		211,717
Equity in net income of subsidiaries		229,120				9,334				—				(238,454		)		—
Net income		208,016				5,686				237,709				(239,694		)		211,717
Amounts attributable to noncontrolling interests (1)		—				—				(3,701		)		—				(3,701		)
Net income available to Arch		208,016				5,686				234,008				(239,694		)		208,016
Preferred dividends		(5,485		)		—				—				—				(5,485		)
Net income available to Arch common shareholders		$	202,531			$	5,686			$	234,008			$	(239,694	)		$	202,531
Comprehensive income available to Arch		$	318,180			$	23,186			$	335,348			$	(358,534	)		$	318,180
(1)     See Note 4.
		Three Months Ended June 30, 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		$	—			$	—			$	758,816			$	—			$	758,816
Net investment income		—				—				74,980				(6,611		)		68,369
Net realized gains		—				—				12,652				—				12,652
Net impairment losses recognized in earnings		—				—				(724		)		—				(724		)
Other underwriting income		—				—				902				—				902
Equity in net income of investment funds accounted for using the equity method		—				—				10,941				—				10,941
Other income		—				—				834				—				834
	Total revenues	—				—				858,401				(6,611		)		851,790
Expenses
Losses and loss adjustment expenses		—				—				418,653				—				418,653
Acquisition expenses		—				—				131,677				—				131,677
Other operating expenses		13,691				773				112,944				—				127,408
Interest expense		5,843				5				6,615				(6,611		)		5,852
Net foreign exchange (gains) losses		—				—				(15,882		)		2,071				(13,811		)
	Total expenses	19,534				778				654,007				(4,540		)		669,779
Income (loss) before income taxes		(19,534		)		(778		)		204,394				(2,071		)		182,011
Income tax benefit (expense)		—				2,750				(7,821		)		—				(5,071		)
Income (loss) before equity in net income of subsidiaries		(19,534		)		1,972				196,573				(2,071		)		176,940
Equity in net income of subsidiaries		196,474				10,703				—				(207,177		)		—
Net income		176,940				12,675				196,573				(209,248		)		176,940
Amounts attributable to noncontrolling interests		—				—				—				—				—
Net income available to Arch		176,940				12,675				196,573				(209,248		)		176,940
Preferred dividends		(5,485		)		—				—				—				(5,485		)
Net income available to Arch common shareholders		$	171,455			$	12,675			$	196,573			$	(209,248	)		$	171,455
Comprehensive income available to Arch		$	(101,945	)		$	(28,981	)		$	(84,383	)		$	113,364			$	(101,945	)
		Six Months Ended June 30, 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		$	—			$	—			$	1,766,932			$	—			$	1,766,932
Net investment income		—				—				157,381				(17,397		)		139,984
Net realized gains		—				—				73,841				—				73,841
Net impairment losses recognized in earnings		—				—				(17,720		)		—				(17,720		)
Other underwriting income		—				—				3,615				—				3,615
Equity in net income of investment funds accounted for using the equity method		—				—				12,493				—				12,493
Other income (loss)		—				—				2,746				—				2,746
	Total revenues	—				—				1,999,288				(17,397		)		1,981,891
Expenses
Losses and loss adjustment expenses		—				—				921,758				—				921,758
Acquisition expenses		—				—				318,500				—				318,500
Other operating expenses		25,557				1,471				275,121				—				302,149
Interest expense		11,707				12,962				21,466				(17,397		)		28,738
Net foreign exchange losses		—				—				6,795				2,062				8,857
	Total expenses	37,264				14,433				1,543,640				(15,335		)		1,580,002
Income (loss) before income taxes		(37,264		)		(14,433		)		455,648				(2,062		)		401,889
Income tax benefit (expense)		—				6,082				(17,109		)		—				(11,027		)
Income (loss) before equity in net income of subsidiaries		(37,264		)		(8,351		)		438,539				(2,062		)		390,862
Equity in net income of subsidiaries		427,780				31,486				—				(459,266		)		—
Net income		390,516				23,135				438,539				(461,328		)		390,862
Amounts attributable to noncontrolling interests (1)		—				—				(346		)		—				(346		)
Net income available to Arch		390,516				23,135				438,193				(461,328		)		390,516
Preferred dividends		(10,969		)		—				—				—				(10,969		)
Net income available to Arch common shareholders		$	379,547			$	23,135			$	438,193			$	(461,328	)		$	379,547
Comprehensive income available to Arch		$	549,435			$	40,406			$	587,467			$	(627,873	)		$	549,435
(1)     See Note 4.
		Six Months Ended June 30, 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		$	—			$	—			$	1,511,586			$	—			$	1,511,586
Net investment income		—				—				147,302				(13,261		)		134,041
Net realized gains		—				—				70,992				—				70,992
Net impairment losses recognized in earnings		—				—				(2,970		)		—				(2,970		)
Other underwriting income		—				—				1,440				—				1,440
Equity in net income of investment funds accounted for using the equity method		—				—				24,764				—				24,764
Other income		—				—				2,078				—				2,078
	Total revenues	—				—				1,755,192				(13,261		)		1,741,931
Expenses
Losses and loss adjustment expenses		—				—				818,056				—				818,056
Acquisition expenses		—				—				259,269				—				259,269
Other operating expenses		23,633				1,550				222,408				—				247,591
Interest expense		11,663				8				13,340				(13,261		)		11,750
Net foreign exchange (gains) losses		—				—				(28,654		)		(9,421		)		(38,075		)
	Total expenses	35,296				1,558				1,284,419				(22,682		)		1,298,591
Income (loss) before income taxes		(35,296		)		(1,558		)		470,773				9,421				443,340
Income tax benefit (expense)		—				2,985				(12,909		)		—				(9,924		)
Income (loss) before equity in net income of subsidiaries		(35,296		)		1,427				457,864				9,421				433,416
Equity in net income of subsidiaries		468,712				29,185				—				(497,897		)		—
Net income		433,416				30,612				457,864				(488,476		)		433,416
Amounts attributable to noncontrolling interests		—				—				—				—				—
Net income available to Arch		433,416				30,612				457,864				(488,476		)		433,416
Preferred dividends		(10,969		)		—				—				—				(10,969		)
Net income available to Arch common shareholders		$	422,447			$	30,612			$	457,864			$	(488,476	)		$	422,447
Comprehensive income available to Arch		$	97,077			$	(22,829	)		$	130,946			$	(108,117	)		$	97,077
				Six Months Ended June 30, 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 Operating Activities			$	10,099			$	424			$	474,565			$	(32,025	)		$	453,063
Investing Activities
Purchases of fixed maturity investments				—				(78,509		)		(14,233,239		)		—				(14,311,748		)
Purchases of equity securities				—				—				(174,687		)		—				(174,687		)
Purchases of other investments				—				—				(1,022,987		)		—				(1,022,987		)
Proceeds from the sales of fixed maturity investments				—				—				13,204,854				—				13,204,854
Proceeds from the sales of equity securities				—				—				98,687				—				98,687
Proceeds from the sales of other investments				—				—				618,707				—				618,707
Proceeds from redemptions and maturities of fixed maturity investments				—				—				432,040				—				432,040
Net (purchases) sales of short-term investments				(31		)		408,779				21,556				—				430,304
Change in investment of securities lending collateral				—				—				18,701				—				18,701
Contributions to subsidiaries				—				(313,207		)		(100,000		)		413,207				—
Intercompany loans issued				—				—				10,250				(10,250		)		—
Purchase of business, net of cash acquired				—				—				(235,578		)		—				(235,578		)
Purchases of furniture, equipment and other assets				(128		)		—				(10,232		)		—				(10,360		)
	Net Cash Provided By (Used For) Investing Activities			(159		)		17,063				(1,371,928		)		402,957				(952,067		)
Financing Activities
Purchases of common shares under share repurchase program				—				—				—				—				—
Proceeds from common shares issued, net				2,521				—				413,207				(413,207		)		2,521
Repayments of intercompany borrowings				—				(10,250		)		—				10,250				—
Change in securities lending collateral				—				—				(18,701		)		—				(18,701		)
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)				—				—				(4,816		)		—				(4,816		)
Dividends paid to parent				—				—				(32,025		)		32,025				—
Other				—				—				4,706				—				4,706
Preferred dividends paid				(10,969		)		—				—				—				(10,969		)
	Net Cash Provided By (Used For) Financing Activities			(8,448		)		(10,250		)		1,378,507				(370,932		)		988,877
Effects of exchange rates changes on foreign currency cash				—				—				2,513				—				2,513
Increase in cash				1,492				7,237				483,657				—				492,386
Cash beginning of year				3,223				509				430,325				—				434,057
Cash end of period				$	4,715			$	7,746			$	913,982			$	—			$	926,443
(1)     See Note 4.
				Six Months Ended June 30, 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) Operating Activities			$	69,021			$	(534	)		$	412,867			$	(93,000	)		$	388,354
Investing Activities
Purchases of fixed maturity investments				—				—				(8,599,697		)		—				(8,599,697		)
Purchases of equity securities				—				—				(272,323		)		—				(272,323		)
Purchases of other investments				—				—				(648,915		)		—				(648,915		)
Proceeds from the sales of fixed maturity investments				—				—				8,468,641				—				8,468,641
Proceeds from the sales of equity securities				—				—				194,212				—				194,212
Proceeds from the sales of other investments				—				—				506,434				—				506,434
Proceeds from redemptions and maturities of fixed maturity investments				—				—				424,953				—				424,953
Net (purchases) sales of short-term investments				(3,028		)		996				(373,114		)		—				(375,146		)
Change in investment of securities lending collateral				—				—				3,221				—				3,221
Contributions to subsidiaries				—				(10,250		)		(250		)		10,500				—
Intercompany loans issued				—				—				(10,250		)		10,250				—
Purchases of furniture, equipment and other assets				(131		)		—				(6,961		)		—				(7,092		)
	Net Cash Provided By (Used For) Investing Activities			(3,159		)		(9,254		)		(314,049		)		20,750				(305,712		)
Financing Activities
Purchases of common shares under share repurchase program				(56,463		)		—				—				—				(56,463		)
Proceeds from common shares issued, net				(517		)		—				10,500				(10,500		)		(517		)
Proceeds from intercompany borrowings				—				10,250				—				(10,250		)		—
Change in securities lending collateral				—				—				(3,221		)		—				(3,221		)
Dividends paid to parent				—				—				(93,000		)		93,000				—
Other				—				—				5,042				—				5,042
Preferred dividends paid				(10,969		)		—				—				—				(10,969		)
	Net Cash Provided By (Used For) Financing Activities			(67,949		)		10,250				(80,679		)		72,250				(66,128		)
Effects of exchange rates changes on foreign currency cash				—				—				(12,436		)		—				(12,436		)
Increase (decrease) in cash				(2,087		)		462				5,703				—				4,078
Cash beginning of year				6,417				612				364,012				—				371,041
Cash end of period				$	4,330			$	1,074			$	369,715			$	—			$	375,119

Legal_Proceedings

Legal Proceedings	6 Months Ended
	Jun. 30, 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 June 30, 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.

Significant_Accounting_Policie1

Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2014
Accounting Policies [Abstract]	'
Basis of presentation	'
	The interim 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 the Company and Watford. 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. In the opinion of management, the accompanying unaudited interim consolidated financial statements reflect all adjustments (consisting of normally recurring accruals) necessary for a fair statement of results on an interim basis. The results of any interim period are not necessarily indicative of the results for a full year or any future periods.
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 expected life 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 operations.
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 charged against revenue 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 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 and anticipated investment income. A premium deficiency 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.
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 due from insureds 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
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 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 realizable values with the related expense recorded in the Company’s results of operations.

Business_Acquired_Tables

Business Acquired (Tables)	6 Months Ended
	Jun. 30, 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_Entities_and1

Variable Interest Entities and Noncontrolling Interests (Tables)	6 Months Ended
	Jun. 30, 2014
Noncontrolling Interest [Abstract]	'
Activity in non-redeemable noncontrolling interests	'
	The following table sets forth activity in the non-redeemable noncontrolling interests:
		Three Months Ended June 30, 2014			Six Months Ended June 30, 2014
	Balance, beginning of period	$	793,496		$	—
	Sale of shares to noncontrolling interests	—			796,903
	Amounts attributable to noncontrolling interests	(1,156		)	(4,563		)
	Balance, end of period	$	792,340		$	792,340

Earnings_Per_Common_Share_Tabl

Earnings Per Common Share (Tables)	6 Months Ended
	Jun. 30, 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:
		Three Months Ended						Six Months Ended
		June 30,						June 30,
		2014			2013			2014			2013
	Numerator:
	Net income	$	211,717		$	176,940		$	390,862		$	433,416
	Amounts attributable to noncontrolling interests	(3,701		)	—			(346		)	—
	Net income available to Arch	208,016			176,940			390,516			433,416
	Preferred dividends	(5,485		)	(5,485		)	(10,969		)	(10,969		)
	Net income available to Arch common shareholders	$	202,531		$	171,455		$	379,547		$	422,447
	Denominator:
	Weighted average common shares outstanding — basic	132,650,634			131,377,274			132,256,462			131,143,885
	Effect of dilutive common share equivalents:
	Nonvested restricted shares	1,144,621			1,088,030			1,236,408			1,181,947
	Stock options (1)	3,094,689			3,383,746			3,224,019			3,298,394
	Weighted average common shares and common share equivalents outstanding — diluted	136,889,944			135,849,050			136,716,889			135,624,226
	Earnings per common share:
	Basic	$	1.53		$	1.31		$	2.87		$	3.22
	Diluted	$	1.48		$	1.26		$	2.78		$	3.11
	_________________________________________________
	-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 the 2014 second quarter and 2013 second quarter, the number of stock options excluded were 978,237 and 1,428,616, respectively. For the six months ended June 30, 2014 and 2013, the number of stock options excluded were 1,318,662 and 1,730,313, respectively.

Segment_Information_Tables

Segment Information (Tables)	6 Months Ended
	Jun. 30, 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 or loss to net income available to common shareholders:
		Three Months Ended
		30-Jun-14
		Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
	Gross premiums written (1)	$	852,231		$	349,841		$	55,476		$	1,256,934		$	54,562		$	1,271,761
	Premiums ceded	(273,349		)	(58,994		)	(5,079		)	(336,808		)	(2,760		)	(299,833		)
	Net premiums written	578,882			290,847			50,397			920,126			51,802			971,928
	Change in unearned premiums	(71,170		)	44,780			436			(25,954		)	(38,822		)	(64,776		)
	Net premiums earned	507,712			335,627			50,833			894,172			12,980			907,152
	Other underwriting income	514			303			1,216			2,033			—			2,033
	Losses and loss adjustment expenses	(311,526		)	(150,325		)	(15,473		)	(477,324		)	(8,194		)	(485,518		)
	Acquisition expenses, net	(76,449		)	(66,035		)	(11,481		)	(153,965		)	(4,193		)	(158,158		)
	Other operating expenses	(85,829		)	(37,666		)	(16,288		)	(139,783		)	(1,635		)	(141,418		)
	Underwriting income (loss)	$	34,422		$	81,904		$	8,807		125,133			(1,042		)	124,091
	Net investment income										72,458			532			72,990
	Net realized gains										50,966			3,178			54,144
	Net impairment losses recognized in earnings										(14,749		)	—			(14,749		)
	Equity in net income of investment funds accounted for using the equity method										9,240			—			9,240
	Other income (loss)										4,850			—			4,850
	Other expenses										(15,279		)	347			(14,932		)
	Interest expense										(14,334		)	—			(14,334		)
	Net foreign exchange gains (losses)										(2,764		)	470			(2,294		)
	Income before income taxes										215,521			3,485			219,006
	Income tax expense										(7,289		)	—			(7,289		)
	Net income										208,232			3,485			211,717
	Dividends attributable to redeemable noncontrolling interests										—			(4,857		)	(4,857		)
	Amounts attributable to noncontrolling interests										—			1,156			1,156
	Net income available to Arch										208,232			(216		)	208,016
	Preferred dividends										(5,485		)	—			(5,485		)
	Net income available to Arch common shareholders										$	202,747		$	(216	)	$	202,531
	Underwriting Ratios
	Loss ratio	61.4		%	44.8		%	30.4		%	53.4		%	63.1		%	53.5		%
	Acquisition expense ratio (2)	15		%	19.7		%	22.6		%	17.2		%	32.3		%	17.4		%
	Other operating expense ratio	16.9		%	11.2		%	32		%	15.6		%	12.6		%	15.6		%
	Combined ratio	93.3		%	75.7		%	85		%	86.2		%	108		%	86.5		%
	Goodwill and intangible assets	$	24,498		$	4,942		$	89,281		$	118,721		$	—		$	118,721
	Total investable assets										$	14,688,881		$	1,114,719		$	15,803,600
	Total assets										21,210,197			1,363,318			22,573,515
	Total liabilities										15,078,943			253,507			15,332,450
	_________________________________________________
	-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.
	-2	The acquisition expense ratio is adjusted to include certain other underwriting income.
		Three Months Ended
		30-Jun-13
		Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
	Gross premiums written (1)	$	703,904		$	318,898		$	18,744		$	1,040,738		$	—		$	1,040,738
	Premiums ceded	(202,336		)	(28,675		)	—			(230,203		)	—			(230,203		)
	Net premiums written	501,568			290,223			18,744			810,535			—			810,535
	Change in unearned premiums	(42,912		)	(2,731		)	(6,076		)	(51,719		)	—			(51,719		)
	Net premiums earned	458,656			287,492			12,668			758,816			—			758,816
	Other underwriting income	529			373			—			902			—			902
	Losses and loss adjustment expenses	(291,192		)	(125,283		)	(2,178		)	(418,653		)	—			(418,653		)
	Acquisition expenses, net	(74,249		)	(53,291		)	(4,137		)	(131,677		)	—			(131,677		)
	Other operating expenses	(80,167		)	(31,902		)	(1,290		)	(113,359		)	—			(113,359		)
	Underwriting income	$	13,577		$	77,389		$	5,063		96,029			—			96,029
	Net investment income										68,369			—			68,369
	Net realized gains										12,652			—			12,652
	Net impairment losses recognized in earnings										(724		)	—			(724		)
	Equity in net income of investment funds accounted for using the equity method										10,941			—			10,941
	Other income (loss)										834			—			834
	Other expenses										(14,049		)	—			(14,049		)
	Interest expense										(5,852		)	—			(5,852		)
	Net foreign exchange gains										13,811			—			13,811
Income before income taxes										182,011			—			182,011
Income tax expense										(5,071		)	—			(5,071		)
Net income										176,940			—			176,940
Dividends attributable to redeemable noncontrolling interests										—			—			—
Amounts attributable to noncontrolling interests										—			—			—
Net income available to Arch										176,940			—			176,940
Preferred dividends										(5,485		)	—			(5,485		)
Net income available to Arch common shareholders										$	171,455		$	—		$	171,455
Underwriting Ratios
Loss ratio	63.5		%	43.6		%	17.2		%	55.2		%	—		%	55.2		%
Acquisition expense ratio (2)	16.1		%	18.5		%	32.7		%	17.3		%	—		%	17.3		%
Other operating expense ratio	17.5		%	11.1		%	10.2		%	14.9		%	—		%	14.9		%
Combined ratio	97.1		%	73.2		%	60.1		%	87.4		%	—		%	87.4		%
Goodwill and intangible assets	$	20,888		$	10,072		$	—		$	30,960		$	—		$	30,960
Total investable assets										$	12,960,811		$	—		$	12,960,811
Total assets										18,917,683			—			18,917,683
Total liabilities										13,683,365			—			13,683,365
_________________________________________________
-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.
-2	The acquisition expense ratio is adjusted to include certain other underwriting income.
	Six Months Ended
	30-Jun-14
	Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
Gross premiums written (1)	$	1,582,877		$	866,894		$	103,383		$	2,552,070		$	86,756		$	2,566,897
Premiums ceded	(458,393		)	(132,121		)	(9,718		)	(599,148		)	(2,760		)	(529,979		)
Net premiums written	1,124,484			734,773			93,665			1,952,922			83,996			2,036,918
Change in unearned premiums	(139,271		)	(57,798		)	(4,067		)	(201,136		)	(68,850		)	(269,986		)
Net premiums earned	985,213			676,975			89,598			1,751,786			15,146			1,766,932
Other underwriting income	1,014			619			1,982			3,615			—			3,615
Losses and loss adjustment expenses	(598,296		)	(289,961		)	(23,951		)	(912,208		)	(9,550		)	(921,758		)
Acquisition expenses, net	(153,381		)	(139,468		)	(20,635		)	(313,484		)	(5,016		)	(318,500		)
Other operating expenses	(166,973		)	(73,861		)	(30,164		)	(270,998		)	(2,744		)	(273,742		)
Underwriting income (loss)	$	67,577		$	174,304		$	16,830		258,711			(2,164		)	256,547
Net investment income										139,451			533			139,984
Net realized gains										70,663			3,178			73,841
Net impairment losses recognized in earnings										(17,720		)	—			(17,720		)
Equity in net income of investment funds accounted for using the equity method										12,493			—			12,493
Other income (loss)										2,746			—			2,746
Other expenses										(26,078		)	(2,329		)	(28,407		)
Interest expense										(28,738		)	—			(28,738		)
Net foreign exchange gains (losses)										(9,420		)	563			(8,857		)
Income before income taxes										402,108			(219		)	401,889
Income tax expense										(11,027		)	—			(11,027		)
Net income (loss)										391,081			(219		)	390,862
Dividends attributable to redeemable noncontrolling interests										—			(4,909		)	(4,909		)
Amounts attributable to noncontrolling interests										—			4,563			4,563
Net income available to Arch										391,081			(565		)	390,516
Preferred dividends										(10,969		)	—			(10,969		)
Net income available to Arch common shareholders										$	380,112		$	(565	)	$	379,547
Underwriting Ratios
Loss ratio	60.7		%	42.8		%	26.7		%	52.1		%	63.1		%	52.2		%
Acquisition expense ratio (2)	15.5		%	20.6		%	23		%	17.8		%	33.1		%	18		%
Other operating expense ratio	16.9		%	10.9		%	33.7		%	15.5		%	18.1		%	15.5		%
Combined ratio	93.1		%	74.3		%	83.4		%	85.4		%	114.3		%	85.7		%
_________________________________________________
-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.
-2	The acquisition expense ratio is adjusted to include certain other underwriting income.
	Six Months Ended
	30-Jun-13
	Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
Gross premiums written (1)	$	1,392,721		$	769,345		$	44,502		$	2,204,437		$	—		$	2,204,437
Premiums ceded	(386,603		)	(56,654		)	—			(441,126		)	—			(441,126		)
Net premiums written	1,006,118			712,691			44,502			1,763,311			—			1,763,311
Change in unearned premiums	(102,497		)	(129,046		)	(20,182		)	(251,725		)	—			(251,725		)
Net premiums earned	903,621			583,645			24,320			1,511,586			—			1,511,586
Other underwriting income	1,054			386			—			1,440			—			1,440
Losses and loss adjustment expenses	(574,659		)	(239,140		)	(4,257		)	(818,056		)	—			(818,056		)
Acquisition expenses, net	(145,007		)	(106,434		)	(7,828		)	(259,269		)	—			(259,269		)
Other operating expenses	(156,482		)	(64,099		)	(2,693		)	(223,274		)	—			(223,274		)
Underwriting income	$	28,527		$	174,358		$	9,542		212,427			—			212,427
Net investment income										134,041			—			134,041
Net realized gains										70,992			—			70,992
Net impairment losses recognized in earnings										(2,970		)	—			(2,970		)
Equity in net income of investment funds accounted for using the equity method										24,764			—			24,764
Other income (loss)										2,078			—			2,078
Other expenses										(24,317		)	—			(24,317		)
Interest expense										(11,750		)	—			(11,750		)
Net foreign exchange gains										38,075			—			38,075
Income before income taxes										443,340			—			443,340
Income tax expense										(9,924		)	—			(9,924		)
Net income										433,416			—			433,416
Dividends attributable to redeemable noncontrolling interests										—			—			—
Amounts attributable to noncontrolling interests										—			—			—
Net income available to Arch										433,416			—			433,416
Preferred dividends										(10,969		)	—			(10,969		)
Net income available to Arch common shareholders										$	422,447		$	—		$	422,447
Underwriting Ratios
Loss ratio	63.6		%	41		%	17.5		%	54.1		%	—		%	54.1		%
Acquisition expense ratio (2)	15.9		%	18.2		%	32.2		%	17.1		%	—		%	17.1		%
Other operating expense ratio	17.3		%	11		%	11.1		%	14.8		%	—		%	14.8		%
Combined ratio	96.8		%	70.2		%	60.8		%	86		%	—		%	86		%
_________________________________________________
-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.
-2	The acquisition expense ratio is adjusted to include certain other underwriting income.

Investment_Information_Tables

Investment Information (Tables)	6 Months Ended
	Jun. 30, 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 investments classified as available for sale:
		Fair			Gross			Gross			Cost or			OTTI
		Value			Unrealized			Unrealized			Amortized			Unrealized
					Gains			Losses			Cost			Losses (2)
	30-Jun-14
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	2,906,484		$	48,736		$	(10,472	)	$	2,868,220		$	(16	)
	Mortgage backed securities	1,138,385			25,925			(8,572		)	1,121,032			(3,602		)
	Municipal bonds	1,413,131			33,549			(1,095		)	1,380,677			—
	Commercial mortgage backed securities	1,119,401			18,238			(6,051		)	1,107,214			—
	U.S. government and government agencies	1,435,974			5,137			(2,162		)	1,432,999			(19		)
	Non-U.S. government securities	1,073,256			14,936			(5,399		)	1,063,719			—
	Asset backed securities	1,710,631			8,988			(6,740		)	1,708,383			(47		)
	Total	10,797,262			155,509			(40,491		)	10,682,244			(3,684		)
	Equity securities	608,820			100,476			(4,750		)	513,094			—
	Other investments	457,567			28,609			—			428,958			—
	Short-term investments	981,359			902			(1,191		)	981,648			—
	Total	$	12,845,008		$	285,496		$	(46,432	)	$	12,605,944		$	(3,684	)
	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 June 30, 2014, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $2.0 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
		Fair			Gross			Fair			Gross			Fair			Gross
		Value			Unrealized			Value			Unrealized			Value			Unrealized
					Losses						Losses						Losses
	30-Jun-14
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	546,004		$	(3,596	)	$	238,526		$	(6,876	)	$	784,530		$	(10,472	)
	Mortgage backed securities	176,720			(1,893		)	142,523			(6,679		)	319,243			(8,572		)
	Municipal bonds	99,417			(290		)	59,847			(805		)	159,264			(1,095		)
	Commercial mortgage backed securities	163,092			(2,219		)	95,877			(3,832		)	258,969			(6,051		)
	U.S. government and government agencies	338,288			(1,783		)	13,616			(379		)	351,904			(2,162		)
	Non-U.S. government securities	361,383			(2,571		)	69,218			(2,828		)	430,601			(5,399		)
	Asset backed securities	362,407			(2,135		)	215,752			(4,605		)	578,159			(6,740		)
	Total	2,047,311			(14,487		)	835,359			(26,004		)	2,882,670			(40,491		)
	Equity securities	81,731			(4,750		)	—			—			81,731			(4,750		)
	Other investments	—			—			—			—			—			—
	Short-term investments	95,699			(1,191		)	—			—			95,699			(1,191		)
	Total	$	2,224,741		$	(20,428	)	$	835,359		$	(26,004	)	$	3,060,100		$	(46,432	)
	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	'
	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.
			30-Jun-14						31-Dec-13
	Maturity		Fair Value			Amortized			Fair Value			Amortized
						Cost						Cost
	Due in one year or less		$	209,116		$	203,646		$	235,330		$	232,652
	Due after one year through five years		4,408,427			4,369,079			3,738,500			3,718,920
	Due after five years through 10 years		1,958,624			1,924,301			1,966,536			1,979,510
	Due after 10 years		252,678			248,589			196,305			198,286
			6,828,845			6,745,615			6,136,671			6,129,368
	Mortgage backed securities		1,138,385			1,121,032			1,133,095			1,139,034
	Commercial mortgage backed securities		1,119,401			1,107,214			1,074,497			1,075,749
	Asset backed securities		1,710,631			1,708,383			1,332,594			1,326,356
	Total		$	10,797,262		$	10,682,244		$	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:
		June 30,			December 31,
		2014			2013
	Available for sale:
	Asian and emerging markets	$	292,772		$	331,984
	Investment grade fixed income	164,795			159,115
	Other	—			7,211
	Total available for sale	457,567			498,310
	Fair value option:
	Term loan investments (par value: $909,538 and $494,502)	953,649			512,076
	Asian and emerging markets	25,321			14,054
	Investment grade fixed income	81,737			75,062
	Other (1)	310,829			172,088
	Total fair value option	1,371,536			773,280
	Total	$	1,829,103		$	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:
		June 30,			December 31,
		2014			2013
	Fixed maturities	$	438,048		$	448,254
	Other investments	1,371,536			773,280
	Short-term investments	231,507			—
	Investments accounted for using the fair value option	$	2,041,091		$	1,221,534
Components of net investment income	'
	The components of net investment income were derived from the following sources:
		Three Months Ended						Six Months Ended
		June 30,						June 30,
		2014			2013			2014			2013
	Fixed maturities	$	65,869		$	62,004		$	128,318		$	124,010
	Term loan investments	6,908			6,026			12,577			10,243
	Equity securities (dividends)	3,271			3,164			6,192			4,587
	Short-term investments	107			364			512			756
	Other (1)	9,099			4,734			13,819			11,033
	Gross investment income	85,254			76,292			161,418			150,629
	Investment expenses	(12,264		)	(7,923		)	(21,434		)	(16,588		)
	Net investment income	$	72,990		$	68,369		$	139,984		$	134,041
	_________________________________________________
	-1	Includes dividends on investment funds and other items.
Summary of net realized gains (losses)	'
	Net realized gains (losses) were as follows, excluding other-than-temporary impairment provisions:
		Three Months Ended						Six Months Ended
		June 30,						June 30,
		2014			2013			2014			2013
	Available for sale securities:
	Gross gains on investment sales	$	56,763		$	68,853		$	122,814		$	135,873
	Gross losses on investment sales	(33,028		)	(54,783		)	(73,041		)	(79,839		)
	Change in fair value of assets and liabilities accounted for using the fair value option:
	Fixed maturities	10,984			(6,702		)	9,341			(1,644		)
	Other investments	17,985			(9,502		)	28,761			449
	Equity securities	—			5			—			704
	Derivative instruments (1)	12,402			15,646			(366		)	15,268
	Other (2)	(10,962		)	(865		)	(13,668		)	181
	Net realized gains (losses)	$	54,144		$	12,652		$	73,841		$	70,992
	_________________________________________________
	-1	See Note 9 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 following table details the net impairment losses recognized in earnings by asset class:
		Three Months Ended						Six Months Ended
		June 30,						June 30,
		2014			2013			2014			2013
	Fixed maturities:
	Mortgage backed securities	$	—		$	—		$	—		$	(15	)
	Corporate bonds	(664		)	—			(664		)	—
	Asset backed securities	(5		)	—			(11		)	(20		)
	Total	(669		)	—			(675		)	(35		)
	Equity securities	(278		)	(724		)	(278		)	(2,935		)
	Other investments	(13,802		)	—			(16,767		)	—
	Net impairment losses recognized in earnings	$	(14,749	)	$	(724	)	$	(17,720	)	$	(2,970	)
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:
		Three Months Ended						Six Months Ended
		June 30,						June 30,
		2014			2013			2014			2013
	Balance at start of period	$	47,256		$	61,956		$	60,062		$	62,001
	Credit loss impairments recognized on securities not previously impaired	—			—			—			33
	Credit loss impairments recognized on securities previously impaired	5			—			11			2
	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	(25,820		)	(507		)	(38,632		)	(587		)
	Balance at end of period	$	21,441		$	61,449		$	21,441		$	61,449
Summary of restricted assets	'
	The following table details the value of the Company’s restricted assets:
		June 30,			December 31,
		2014			2013
	Assets used for collateral or guarantees:
	Affiliated transactions	$	4,352,540		$	4,060,533
	Third party agreements	943,551			856,890
	Deposits with U.S. regulatory authorities	344,087			302,809
	Deposits with non-U.S. regulatory authorities	—			6,546
	Trust funds	81,942			75,264
	Total restricted assets	$	5,722,120		$	5,302,042

Fair_Value_Tables

Fair Value (Tables)	6 Months Ended
	Jun. 30, 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 June 30, 2014:
					Fair Value Measurement Using:
		Fair			Quoted Prices in			Significant			Significant
		Value			Active Markets			Other			Unobservable
					for Identical			Observable			Inputs
					Assets			Inputs			(Level 3)
					(Level 1)			(Level 2)
	Assets measured at fair value:
	Available for sale securities:
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	2,906,484		$	—		$	2,906,484		$	—
	Mortgage backed securities	1,138,385			—			1,138,385			—
	Municipal bonds	1,413,131			—			1,413,131			—
	Commercial mortgage backed securities	1,119,401			—			1,119,401			—
	U.S. government and government agencies	1,435,974			1,435,974			—			—
	Non-U.S. government securities	1,073,256			—			1,073,256			—
	Asset backed securities	1,710,631			—			1,710,631			—
	Total	10,797,262			1,435,974			9,361,288			—
	Equity securities	608,820			608,820			—			—
	Other investments	457,567			—			354,354			103,213
	Short-term investments	981,359			948,468			32,891			—
	Fair value option:
	Corporate bonds	343,607			—			343,607			—
	Non-U.S. government bonds	60,268			—			60,268			—
	Mortgage backed securities	16,807			—			16,807			—
	Asset backed securities	17,366			—			17,366			—
	Other investments	1,371,536			—			920,500			451,036
	Short-term investments	231,507			231,507			—			—
	Total	2,041,091			231,507			1,358,548			451,036
	Total assets measured at fair value	$	14,886,099		$	3,224,769		$	11,107,081		$	554,249
	Liabilities measured at fair value:
	Contingent consideration liability	$	53,099		$	—		$	—		$	53,099
	Total liabilities measured at fair value	$	53,099		$	—		$	—		$	53,099
	_________________________________________________
	-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:
		Fair			Quoted Prices in			Significant			Significant
		Value			Active Markets			Other			Unobservable
					for Identical			Observable			Inputs
					Assets			Inputs			(Level 3)
					(Level 1)			(Level 2)
	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:
	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 tables present a reconciliation of the beginning and ending balances for all financial assets and liabilities measured at fair value on a recurring basis using Level 3 inputs:
		Fair Value Measurements Using:
		Significant Unobservable Inputs (Level 3)
		Assets												Liabilities
	s	Available-For-Sale						Fair Value Option
		Corporate			Other			Other			Total			Contingent Consideration Liability
		Bonds			Investments			Investments
	Three Months Ended June 30, 2014
	Balance at beginning of period	$	—		$	104,335		$	419,146		$	523,481		$	43,156
	Total gains or (losses) (realized/unrealized)
	Included in earnings (1)	—			(1,722		)	16,136			14,414			9,943
	Included in other comprehensive income	—			600			932			1,532			—
	Purchases, issuances, sales and settlements
	Purchases	—			—			66,615			66,615			—
	Issuances	—			—			—			—			—
	Sales	—			—			—			—			—
	Settlements	—			—			(51,793		)	(51,793		)	—
	Transfers in and/or out of Level 3	—			—			—			—			—
	Balance at end of period	$	—		$	103,213		$	451,036		$	554,249		$	53,099
	Three Months Ended June 30, 2013
	Balance at beginning of period	$	97,298		$	185,934		$	179,370		$	462,602		$	—
	Total gains or (losses) (realized/unrealized)
	Included in earnings (1)	3,187			—			(2,075		)	1,112			—
	Included in other comprehensive income	(1,363		)	3,959			—			2,596			—
	Purchases, issuances, sales and settlements
	Purchases	—			—			118,916			118,916			—
	Issuances	—			—			—			—			—
	Sales	(96,655		)	—			—			(96,655		)	—
	Settlements	(107		)	—			(6,022		)	(6,129		)	—
	Transfers in and/or out of Level 3	—			—			—			—			—
	Balance at end of period	$	2,360		$	189,893		$	290,189		$	482,442		$	—
	_________________________________________________
	-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. Gains or losses on the contingent consideration liability were included in net realized gains (losses).
		Fair Value Measurements Using:
		Significant Unobservable Inputs (Level 3)
		Assets												Liabilities
	s	Available-For-Sale						Fair Value Option
		Corporate			Other			Other			Total			Contingent Consideration Liability
		Bonds			Investments			Investments
	Six Months Ended June 30, 2014
	Balance at beginning of period	$	2,045		$	170,420		$	377,525		$	549,990		$	—
	Total gains or (losses) (realized/unrealized)
	Included in earnings (1)	—			553			22,446			22,999			11,337
	Included in other comprehensive income	—			(1,596		)	932			(664		)	—
	Purchases, issuances, sales and settlements
	Purchases	—			—			106,725			106,725			—
	Issuances	—			—			—			—			41,762
	Sales	(2,045		)	(66,164		)	—			(68,209		)	—
	Settlements	—			—			(56,592		)	(56,592		)	—
	Transfers in and/or out of Level 3	—			—			—			—			—
	Balance at end of period	$	—		$	103,213		$	451,036		$	554,249		$	53,099
	Six Months Ended June 30, 2013
	Balance at beginning of period	$	98,404		$	184,202		$	195,350		$	477,956		$	—
	Total gains or (losses) (realized/unrealized)
	Included in earnings (1)	4,679			4,762			(1,702		)	7,739			—
	Included in other comprehensive income	(3,051		)	5,871			—			2,820			—
	Purchases, issuances, sales and settlements
	Purchases	—			5,000			160,570			165,570			—
	Issuances	—			—			—			—			—
	Sales	(96,655		)	—			—			(96,655		)	—
	Settlements	(1,017		)	(9,942		)	(64,029		)	(74,988		)	—
	Transfers in and/or out of Level 3	—			—			—			—			—
	Balance at end of period	$	2,360		$	189,893		$	290,189		$	482,442		$	—
_________________________________________________
-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. Gains or losses on the contingent consideration liability were included in net realized gains (losses).

Derivative_Instruments_Tables

Derivative Instruments (Tables)	6 Months Ended
	Jun. 30, 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)
	30-Jun-14
	Futures contracts	$	986			$	(198	)		$	788			$	783,914
	Foreign currency forward contracts	856				(1,949		)		(1,093		)		279,559
	TBAs	160,074				(149,782		)		10,292				296,380
	Other	1,100				(722		)		378				504,709
	Total	$	163,016			$	(152,651	)		$	10,365
	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
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:
			Three Months Ended								Six Months Ended
	Derivatives not designated as		June 30,								June 30,
	hedging instruments		2014				2013				2014				2013
	Futures contracts		$	15,185			$	10,623			$	5,323			$	9,794
	Foreign currency forward contracts		(1,811		)		7,605				(4,068		)		6,880
	TBAs		(270		)		(3,751		)		(307		)		(3,210		)
	Other		(702		)		1,169				(1,314		)		1,804
	Total		$	12,402			$	15,646			$	(366	)		$	15,268

Other_Comprehensive_Income_Los1

Other Comprehensive Income (Loss) (Tables)	6 Months Ended
	Jun. 30, 2014
Comprehensive Income Note Disclosure [Abstract]	'
Details about amounts reclassified from AOCI	'
	The following table presents details about amounts reclassified from accumulated other comprehensive income:
					Amounts Reclassed from AOCI
			Consolidated Statement of Income		Three Months Ended								Six Months Ended
	Details About		Line Item That Includes		June 30,								June 30,
	AOCI Components		Reclassification		2014				2013				2014			2013
	Unrealized appreciation on available-for-sale investments
			Net realized gains		$	23,735			$	14,634			$	49,773		$	59,001
			Other-than-temporary impairment losses		(14,749		)		(724		)		(17,720		)	(2,972		)
			Total before tax		8,986				13,910				32,053			56,029
			Income tax (expense) benefit		(701		)		6				(2,519		)	(3,412		)
			Net of tax		$	8,285			$	13,916			$	29,534		$	52,617
Schedule of comprehensive income (loss)	'
	The following table presents the tax effects allocated to each component of other comprehensive income (loss):
		Before Tax Amount				Tax Expense (Benefit)				Net of Tax Amount
	Three Months Ended June 30, 2014
	Unrealized appreciation (decline) in value of investments:
	Unrealized holding gains (losses) arising during period	$	115,150			$	6,722			$	108,428
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	—				—				—
	Less reclassification of net realized gains included in net income	8,986				701				8,285
	Foreign currency translation adjustments	10,021				—				10,021
	Other comprehensive income (loss)	$	116,185			$	6,021			$	110,164
	Three Months Ended June 30, 2013
	Unrealized appreciation (decline) in value of investments:
	Unrealized holding gains (losses) arising during period	$	(281,388	)		$	(21,826	)		$	(259,562	)
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	—				—				—
	Less reclassification of net realized gains included in net income	13,910				(6		)		13,916
	Foreign currency translation adjustments	(5,407		)		—				(5,407		)
	Other comprehensive income (loss)	$	(300,705	)		$	(21,820	)		$	(278,885	)
	Six Months Ended June 30, 2014
	Unrealized appreciation (decline) in value of investments:
	Unrealized holding gains (losses) arising during period	$	190,550			$	10,769			$	179,781
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	—				—				—
	Less reclassification of net realized gains included in net income	32,053				2,519				29,534
	Foreign currency translation adjustments	8,672				—				8,672
	Other comprehensive income (loss)	$	167,169			$	8,250			$	158,919
	Six Months Ended June 30, 2013
	Unrealized appreciation (decline) in value of investments:
	Unrealized holding gains (losses) arising during period	$	(271,041	)		$	(20,950	)		$	(250,091	)
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	(2		)		—				(2		)
	Less reclassification of net realized gains included in net income	56,029				3,412				52,617
	Foreign currency translation adjustments	(33,629		)		—				(33,629		)
	Other comprehensive income (loss)	$	(360,701	)		$	(24,362	)		$	(336,339	)

Guarantor_Financial_Informatio1

Guarantor Financial Information (Tables)	6 Months Ended
	Jun. 30, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]	'
Condensed consolidating balance sheet	'
			30-Jun-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		$	2,561			$	78,702			$	15,081,872			$	—			$	15,163,135
	Cash		4,715				7,746				913,982				—				926,443
	Investments in subsidiaries		6,629,043				1,639,914				—				(8,268,957		)		—
	Due from subsidiaries and affiliates		636				—				411,687				(412,323		)		—
	Premiums receivable		—				—				1,477,354				(378,662		)		1,098,692
	Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses		—				—				5,634,614				(3,838,211		)		1,796,403
	Contractholder receivables		—				—				1,234,392				—				1,234,392
	Prepaid reinsurance premiums		—				—				1,407,938				(977,724		)		430,214
	Deferred acquisition costs, net		—				—				399,385				—				399,385
	Other assets		8,025				64,629				2,022,687				(570,490		)		1,524,851
		Total assets	$	6,644,980			$	1,790,991			$	28,583,911			$	(14,446,367	)		$	22,573,515
	Liabilities
	Reserve for losses and loss adjustment expenses		$	—			$	—			$	12,848,119			$	(3,829,130	)		$	9,018,989
	Unearned premiums		—				—				3,277,416				(977,724		)		2,299,692
	Reinsurance balances payable		—				—				804,239				(540,892		)		263,347
	Contractholder payables		—				—				1,234,392				—				1,234,392
	Deposit accounting liabilities		—				—				715,369				(318,032		)		397,337
	Senior notes		300,000				500,000				—				—				800,000
	Revolving credit agreement borrowings		100,000				—				—				—				100,000
	Due to subsidiaries and affiliates		4,954				1,189				406,180				(412,323		)		—
	Other liabilities		10,627				45,057				1,262,318				(99,309		)		1,218,693
		Total liabilities	415,581				546,246				20,548,033				(6,177,410		)		15,332,450
	Redeemable noncontrolling interests (1)		—				—				219,326				—				219,326
	Shareholders’ Equity
	Total shareholders’ equity available to Arch		6,229,399				1,244,745				7,024,212				(8,268,957		)		6,229,399
	Non-redeemable noncontrolling interests (1)		—				—				792,340				—				792,340
		Total shareholders’ equity	6,229,399				1,244,745				7,816,552				(8,268,957		)		7,021,739
		Total liabilities, noncontrolling interests and shareholders’ equity	$	6,644,980			$	1,790,991			$	28,583,911			$	(14,446,367	)		$	22,573,515
	(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	'
			Three Months Ended June 30, 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		$	—			$	—			$	907,152			$	—			$	907,152
	Net investment income		—				—				80,380				(7,390		)		72,990
	Net realized gains		—				—				54,144				—				54,144
	Net impairment losses recognized in earnings		—				—				(14,749		)		—				(14,749		)
	Other underwriting income		—				—				2,033				—				2,033
	Equity in net income of investment funds accounted for using the equity method		—				—				9,240				—				9,240
	Other income (loss)		—				—				4,850				—				4,850
		Total revenues	—				—				1,043,050				(7,390		)		1,035,660
	Expenses
	Losses and loss adjustment expenses		—				—				485,518				—				485,518
	Acquisition expenses		—				—				158,158				—				158,158
	Other operating expenses		15,250				494				140,606				—				156,350
	Interest expense		5,854				6,448				9,422				(7,390		)		14,334
	Net foreign exchange losses		—				—				1,054				1,240				2,294
		Total expenses	21,104				6,942				794,758				(6,150		)		816,654
	Income (loss) before income taxes		(21,104		)		(6,942		)		248,292				(1,240		)		219,006
	Income tax benefit (expense)		—				3,294				(10,583		)		—				(7,289		)
	Income (loss) before equity in net income of subsidiaries		(21,104		)		(3,648		)		237,709				(1,240		)		211,717
	Equity in net income of subsidiaries		229,120				9,334				—				(238,454		)		—
	Net income		208,016				5,686				237,709				(239,694		)		211,717
	Amounts attributable to noncontrolling interests (1)		—				—				(3,701		)		—				(3,701		)
	Net income available to Arch		208,016				5,686				234,008				(239,694		)		208,016
	Preferred dividends		(5,485		)		—				—				—				(5,485		)
	Net income available to Arch common shareholders		$	202,531			$	5,686			$	234,008			$	(239,694	)		$	202,531
	Comprehensive income available to Arch		$	318,180			$	23,186			$	335,348			$	(358,534	)		$	318,180
	(1)     See Note 4.
			Three Months Ended June 30, 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		$	—			$	—			$	758,816			$	—			$	758,816
	Net investment income		—				—				74,980				(6,611		)		68,369
	Net realized gains		—				—				12,652				—				12,652
	Net impairment losses recognized in earnings		—				—				(724		)		—				(724		)
	Other underwriting income		—				—				902				—				902
	Equity in net income of investment funds accounted for using the equity method		—				—				10,941				—				10,941
	Other income		—				—				834				—				834
		Total revenues	—				—				858,401				(6,611		)		851,790
	Expenses
	Losses and loss adjustment expenses		—				—				418,653				—				418,653
	Acquisition expenses		—				—				131,677				—				131,677
	Other operating expenses		13,691				773				112,944				—				127,408
	Interest expense		5,843				5				6,615				(6,611		)		5,852
	Net foreign exchange (gains) losses		—				—				(15,882		)		2,071				(13,811		)
		Total expenses	19,534				778				654,007				(4,540		)		669,779
	Income (loss) before income taxes		(19,534		)		(778		)		204,394				(2,071		)		182,011
	Income tax benefit (expense)		—				2,750				(7,821		)		—				(5,071		)
	Income (loss) before equity in net income of subsidiaries		(19,534		)		1,972				196,573				(2,071		)		176,940
	Equity in net income of subsidiaries		196,474				10,703				—				(207,177		)		—
	Net income		176,940				12,675				196,573				(209,248		)		176,940
	Amounts attributable to noncontrolling interests		—				—				—				—				—
	Net income available to Arch		176,940				12,675				196,573				(209,248		)		176,940
	Preferred dividends		(5,485		)		—				—				—				(5,485		)
	Net income available to Arch common shareholders		$	171,455			$	12,675			$	196,573			$	(209,248	)		$	171,455
	Comprehensive income available to Arch		$	(101,945	)		$	(28,981	)		$	(84,383	)		$	113,364			$	(101,945	)
			Six Months Ended June 30, 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		$	—			$	—			$	1,766,932			$	—			$	1,766,932
	Net investment income		—				—				157,381				(17,397		)		139,984
	Net realized gains		—				—				73,841				—				73,841
Net impairment losses recognized in earnings		—				—				(17,720		)		—				(17,720		)
Other underwriting income		—				—				3,615				—				3,615
Equity in net income of investment funds accounted for using the equity method		—				—				12,493				—				12,493
Other income (loss)		—				—				2,746				—				2,746
	Total revenues	—				—				1,999,288				(17,397		)		1,981,891
Expenses
Losses and loss adjustment expenses		—				—				921,758				—				921,758
Acquisition expenses		—				—				318,500				—				318,500
Other operating expenses		25,557				1,471				275,121				—				302,149
Interest expense		11,707				12,962				21,466				(17,397		)		28,738
Net foreign exchange losses		—				—				6,795				2,062				8,857
	Total expenses	37,264				14,433				1,543,640				(15,335		)		1,580,002
Income (loss) before income taxes		(37,264		)		(14,433		)		455,648				(2,062		)		401,889
Income tax benefit (expense)		—				6,082				(17,109		)		—				(11,027		)
Income (loss) before equity in net income of subsidiaries		(37,264		)		(8,351		)		438,539				(2,062		)		390,862
Equity in net income of subsidiaries		427,780				31,486				—				(459,266		)		—
Net income		390,516				23,135				438,539				(461,328		)		390,862
Amounts attributable to noncontrolling interests (1)		—				—				(346		)		—				(346		)
Net income available to Arch		390,516				23,135				438,193				(461,328		)		390,516
Preferred dividends		(10,969		)		—				—				—				(10,969		)
Net income available to Arch common shareholders		$	379,547			$	23,135			$	438,193			$	(461,328	)		$	379,547
Comprehensive income available to Arch		$	549,435			$	40,406			$	587,467			$	(627,873	)		$	549,435
(1)     See Note 4.
		Six Months Ended June 30, 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		$	—			$	—			$	1,511,586			$	—			$	1,511,586
Net investment income		—				—				147,302				(13,261		)		134,041
Net realized gains		—				—				70,992				—				70,992
Net impairment losses recognized in earnings		—				—				(2,970		)		—				(2,970		)
Other underwriting income		—				—				1,440				—				1,440
Equity in net income of investment funds accounted for using the equity method		—				—				24,764				—				24,764
Other income		—				—				2,078				—				2,078
	Total revenues	—				—				1,755,192				(13,261		)		1,741,931
Expenses
Losses and loss adjustment expenses		—				—				818,056				—				818,056
Acquisition expenses		—				—				259,269				—				259,269
Other operating expenses		23,633				1,550				222,408				—				247,591
Interest expense		11,663				8				13,340				(13,261		)		11,750
Net foreign exchange (gains) losses		—				—				(28,654		)		(9,421		)		(38,075		)
	Total expenses	35,296				1,558				1,284,419				(22,682		)		1,298,591
Income (loss) before income taxes		(35,296		)		(1,558		)		470,773				9,421				443,340
Income tax benefit (expense)		—				2,985				(12,909		)		—				(9,924		)
Income (loss) before equity in net income of subsidiaries		(35,296		)		1,427				457,864				9,421				433,416
Equity in net income of subsidiaries		468,712				29,185				—				(497,897		)		—
Net income		433,416				30,612				457,864				(488,476		)		433,416
Amounts attributable to noncontrolling interests		—				—				—				—				—
Net income available to Arch		433,416				30,612				457,864				(488,476		)		433,416
Preferred dividends		(10,969		)		—				—				—				(10,969		)
Net income available to Arch common shareholders		$	422,447			$	30,612			$	457,864			$	(488,476	)		$	422,447
Comprehensive income available to Arch		$	97,077			$	(22,829	)		$	130,946			$	(108,117	)		$	97,077
Condensed consolidating statement of cash flows	'
					Six Months Ended June 30, 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 Operating Activities			$	10,099			$	424			$	474,565			$	(32,025	)		$	453,063
	Investing Activities
	Purchases of fixed maturity investments				—				(78,509		)		(14,233,239		)		—				(14,311,748		)
	Purchases of equity securities				—				—				(174,687		)		—				(174,687		)
	Purchases of other investments				—				—				(1,022,987		)		—				(1,022,987		)
	Proceeds from the sales of fixed maturity investments				—				—				13,204,854				—				13,204,854
	Proceeds from the sales of equity securities				—				—				98,687				—				98,687
	Proceeds from the sales of other investments				—				—				618,707				—				618,707
	Proceeds from redemptions and maturities of fixed maturity investments				—				—				432,040				—				432,040
	Net (purchases) sales of short-term investments				(31		)		408,779				21,556				—				430,304
	Change in investment of securities lending collateral				—				—				18,701				—				18,701
	Contributions to subsidiaries				—				(313,207		)		(100,000		)		413,207				—
	Intercompany loans issued				—				—				10,250				(10,250		)		—
	Purchase of business, net of cash acquired				—				—				(235,578		)		—				(235,578		)
	Purchases of furniture, equipment and other assets				(128		)		—				(10,232		)		—				(10,360		)
		Net Cash Provided By (Used For) Investing Activities			(159		)		17,063				(1,371,928		)		402,957				(952,067		)
	Financing Activities
	Purchases of common shares under share repurchase program				—				—				—				—				—
	Proceeds from common shares issued, net				2,521				—				413,207				(413,207		)		2,521
	Repayments of intercompany borrowings				—				(10,250		)		—				10,250				—
	Change in securities lending collateral				—				—				(18,701		)		—				(18,701		)
	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)				—				—				(4,816		)		—				(4,816		)
	Dividends paid to parent				—				—				(32,025		)		32,025				—
	Other				—				—				4,706				—				4,706
	Preferred dividends paid				(10,969		)		—				—				—				(10,969		)
		Net Cash Provided By (Used For) Financing Activities			(8,448		)		(10,250		)		1,378,507				(370,932		)		988,877
	Effects of exchange rates changes on foreign currency cash				—				—				2,513				—				2,513
	Increase in cash				1,492				7,237				483,657				—				492,386
	Cash beginning of year				3,223				509				430,325				—				434,057
	Cash end of period				$	4,715			$	7,746			$	913,982			$	—			$	926,443
	(1)     See Note 4.
					Six Months Ended June 30, 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) Operating Activities			$	69,021			$	(534	)		$	412,867			$	(93,000	)		$	388,354
	Investing Activities
	Purchases of fixed maturity investments				—				—				(8,599,697		)		—				(8,599,697		)
	Purchases of equity securities				—				—				(272,323		)		—				(272,323		)
	Purchases of other investments				—				—				(648,915		)		—				(648,915		)
	Proceeds from the sales of fixed maturity investments				—				—				8,468,641				—				8,468,641
	Proceeds from the sales of equity securities				—				—				194,212				—				194,212
	Proceeds from the sales of other investments				—				—				506,434				—				506,434
	Proceeds from redemptions and maturities of fixed maturity investments				—				—				424,953				—				424,953
	Net (purchases) sales of short-term investments				(3,028		)		996				(373,114		)		—				(375,146		)
	Change in investment of securities lending collateral				—				—				3,221				—				3,221
	Contributions to subsidiaries				—				(10,250		)		(250		)		10,500				—
	Intercompany loans issued				—				—				(10,250		)		10,250				—
	Purchases of furniture, equipment and other assets				(131		)		—				(6,961		)		—				(7,092		)
		Net Cash Provided By (Used For) Investing Activities			(3,159		)		(9,254		)		(314,049		)		20,750				(305,712		)
	Financing Activities
	Purchases of common shares under share repurchase program				(56,463		)		—				—				—				(56,463		)
	Proceeds from common shares issued, net				(517		)		—				10,500				(10,500		)		(517		)
	Proceeds from intercompany borrowings				—				10,250				—				(10,250		)		—
	Change in securities lending collateral				—				—				(3,221		)		—				(3,221		)
	Dividends paid to parent				—				—				(93,000		)		93,000				—
	Other				—				—				5,042				—				5,042
Preferred dividends paid				(10,969		)		—				—				—				(10,969		)
	Net Cash Provided By (Used For) Financing Activities			(67,949		)		10,250				(80,679		)		72,250				(66,128		)
Effects of exchange rates changes on foreign currency cash				—				—				(12,436		)		—				(12,436		)
Increase (decrease) in cash				(2,087		)		462				5,703				—				4,078
Cash beginning of year				6,417				612				364,012				—				371,041
Cash end of period				$	4,330			$	1,074			$	369,715			$	—			$	375,119

General_Details

General (Details) (Watford Holdings Ltd, Variable Interest Entity, Primary Beneficiary, USD $)	0 Months Ended	1 Months Ended
In Millions, unless otherwise specified	Mar. 20, 2014	Mar. 31, 2014
Watford Holdings Ltd | Variable Interest Entity, Primary Beneficiary	'	'
Variable Interest Entity [Line Items]	'	'
Percentage ownership	11.00%	'
Initial investment contribution amount	$100	$100

Business_Acquired_Summary_of_f

Business Acquired - Summary of fair value of net assets acquired and allocation of purchase price(Details) (CMG Mortgage Insurance Company, USD $)	0 Months Ended
In Thousands, unless otherwise specified	Jan. 30, 2014
Business Acquisition [Line Items]	'
Cash paid	$245,157
Contingent consideration liability	41,762
Total purchase price (a)	286,919
Cash	9,579
Investments, at fair value	312,093
Other assets acquired	21,691
Total assets acquired	437,650
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 life of finite lived intangible liabilities acquired	'9 years
Insurance licenses	'
Business Acquisition [Line Items]	'
Intangible asset -- insurance licenses	16,858
Acquired insurance contracts	'
Business Acquisition [Line Items]	'
Intangible asset	46,473
Useful life of finite lived intangible assets acquired	'5 years
Operating platform	'
Business Acquisition [Line Items]	'
Intangible asset	29,900
Useful life of finite lived intangible assets acquired	'5 years
Favorable lease contract	'
Business Acquisition [Line Items]	'
Intangible asset	$1,056
Useful life of finite lived intangible assets acquired	'5 years

Business_Acquired_Narrative_De

Business Acquired - Narrative (Details) (USD $)	0 Months Ended	6 Months Ended	0 Months Ended			6 Months Ended				5 Months Ended
	Jan. 30, 2014	Jun. 30, 2014	Jan. 30, 2014	Jan. 30, 2014	Jan. 30, 2014	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014
	CMG Mortgage Insurance Company	CMG Mortgage Insurance Company	Stock Purchase Agreement (SPA)	Asset Purchase Agreement (APA)	Maximum	Maximum	Minimum	New mortgage insurance business (post closing)	Contingent consideration liability	Net intangible assets acquired
			CMG Mortgage Insurance Company	CMG Mortgage Insurance Company	CMG Mortgage Insurance Company	CMG Mortgage Insurance Company	CMG Mortgage Insurance Company	CMG Mortgage Insurance Company	CMG Mortgage Insurance Company
Business Acquisition [Line Items]	'	'	'	'	'	'	'	'	'	'
Amortization of Intangible Assets	'	'	'	'	'	'	'	'	'	$9,300,000
Percentage of voting interests acquired	100.00%	'	'	'	'	'	'	'	'	'
Quota share agreement percentage	'	100.00%	'	'	'	'	'	20.00%	'	'
Quota share reinsurance agreement limit	'	'	'	'	'	'	'	25,000,000,000	'	'
Cash paid	245,157,000	'	160,600,000	84,600,000	'	'	'	'	'	'
Percentage of book value	'	'	80.00%	'	150.00%	'	'	'	'	'
Contingent consideration, upper bound	'	136,900,000	'	'	'	'	'	'	'	'
Discount rate, percentage	15.00%	'	'	'	'	'	'	'	'	'
Extension period	'	'	'	'	'	'3 years	'1 year	'	'	'
Contingent consideration liability	41,762,000	'	'	'	'	'	'	'	53,099,000	'
Goodwill	13,852,000	'	'	'	'	'	'	'	'	'
Goodwill, deferred tax asset	'	$48,000,000	'	'	'	'	'	'	'	'

Significant_Accounting_Policie2

Significant Accounting Policies (Details)	6 Months Ended
	Jun. 30, 2014
Significant Accounting Policies [Line Items]	'
Premium revenue recognition period	'12 months
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

Variable_Interest_Entities_and2

Variable Interest Entities and Noncontrolling Interests - Variable interest entity (Details) (Variable Interest Entity, Primary Beneficiary, Watford Holdings Ltd, USD $)	0 Months Ended	1 Months Ended
	Mar. 20, 2014	Mar. 31, 2014
Variable Interest Entity, Primary Beneficiary | Watford Holdings Ltd	'	'
Variable Interest Entity [Line Items]	'	'
Initial capitalization amount	'	$1,100,000,000
Proceeds from issuance of common shares	'	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	$100,000,000
Ownership percentage	'	11.00%

Variable_Interest_Entities_and3

Variable Interest Entities and Noncontrolling Interests - Non-redeemable noncontrolling interests (Details) (USD $)	3 Months Ended				6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Noncontrolling Interest [Line Items]	'		'		'		'
Non-redeemable noncontrolling interests, beginning of year	'		'		$0		'
Net (loss) income attributable to noncontrolling interests	1,156		0		4,563		0
Non-redeemable noncontrolling interests, end of period	792,340	[1]	0	[1]	792,340	[1]	0	[1]
Watford Holdings Ltd | Noncontrolling Interest	'		'		'		'
Noncontrolling Interest [Line Items]	'		'		'		'
Non-redeemable noncontrolling interests, beginning of year	793,496	[1]	'		0	[1]	'
Sale of shares to noncontrolling interests	0		'		796,903		'
Net (loss) income attributable to noncontrolling interests	-1,156		'		-4,563		'
Non-redeemable noncontrolling interests, end of period	$792,340	[1]	'		$792,340	[1]	'
Common shares | Watford Holdings Ltd | Noncontrolling Interest	'		'		'		'
Noncontrolling Interest [Line Items]	'		'		'		'
Noncontrolling interest, ownership percentage by noncontrolling owners	89.00%		'		89.00%		'
[1]	See Note 4.

Variable_Interest_Entities_and4

Variable Interest Entities and Noncontrolling Interests - Redeemable noncontrolling interests (Details) (Watford Holdings Ltd, USD $)	6 Months Ended
	Jun. 30, 2014
Redeemable Noncontrolling Interest [Line Items]	'
Par value per share	$0.01
Liquidation preference per share	$25
Issue price per share	$24.50
Preference shares, number of shares issued	9,065,200
Preference shares, dividend rate (percentage)	8.50%
3 month US dollar LIBOR	'
Redeemable Noncontrolling Interest [Line Items]	'
Preference shares, dividend rate (percentage)	1.00%
Cumulative redeemable preference shares	'
Redeemable Noncontrolling Interest [Line Items]	'
Noncontrolling interest, ownership percentage by noncontrolling owners	100.00%

Earnings_Per_Common_Share_Deta

Earnings Per Common Share (Details) (USD $)	3 Months Ended				6 Months Ended
In Thousands, except Share data, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Numerator:	'		'		'		'
Net income	$211,717		$176,940		$390,862		$433,416
Amounts attributable to noncontrolling interests	-3,701	[1]	0	[1]	-346	[1]	0	[1]
Net income available to Arch	208,016		176,940		390,516		433,416
Preferred dividends	-5,485		-5,485		-10,969		-10,969
Net income available to Arch common shareholders	$202,531		$171,455		$379,547		$422,447
Denominator:	'		'		'		'
Weighted average common shares outstanding b basic	132,650,634		131,377,274		132,256,462		131,143,885
Effect of dilutive common share equivalents:	'		'		'		'
Nonvested restricted shares	1,144,621		1,088,030		1,236,408		1,181,947
Stock options	3,094,689		3,383,746		3,224,019		3,298,394
Weighted average common shares and common share equivalents outstanding b diluted	136,889,944		135,849,050		136,716,889		135,624,226
Earnings per common share:	'		'		'		'
Basic (per share)	$1.53		$1.31		$2.87		$3.22
Diluted (per share)	$1.48		$1.26		$2.78		$3.11
Stock Options	'		'		'		'
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'		'		'		'
Antidilutive securities excluded from computation of earnings per common share (shares)	978,237		1,428,616		1,318,662		1,730,313
[1]	See Note 4.

Segment_Information_Summary_of

Segment Information - Summary of underwriting income or loss by segment (Details) (USD $)	3 Months Ended				6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013		Dec. 31, 2013
Segment Reporting Information [Line Items]	'		'		'		'		'
Gross premiums written	$1,271,761		$1,040,738		$2,566,897		$2,204,437		'
Premiums ceded	-299,833		-230,203		-529,979		-441,126		'
Net premiums written	971,928		810,535		2,036,918		1,763,311		'
Change in unearned premiums	-64,776		-51,719		-269,986		-251,725		'
Net premiums earned	907,152		758,816		1,766,932		1,511,586		'
Other underwriting income	2,033		902		3,615		1,440		'
Losses and loss adjustment expenses	-485,518		-418,653		-921,758		-818,056		'
Acquisition expenses, net	-158,158		-131,677		-318,500		-259,269		'
Other operating expenses	-141,418		-113,359		-273,742		-223,274		'
Underwriting income (loss)	124,091		96,029		256,547		212,427		'
Net investment income	72,990		68,369		139,984		134,041		'
Net realized gains	54,144		12,652		73,841		70,992		'
Net impairment losses recognized in earnings	-14,749		-724		-17,720		-2,970		'
Equity in net income (loss) of investment funds accounted for using the equity method	9,240		10,941		12,493		24,764		'
Other income (loss)	4,850		834		2,746		2,078		'
Other expenses	-14,932		-14,049		-28,407		-24,317		'
Interest expense	-14,334		-5,852		-28,738		-11,750		'
Net foreign exchange gains (losses)	-2,294		13,811		-8,857		38,075		'
Income before income taxes	219,006		182,011		401,889		443,340		'
Income tax expense	-7,289		-5,071		-11,027		-9,924		'
Net income	211,717		176,940		390,862		433,416		'
Dividends attributable to redeemable noncontrolling interests	-4,857		0		-4,909		0		'
Amounts attributable to noncontrolling interests	1,156		0		4,563		0		'
Net income available to Arch	208,016		176,940		390,516		433,416		'
Preferred dividends	-5,485		-5,485		-10,969		-10,969		'
Net income available to Arch common shareholders	202,531		171,455		379,547		422,447		'
Underwriting Ratios	'		'		'		'		'
Loss ratio	53.50%		55.20%		52.20%		54.10%		'
Acquisition expense ratio	17.40%	[1]	17.30%	[1]	18.00%	[1]	17.10%	[1]	'
Other operating expense ratio	15.60%		14.90%		15.50%		14.80%		'
Combined ratio	86.50%		87.40%		85.70%		86.00%		'
Goodwill and intangible assets	118,721		30,960		118,721		30,960		27,319
Total investable assets	15,803,600		12,960,811		15,803,600		12,960,811		'
Total assets	22,573,515		18,917,683		22,573,515		18,917,683		19,566,094
Total liabilities	15,332,450		13,683,365		15,332,450		13,683,365		13,918,598
Underwriting segments	'		'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'		'
Gross premiums written	1,256,934	[2]	1,040,738	[2]	2,552,070	[2]	2,204,437	[2]	'
Premiums ceded	-336,808		-230,203		-599,148		-441,126		'
Net premiums written	920,126		810,535		1,952,922		1,763,311		'
Change in unearned premiums	-25,954		-51,719		-201,136		-251,725		'
Net premiums earned	894,172		758,816		1,751,786		1,511,586		'
Other underwriting income	2,033		902		3,615		1,440		'
Losses and loss adjustment expenses	-477,324		-418,653		-912,208		-818,056		'
Acquisition expenses, net	-153,965		-131,677		-313,484		-259,269		'
Other operating expenses	-139,783		-113,359		-270,998		-223,274		'
Underwriting income (loss)	125,133		96,029		258,711		212,427		'
Net investment income	72,458		68,369		139,451		134,041		'
Net realized gains	50,966		12,652		70,663		70,992		'
Net impairment losses recognized in earnings	-14,749		-724		-17,720		-2,970		'
Equity in net income (loss) of investment funds accounted for using the equity method	9,240		10,941		12,493		24,764		'
Other income (loss)	4,850		834		2,746		2,078		'
Other expenses	-15,279		-14,049		-26,078		-24,317		'
Interest expense	-14,334		-5,852		-28,738		-11,750		'
Net foreign exchange gains (losses)	-2,764		13,811		-9,420		38,075		'
Income before income taxes	215,521		182,011		402,108		443,340		'
Income tax expense	-7,289		-5,071		-11,027		-9,924		'
Net income	208,232		176,940		391,081		433,416		'
Dividends attributable to redeemable noncontrolling interests	0		0		0		0		'
Amounts attributable to noncontrolling interests	0		0		0		0		'
Net income available to Arch	208,232		176,940		391,081		433,416		'
Preferred dividends	-5,485		-5,485		-10,969		-10,969		'
Net income available to Arch common shareholders	202,747		171,455		380,112		422,447		'
Underwriting Ratios	'		'		'		'		'
Loss ratio	53.40%		55.20%		52.10%		54.10%		'
Acquisition expense ratio	17.20%	[1]	17.30%	[1]	17.80%	[1]	17.10%	[1]	'
Other operating expense ratio	15.60%		14.90%		15.50%		14.80%		'
Combined ratio	86.20%		87.40%		85.40%		86.00%		'
Goodwill and intangible assets	118,721		30,960		118,721		30,960		'
Total investable assets	14,688,881		12,960,811		14,688,881		12,960,811		'
Total assets	21,210,197		18,917,683		21,210,197		18,917,683		'
Total liabilities	15,078,943		13,683,365		15,078,943		13,683,365		'
Underwriting segments | Insurance	'		'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'		'
Gross premiums written	852,231	[2]	703,904	[2]	1,582,877	[2]	1,392,721	[2]	'
Premiums ceded	-273,349		-202,336		-458,393		-386,603		'
Net premiums written	578,882		501,568		1,124,484		1,006,118		'
Change in unearned premiums	-71,170		-42,912		-139,271		-102,497		'
Net premiums earned	507,712		458,656		985,213		903,621		'
Other underwriting income	514		529		1,014		1,054		'
Losses and loss adjustment expenses	-311,526		-291,192		-598,296		-574,659		'
Acquisition expenses, net	-76,449		-74,249		-153,381		-145,007		'
Other operating expenses	-85,829		-80,167		-166,973		-156,482		'
Underwriting income (loss)	34,422		13,577		67,577		28,527		'
Underwriting Ratios	'		'		'		'		'
Loss ratio	61.40%		63.50%		60.70%		63.60%		'
Acquisition expense ratio	15.00%	[1]	16.10%	[1]	15.50%	[1]	15.90%	[1]	'
Other operating expense ratio	16.90%		17.50%		16.90%		17.30%		'
Combined ratio	93.30%		97.10%		93.10%		96.80%		'
Goodwill and intangible assets	24,498		20,888		24,498		20,888		'
Underwriting segments | Reinsurance	'		'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'		'
Gross premiums written	349,841	[2]	318,898	[2]	866,894	[2]	769,345	[2]	'
Premiums ceded	-58,994		-28,675		-132,121		-56,654		'
Net premiums written	290,847		290,223		734,773		712,691		'
Change in unearned premiums	44,780		-2,731		-57,798		-129,046		'
Net premiums earned	335,627		287,492		676,975		583,645		'
Other underwriting income	303		373		619		386		'
Losses and loss adjustment expenses	-150,325		-125,283		-289,961		-239,140		'
Acquisition expenses, net	-66,035		-53,291		-139,468		-106,434		'
Other operating expenses	-37,666		-31,902		-73,861		-64,099		'
Underwriting income (loss)	81,904		77,389		174,304		174,358		'
Underwriting Ratios	'		'		'		'		'
Loss ratio	44.80%		43.60%		42.80%		41.00%		'
Acquisition expense ratio	19.70%	[1]	18.50%	[1]	20.60%	[1]	18.20%	[1]	'
Other operating expense ratio	11.20%		11.10%		10.90%		11.00%		'
Combined ratio	75.70%		73.20%		74.30%		70.20%		'
Goodwill and intangible assets	4,942		10,072		4,942		10,072		'
Underwriting segments | Mortgage	'		'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'		'
Gross premiums written	55,476	[2]	18,744	[2]	103,383	[2]	44,502	[2]	'
Premiums ceded	-5,079		0		-9,718		0		'
Net premiums written	50,397		18,744		93,665		44,502		'
Change in unearned premiums	436		-6,076		-4,067		-20,182		'
Net premiums earned	50,833		12,668		89,598		24,320		'
Other underwriting income	1,216		0		1,982		0		'
Losses and loss adjustment expenses	-15,473		-2,178		-23,951		-4,257		'
Acquisition expenses, net	-11,481		-4,137		-20,635		-7,828		'
Other operating expenses	-16,288		-1,290		-30,164		-2,693		'
Underwriting income (loss)	8,807		5,063		16,830		9,542		'
Underwriting Ratios	'		'		'		'		'
Loss ratio	30.40%		17.20%		26.70%		17.50%		'
Acquisition expense ratio	22.60%	[1]	32.70%	[1]	23.00%	[1]	32.20%	[1]	'
Other operating expense ratio	32.00%		10.20%		33.70%		11.10%		'
Combined ratio	85.00%		60.10%		83.40%		60.80%		'
Goodwill and intangible assets	89,281		0		89,281		0		'
Other	'		'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'		'
Gross premiums written	54,562	[2]	0	[2]	86,756	[2]	0	[2]	'
Premiums ceded	-2,760		0		-2,760		0		'
Net premiums written	51,802		0		83,996		0		'
Change in unearned premiums	-38,822		0		-68,850		0		'
Net premiums earned	12,980		0		15,146		0		'
Other underwriting income	0		0		0		0		'
Losses and loss adjustment expenses	-8,194		0		-9,550		0		'
Acquisition expenses, net	-4,193		0		-5,016		0		'
Other operating expenses	-1,635		0		-2,744		0		'
Underwriting income (loss)	-1,042		0		-2,164		0		'
Net investment income	532		0		533		0		'
Net realized gains	3,178		0		3,178		0		'
Net impairment losses recognized in earnings	0		0		0		0		'
Equity in net income (loss) of investment funds accounted for using the equity method	0		0		0		0		'
Other income (loss)	0		0		0		0		'
Other expenses	347		0		-2,329		0		'
Interest expense	0		0		0		0		'
Net foreign exchange gains (losses)	470		0		563		0		'
Income before income taxes	3,485		0		-219		0		'
Income tax expense	0		0		0		0		'
Net income	3,485		0		-219		0		'
Dividends attributable to redeemable noncontrolling interests	-4,857		0		-4,909		0		'
Amounts attributable to noncontrolling interests	1,156		0		4,563		0		'
Net income available to Arch	-216		0		-565		0		'
Preferred dividends	0		0		0		0		'
Net income available to Arch common shareholders	-216		0		-565		0		'
Underwriting Ratios	'		'		'		'		'
Loss ratio	63.10%		0.00%		63.10%		0.00%		'
Acquisition expense ratio	32.30%	[1]	0.00%	[1]	33.10%	[1]	0.00%	[1]	'
Other operating expense ratio	12.60%		0.00%		18.10%		0.00%		'
Combined ratio	108.00%		0.00%		114.30%		0.00%		'
Goodwill and intangible assets	0		0		0		0		'
Total investable assets	1,114,719		0		1,114,719		0		'
Total assets	1,363,318		0		1,363,318		0		'
Total liabilities	$253,507		$0		$253,507		$0		'
[1]	The acquisition expense ratio is adjusted to include certain other underwriting income.
[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_Narrative_

Segment Information - Narrative (Details)	6 Months Ended
	Jun. 30, 2014
	segment
Underwriting segments	'
Segment Reporting Information [Line Items]	'
Number of underwriting segments	3
Other	'
Segment Reporting Information [Line Items]	'
Number of underwriting segments	2

Investment_Information_Summary

Investment Information - Summary of available for sale securities (Details) (USD $)	6 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Available for sale securities:	'		'
Estimated Fair Value	$12,845,008		$12,150,358
Gross Unrealized Gains	285,496		232,673
Gross Unrealized Losses	-46,432		-152,368
Cost or Amortized Cost	12,605,944		12,070,053
OTTI Unrealized Losses	-3,684	[1]	-12,942	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements	'		'
Available for sale securities:	'		'
Estimated Fair Value	10,797,262		9,676,857
Gross Unrealized Gains	155,509	[2]	133,450	[2]
Gross Unrealized Losses	-40,491	[2]	-127,100	[2]
Cost or Amortized Cost	10,682,244	[2]	9,670,507	[2]
OTTI Unrealized Losses	-3,684	[1],[2]	-12,942	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds	'		'
Available for sale securities:	'		'
Estimated Fair Value	2,906,484		2,267,263
Gross Unrealized Gains	48,736	[2]	35,289	[2]
Gross Unrealized Losses	-10,472	[2]	-35,537	[2]
Cost or Amortized Cost	2,868,220	[2]	2,267,511	[2]
OTTI Unrealized Losses	-16	[1],[2]	-16	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities	'		'
Available for sale securities:	'		'
Estimated Fair Value	1,138,385		1,133,095
Gross Unrealized Gains	25,925	[2]	16,270	[2]
Gross Unrealized Losses	-8,572	[2]	-22,209	[2]
Cost or Amortized Cost	1,121,032	[2]	1,139,034	[2]
OTTI Unrealized Losses	-3,602	[1],[2]	-9,269	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds	'		'
Available for sale securities:	'		'
Estimated Fair Value	1,413,131		1,481,738
Gross Unrealized Gains	33,549	[2]	29,378	[2]
Gross Unrealized Losses	-1,095	[2]	-9,730	[2]
Cost or Amortized Cost	1,380,677	[2]	1,462,090	[2]
OTTI Unrealized Losses	0	[1],[2]	-17	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities	'		'
Available for sale securities:	'		'
Estimated Fair Value	1,119,401		1,074,497
Gross Unrealized Gains	18,238	[2]	13,972	[2]
Gross Unrealized Losses	-6,051	[2]	-15,224	[2]
Cost or Amortized Cost	1,107,214	[2]	1,075,749	[2]
OTTI Unrealized Losses	0	[1],[2]	-199	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies	'		'
Available for sale securities:	'		'
Estimated Fair Value	1,435,974		1,301,809
Gross Unrealized Gains	5,137	[2]	3,779	[2]
Gross Unrealized Losses	-2,162	[2]	-11,242	[2]
Cost or Amortized Cost	1,432,999	[2]	1,309,272	[2]
OTTI Unrealized Losses	-19	[1],[2]	-19	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities	'		'
Available for sale securities:	'		'
Estimated Fair Value	1,073,256		1,085,861
Gross Unrealized Gains	14,936	[2]	14,729	[2]
Gross Unrealized Losses	-5,399	[2]	-19,363	[2]
Cost or Amortized Cost	1,063,719	[2]	1,090,495	[2]
OTTI Unrealized Losses	0	[1],[2]	0	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities	'		'
Available for sale securities:	'		'
Estimated Fair Value	1,710,631		1,332,594
Gross Unrealized Gains	8,988	[2]	20,033	[2]
Gross Unrealized Losses	-6,740	[2]	-13,795	[2]
Cost or Amortized Cost	1,708,383	[2]	1,326,356	[2]
OTTI Unrealized Losses	-47	[1],[2]	-3,422	[1],[2]
Equity securities	'		'
Available for sale securities:	'		'
Estimated Fair Value	608,820		496,824
Gross Unrealized Gains	100,476		69,487
Gross Unrealized Losses	-4,750		-5,938
Cost or Amortized Cost	513,094		433,275
OTTI Unrealized Losses	0	[1]	0	[1]
Other investments	'		'
Available for sale securities:	'		'
Estimated Fair Value	457,567		498,310
Gross Unrealized Gains	28,609		28,082
Gross Unrealized Losses	0		-18,459
Cost or Amortized Cost	428,958		488,687
OTTI Unrealized Losses	0	[1]	0	[1]
Short-term investments	'		'
Available for sale securities:	'		'
Estimated Fair Value	981,359		1,478,367
Gross Unrealized Gains	902		1,654
Gross Unrealized Losses	-1,191		-871
Cost or Amortized Cost	981,648		1,477,584
OTTI Unrealized Losses	$0	[1]	$0	[1]
[1]	Represents the total other-than-temporary impairments (bOTTIb) recognized in accumulated other comprehensive income (bAOCIb). It does not include the change in fair value subsequent to the impairment measurement date. At June 30, 2014, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $2.0 million, compared to a net unrealized gain of $6.0 million at December 31, 2013.
[2]	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 bbSecurities Lending Agreements.b

Investment_Information_Aging_o

Investment Information - Aging of available for sale securities in an unrealized loss position (Details) (USD $)	Jun. 30, 2014		Dec. 31, 2013
In Thousands, unless otherwise specified
Available for sale securities:	'		'
Estimated Fair Value - Less than 12 Months	$2,224,741		$6,158,245
Gross Unrealized Losses - Less than 12 Months	-20,428		-137,655
Estimated Fair Value - 12 Months or More	835,359		258,290
Gross Unrealized Losses - 12 Months or More	-26,004		-14,713
Estimated Fair Value - Total	3,060,100		6,416,535
Gross Unrealized Losses - Total	-46,432		-152,368
Fixed maturities and fixed maturities pledged under securities lending agreements	'		'
Available for sale securities:	'		'
Estimated Fair Value - Less than 12 Months	2,047,311	[1]	5,887,621	[1]
Gross Unrealized Losses - Less than 12 Months	-14,487	[1]	-115,071	[1]
Estimated Fair Value - 12 Months or More	835,359	[1]	210,974	[1]
Gross Unrealized Losses - 12 Months or More	-26,004	[1]	-12,029	[1]
Estimated Fair Value - Total	2,882,670	[1]	6,098,595	[1]
Gross Unrealized Losses - Total	-40,491	[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	546,004	[1]	1,183,625	[1]
Gross Unrealized Losses - Less than 12 Months	-3,596	[1]	-32,837	[1]
Estimated Fair Value - 12 Months or More	238,526	[1]	46,673	[1]
Gross Unrealized Losses - 12 Months or More	-6,876	[1]	-2,700	[1]
Estimated Fair Value - Total	784,530	[1]	1,230,298	[1]
Gross Unrealized Losses - Total	-10,472	[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	176,720	[1]	778,693	[1]
Gross Unrealized Losses - Less than 12 Months	-1,893	[1]	-20,253	[1]
Estimated Fair Value - 12 Months or More	142,523	[1]	43,634	[1]
Gross Unrealized Losses - 12 Months or More	-6,679	[1]	-1,956	[1]
Estimated Fair Value - Total	319,243	[1]	822,327	[1]
Gross Unrealized Losses - Total	-8,572	[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	99,417	[1]	589,009	[1]
Gross Unrealized Losses - Less than 12 Months	-290	[1]	-9,422	[1]
Estimated Fair Value - 12 Months or More	59,847	[1]	6,092	[1]
Gross Unrealized Losses - 12 Months or More	-805	[1]	-308	[1]
Estimated Fair Value - Total	159,264	[1]	595,101	[1]
Gross Unrealized Losses - Total	-1,095	[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	163,092	[1]	677,617	[1]
Gross Unrealized Losses - Less than 12 Months	-2,219	[1]	-15,110	[1]
Estimated Fair Value - 12 Months or More	95,877	[1]	1,612	[1]
Gross Unrealized Losses - 12 Months or More	-3,832	[1]	-114	[1]
Estimated Fair Value - Total	258,969	[1]	679,229	[1]
Gross Unrealized Losses - Total	-6,051	[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	338,288	[1]	1,144,809	[1]
Gross Unrealized Losses - Less than 12 Months	-1,783	[1]	-11,242	[1]
Estimated Fair Value - 12 Months or More	13,616	[1]	0	[1]
Gross Unrealized Losses - 12 Months or More	-379	[1]	0	[1]
Estimated Fair Value - Total	351,904	[1]	1,144,809	[1]
Gross Unrealized Losses - Total	-2,162	[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	361,383	[1]	821,506	[1]
Gross Unrealized Losses - Less than 12 Months	-2,571	[1]	-15,776	[1]
Estimated Fair Value - 12 Months or More	69,218	[1]	24,334	[1]
Gross Unrealized Losses - 12 Months or More	-2,828	[1]	-3,587	[1]
Estimated Fair Value - Total	430,601	[1]	845,840	[1]
Gross Unrealized Losses - Total	-5,399	[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	362,407	[1]	692,362	[1]
Gross Unrealized Losses - Less than 12 Months	-2,135	[1]	-10,431	[1]
Estimated Fair Value - 12 Months or More	215,752	[1]	88,629	[1]
Gross Unrealized Losses - 12 Months or More	-4,605	[1]	-3,364	[1]
Estimated Fair Value - Total	578,159	[1]	780,991	[1]
Gross Unrealized Losses - Total	-6,740	[1]	-13,795	[1]
Equity securities	'		'
Available for sale securities:	'		'
Estimated Fair Value - Less than 12 Months	81,731		76,563
Gross Unrealized Losses - Less than 12 Months	-4,750		-5,938
Estimated Fair Value - 12 Months or More	0		0
Gross Unrealized Losses - 12 Months or More	0		0
Estimated Fair Value - Total	81,731		76,563
Gross Unrealized Losses - Total	-4,750		-5,938
Other investments	'		'
Available for sale securities:	'		'
Estimated Fair Value - Less than 12 Months	0		165,891
Gross Unrealized Losses - Less than 12 Months	0		-15,775
Estimated Fair Value - 12 Months or More	0		47,316
Gross Unrealized Losses - 12 Months or More	0		-2,684
Estimated Fair Value - Total	0		213,207
Gross Unrealized Losses - Total	0		-18,459
Short-term investments	'		'
Available for sale securities:	'		'
Estimated Fair Value - Less than 12 Months	95,699		28,170
Gross Unrealized Losses - Less than 12 Months	-1,191		-871
Estimated Fair Value - 12 Months or More	0		0
Gross Unrealized Losses - 12 Months or More	0		0
Estimated Fair Value - Total	95,699		28,170
Gross Unrealized Losses - Total	($1,191)		($871)
[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 bbSecurities Lending Agreements.b

Investment_Information_Maturit

Investment Information - Maturity profile of available for sale securities) (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Estimated Fair Value:	'	'
Estimated fair value	$10,714,532	$9,571,776
Amortized Cost:	'	'
Amortized cost	10,599,911	9,564,634
Fixed maturities and fixed maturities pledged under securities lending agreements	'	'
Estimated Fair Value:	'	'
Due in one year or less	209,116	235,330
Due after one year through five years	4,408,427	3,738,500
Due after five years through 10 years	1,958,624	1,966,536
Due after 10 years	252,678	196,305
Single maturity date	6,828,845	6,136,671
Estimated fair value	10,797,262	9,676,857
Amortized Cost:	'	'
Due in one year or less	203,646	232,652
Due after one year through five years	4,369,079	3,718,920
Due after five years through 10 years	1,924,301	1,979,510
Due after 10 years	248,589	198,286
Single maturity date	6,745,615	6,129,368
Amortized cost	10,682,244	9,670,507
Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities	'	'
Estimated Fair Value:	'	'
Securities without single maturity date	1,138,385	1,133,095
Amortized Cost:	'	'
Securities without single maturity date	1,121,032	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	1,119,401	1,074,497
Amortized Cost:	'	'
Securities without single maturity date	1,107,214	1,075,749
Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities	'	'
Estimated Fair Value:	'	'
Securities without single maturity date	1,710,631	1,332,594
Amortized Cost:	'	'
Securities without single maturity date	$1,708,383	$1,326,356

Investment_Information_Other_i

Investment Information - Other investments (Details) (USD $)	Jun. 30, 2014		Dec. 31, 2013
In Thousands, unless otherwise specified
Schedule Of Other Investments [Line Items]	'		'
Other investments and fair value option investments	$1,829,103		$1,271,590
Available for sale	'		'
Schedule Of Other Investments [Line Items]	'		'
Other investments and fair value option investments	457,567		498,310
Available for sale | Asian and emerging markets	'		'
Schedule Of Other Investments [Line Items]	'		'
Other investments and fair value option investments	292,772		331,984
Available for sale | Investment grade fixed income	'		'
Schedule Of Other Investments [Line Items]	'		'
Other investments and fair value option investments	164,795		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	1,371,536		773,280
Fair value option | Term loan investments	'		'
Schedule Of Other Investments [Line Items]	'		'
Other investments and fair value option investments	953,649		512,076
Other investments par	909,538		494,502
Fair value option | Asian and emerging markets	'		'
Schedule Of Other Investments [Line Items]	'		'
Other investments and fair value option investments	25,321		14,054
Fair value option | Investment grade fixed income	'		'
Schedule Of Other Investments [Line Items]	'		'
Other investments and fair value option investments	81,737		75,062
Fair value option | Other	'		'
Schedule Of Other Investments [Line Items]	'		'
Other investments and fair value option investments	$310,829	[1]	$172,088	[1]
[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 $)	Jun. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Fair Value, Option, Quantitative Disclosures [Line Items]	'	'
Investments accounted for using the fair value option	$2,041,091	$1,221,534
Fixed Maturities	'	'
Fair Value, Option, Quantitative Disclosures [Line Items]	'	'
Investments accounted for using the fair value option	438,048	448,254
Other investments	'	'
Fair Value, Option, Quantitative Disclosures [Line Items]	'	'
Investments accounted for using the fair value option	1,371,536	773,280
Short-term investments	'	'
Fair Value, Option, Quantitative Disclosures [Line Items]	'	'
Investments accounted for using the fair value option	$231,507	$0

Investment_Information_Net_inv

Investment Information - Net investment income (Details) (USD $)	3 Months Ended				6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Net investment income:	'		'		'		'
Gross investment income	$85,254		$76,292		$161,418		$150,629
Investment expenses	-12,264		-7,923		-21,434		-16,588
Net investment income	72,990		68,369		139,984		134,041
Fixed Maturities	'		'		'		'
Net investment income:	'		'		'		'
Gross investment income	65,869		62,004		128,318		124,010
Term loan investments	'		'		'		'
Net investment income:	'		'		'		'
Gross investment income	6,908		6,026		12,577		10,243
Equity securities (dividends)	'		'		'		'
Net investment income:	'		'		'		'
Gross investment income	3,271		3,164		6,192		4,587
Short-term investments	'		'		'		'
Net investment income:	'		'		'		'
Gross investment income	107		364		512		756
Other	'		'		'		'
Net investment income:	'		'		'		'
Gross investment income	$9,099	[1]	$4,734	[1]	$13,819	[1]	$11,033	[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				6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Net realized gains and losses:	'		'		'		'
Available for sale securities, gross gains on investment sales	$56,763		$68,853		$122,814		$135,873
Available for sale securities, gross losses on investment sales	-33,028		-54,783		-73,041		-79,839
Derivative instruments	12,402	[1]	15,646	[1]	-366	[1]	15,268	[1]
Other	-10,962	[2]	-865	[2]	-13,668	[2]	181	[2]
Net realized gains	54,144		12,652		73,841		70,992
Fixed Maturities	'		'		'		'
Net realized gains and losses:	'		'		'		'
Change in fair value of assets and liabilities accounted for using the fair value option	10,984		-6,702		9,341		-1,644
Other investments	'		'		'		'
Net realized gains and losses:	'		'		'		'
Change in fair value of assets and liabilities accounted for using the fair value option	17,985		-9,502		28,761		449
Equity securities	'		'		'		'
Net realized gains and losses:	'		'		'		'
Change in fair value of assets and liabilities accounted for using the fair value option	$0		$5		$0		$704
[1]	See Note 9 for information on the Companybs 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		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]	'	'	'	'
Net impairment losses recognized in earnings	($14,749)	($724)	($17,720)	($2,970)
Fixed Maturities	'	'	'	'
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]	'	'	'	'
Net impairment losses recognized in earnings	-669	0	-675	-35
Fixed Maturities | Mortgage backed securities	'	'	'	'
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]	'	'	'	'
Net impairment losses recognized in earnings	0	0	0	-15
Fixed Maturities | Asset backed securities	'	'	'	'
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]	'	'	'	'
Net impairment losses recognized in earnings	-5	0	-11	-20
Fixed Maturities | Corporate bonds	'	'	'	'
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]	'	'	'	'
Net impairment losses recognized in earnings	-664	0	-664	0
Equity securities	'	'	'	'
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]	'	'	'	'
Net impairment losses recognized in earnings	-278	-724	-278	-2,935
Other investments	'	'	'	'
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]	'	'	'	'
Net impairment losses recognized in earnings	($13,802)	$0	($16,767)	$0

Investment_Information_Other_t1

Investment Information - Other than temporary impairments rollforward (Details) (USD $)	3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Rollforward:	'	'	'	'
Balance at start of period	$47,256	$61,956	$60,062	$62,001
Credit loss impairments recognized on securities not previously impaired	0	0	0	33
Credit loss impairments recognized on securities previously impaired	5	0	11	2
Reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security	0	0	0	0
Reductions for securities sold during the period	-25,820	-507	-38,632	-587
Balance at end of period	$21,441	$61,449	$21,441	$61,449

Investment_Information_Restric

Investment Information - Restricted assets (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Restricted Assets [Line Items]	'	'
Restricted assets	$5,722,120	$5,302,042
Collateral or guarantees - affiliated transactions	'	'
Restricted Assets [Line Items]	'	'
Restricted assets	4,352,540	4,060,533
Collateral or guarantees - third party agreements	'	'
Restricted Assets [Line Items]	'	'
Restricted assets	943,551	856,890
Deposits with US regulatory authorities	'	'
Restricted Assets [Line Items]	'	'
Restricted assets	344,087	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	$81,942	$75,264

Investment_Information_Narrati

Investment Information - Narrative (Details) (USD $)	6 Months Ended		12 Months Ended
	Jun. 30, 2014		Dec. 31, 2013		Jun. 30, 2013
	lots		lots
Narrative items:	'		'		'
Investable assets	$15,803,600,000		'		$12,960,811,000
OTTI unrealized losses at current fair value	2,000,000		6,000,000		'
Continuous unrealized loss, qualitative disclosures:	'		'		'
Number of positions in an unrealized loss position (lots)	1,070		2,080		'
Total number of positions (lots)	4,450		4,400		'
Largest single loss	3,000,000		3,500,000		'
Securities lending, qualitative disclosures:	'		'		'
Fair value of fixed maturities and short-term investments pledged under securities lending agreements	87,000,000		105,100,000		'
Amortized cost of fixed maturities and short-term investments pledged under securities lending agreements	86,600,000		105,900,000		'
Fair value of securities backing securities lending program	82,603,000		100,584,000		'
Approximate fair value of sub-prime securities backing the securities lending program	6,200,000		6,300,000		'
OTTI Unrealized Losses	3,684,000	[1]	12,942,000	[1]	'
Underwriting segments	'		'		'
Narrative items:	'		'		'
Investable assets	14,688,881,000		'		12,960,811,000
Other	'		'		'
Narrative items:	'		'		'
Investable assets	$1,114,719,000		'		$0
[1]	Represents the total other-than-temporary impairments (bOTTIb) recognized in accumulated other comprehensive income (bAOCIb). It does not include the change in fair value subsequent to the impairment measurement date. At June 30, 2014, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $2.0 million, compared to a net unrealized gain of $6.0 million at December 31, 2013.

Fair_Value_Fair_Value_Hierarch

Fair Value - Fair Value Hierarchy (Details) (Recurring, USD $)	Jun. 30, 2014		Dec. 31, 2013
In Thousands, unless otherwise specified
Estimated Fair Value	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Assets measured at fair value	$14,886,099		$13,371,892
Liabilities measured at fair value	53,099		'
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	10,797,262		9,676,857
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	2,906,484		2,267,263
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	1,138,385		1,133,095
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	1,413,131		1,481,738
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	1,119,401		1,074,497
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	1,435,974		1,301,809
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	1,073,256		1,085,861
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	1,710,631		1,332,594
Estimated Fair Value | Equity securities	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Assets measured at fair value	608,820		496,824
Estimated Fair Value | Other investments	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Assets measured at fair value	457,567		498,310
Estimated Fair Value | Short-term investments	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Assets measured at fair value	981,359		1,478,367
Estimated Fair Value | Fair value option	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Assets measured at fair value	2,041,091		1,221,534
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	343,607		334,065
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	16,807		41,033
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	60,268		73,156
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	17,366		'
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	1,371,536		773,280
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	231,507		'
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	3,224,769		3,226,291
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		'
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	1,435,974	[1]	1,301,809	[1]
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]
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]
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]
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]
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	1,435,974	[1]	1,301,809	[1]
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]
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]
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	608,820		496,738
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
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	948,468		1,427,744
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	231,507		0
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
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
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
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		'
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
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	231,507		'
Significant Other Observable Inputs (Level 2)	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Assets measured at fair value	11,107,081		9,595,611
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 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	9,361,288	[1]	8,373,003	[1]
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	2,906,484	[1]	2,265,218	[1]
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	1,138,385	[1]	1,133,095	[1]
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	1,413,131	[1]	1,481,738	[1]
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	1,119,401	[1]	1,074,497	[1]
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]
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	1,073,256	[1]	1,085,861	[1]
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	1,710,631	[1]	1,332,594	[1]
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
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	354,354		327,890
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	32,891		50,623
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	1,358,548		844,009
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	343,607		334,065
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	16,807		41,033
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	60,268		73,156
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	17,366		'
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	920,500		395,755
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	0		'
Significant Unobservable Inputs (Level 3)	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Assets measured at fair value	554,249		549,990
Liabilities measured at fair value	53,099		'
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	53,099		'
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	0	[1]	2,045	[1]
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]
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]
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]
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]
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]
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]
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	0	[1]	0	[1]
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
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	103,213		170,420
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
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	451,036		377,525
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
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
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
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		'
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	451,036		377,525
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		'
[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 $)	3 Months Ended				6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014	Jun. 30, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'		'	'
Balance at beginning of period	$523,481		$462,602		$549,990	$477,956
Total gains or (losses) (realized/unrealized) - included in earnings	14,414	[1]	1,112	[1]	22,999	7,739
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	1,532		2,596		-664	2,820
Purchases, issuances, sales and settlements	'		'		'	'
Purchases	66,615		118,916		106,725	165,570
Issuances	0		0		0	0
Sales	0		-96,655		-68,209	-96,655
Settlements	-51,793		-6,129		-56,592	-74,988
Transfers in and/or out of Level 3	0		0		0	0
Balance at end of period	554,249		482,442		554,249	482,442
Contingent consideration liability	'		'		'	'
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'		'	'
Balance at beginning of period	43,156		0		0	0
Total gains or (losses) (realized/unrealized) - included in earnings	9,943	[1]	0	[1]	11,337	0
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	0		0		0	0
Purchases, issuances, sales and settlements	'		'		'	'
Purchases	0		0		0	0
Issuances	0		0		41,762	0
Sales	0		0		0	0
Settlements	0		0		0	0
Transfers in and/or out of Level 3	0		0		0	0
Balance at end of period	53,099		0		53,099	0
Available for sale | Corporate bonds	'		'		'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'		'	'
Balance at beginning of period	0		97,298		2,045	98,404
Total gains or (losses) (realized/unrealized) - included in earnings	0	[1]	3,187	[1]	0	4,679
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	0		-1,363		0	-3,051
Purchases, issuances, sales and settlements	'		'		'	'
Purchases	0		0		0	0
Issuances	0		0		0	0
Sales	0		-96,655		-2,045	-96,655
Settlements	0		-107		0	-1,017
Transfers in and/or out of Level 3	0		0		0	0
Balance at end of period	0		2,360		0	2,360
Available for sale | Other investments	'		'		'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'		'	'
Balance at beginning of period	104,335		185,934		170,420	184,202
Total gains or (losses) (realized/unrealized) - included in earnings	-1,722	[1]	0	[1]	553	4,762
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	600		3,959		-1,596	5,871
Purchases, issuances, sales and settlements	'		'		'	'
Purchases	0		0		0	5,000
Issuances	0		0		0	0
Sales	0		0		-66,164	0
Settlements	0		0		0	-9,942
Transfers in and/or out of Level 3	0		0		0	0
Balance at end of period	103,213		189,893		103,213	189,893
Fair value option | Other investments	'		'		'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'		'	'
Balance at beginning of period	419,146		179,370		377,525	195,350
Total gains or (losses) (realized/unrealized) - included in earnings	16,136	[1]	-2,075	[1]	22,446	-1,702
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	932		0		932	0
Purchases, issuances, sales and settlements	'		'		'	'
Purchases	66,615		118,916		106,725	160,570
Issuances	0		0		0	0
Sales	0		0		0	0
Settlements	-51,793		-6,022		-56,592	-64,029
Transfers in and/or out of Level 3	0		0		0	0
Balance at end of period	$451,036		$290,189		$451,036	$290,189
[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.

Fair_Value_Narrative_Details

Fair Value - Narrative (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Fair Value Disclosures [Abstract]	'	'	'	'	'
Total assets and liabilities measured at fair value	$14,940,000,000	'	$14,940,000,000	'	$13,370,000,000
Total assets and liabilities measured at fair value priced using non-binding broker quotes	635,500,000	'	635,500,000	'	601,900,000
Total assets and liabilities measured at fair value priced using non-binding broker quotes (percentage)	4.30%	'	4.30%	'	4.50%
Gains (losses) including in earnings attributable to the change in unrealized gains or losses related to assets still held at balance sheet date	14,400,000	-2,200,000	17,800,000	4,400,000	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'	'	'	'
Senior notes	800,000,000	'	800,000,000	'	800,000,000
Reportable Legal Entities | ACGL (Parent Guarantor)	'	'	'	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'	'	'	'
Senior notes	300,000,000	'	300,000,000	'	300,000,000
Estimated fair value of senior notes	412,600,000	'	412,600,000	'	'
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)	'	'	'	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'	'	'	'
Senior notes	500,000,000	'	500,000,000	'	500,000,000
Estimated fair value of senior notes	$540,100,000	'	$540,100,000	'	'

Derivative_Instruments_Details

Derivative Instruments (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013		Dec. 31, 2013
Derivative offsetting [Abstract]	'		'		'		'		'
Derivative assets subject to master netting agreements	$77,800,000		'		$77,800,000		'		$28,000,000
Derivative liabilities subject to master netting agreements	36,500,000		'		36,500,000		'		14,600,000
Net realized gains (losses) on derivative instruments	'		'		'		'		'
Net realized gains (losses) on derivative instruments	12,402,000	[1]	15,646,000	[1]	-366,000	[1]	15,268,000	[1]	'
Not Designated as Hedging Instrument [Member]	'		'		'		'		'
Derivative [Line Items]	'		'		'		'		'
Asset derivatives - fair value	163,016,000		'		163,016,000		'		39,859,000
Liability derivatives - fair value	-152,651,000		'		-152,651,000		'		-26,472,000
Net derivatives - fair value	10,365,000		'		10,365,000		'		13,387,000
Net realized gains (losses) on derivative instruments	'		'		'		'		'
Net realized gains (losses) on derivative instruments	12,402,000		15,646,000		-366,000		15,268,000		'
Not Designated as Hedging Instrument [Member] | Futures contracts	'		'		'		'		'
Net realized gains (losses) on derivative instruments	'		'		'		'		'
Net realized gains (losses) on derivative instruments	15,185,000		10,623,000		5,323,000		9,794,000		'
Not Designated as Hedging Instrument [Member] | Futures contracts | Other investments available for sale, at fair value	'		'		'		'		'
Derivative [Line Items]	'		'		'		'		'
Asset derivatives - fair value	986,000		'		986,000		'		461,000
Liability derivatives - fair value	-198,000		'		-198,000		'		-110,000
Net derivatives - fair value	788,000		'		788,000		'		351,000
Net derivatives - notional value	783,914,000	[2]	'		783,914,000	[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	-1,811,000		7,605,000		-4,068,000		6,880,000		'
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	856,000		'		856,000		'		5,023,000
Liability derivatives - fair value	-1,949,000		'		-1,949,000		'		-3,090,000
Net derivatives - fair value	-1,093,000		'		-1,093,000		'		1,933,000
Net derivatives - notional value	279,559,000	[2]	'		279,559,000	[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	-270,000		-3,751,000		-307,000		-3,210,000		'
Not Designated as Hedging Instrument [Member] | TBAs | Fixed maturities available for sale, at fair value	'		'		'		'		'
Derivative [Line Items]	'		'		'		'		'
Asset derivatives - fair value	160,074,000		'		160,074,000		'		33,455,000
Liability derivatives - fair value	-149,782,000		'		-149,782,000		'		-21,731,000
Net derivatives - fair value	10,292,000		'		10,292,000		'		11,724,000
Net derivatives - notional value	296,380,000	[2]	'		296,380,000	[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	-702,000		1,169,000		-1,314,000		1,804,000		'
Not Designated as Hedging Instrument [Member] | Other | Other investments available for sale, at fair value	'		'		'		'		'
Derivative [Line Items]	'		'		'		'		'
Asset derivatives - fair value	1,100,000		'		1,100,000		'		920,000
Liability derivatives - fair value	-722,000		'		-722,000		'		-1,541,000
Net derivatives - fair value	378,000		'		378,000		'		-621,000
Net derivatives - notional value	$504,709,000	[2]	'		$504,709,000	[2]	'		$347,916,000	[2]
[1]	See Note 9 for information on the Companybs derivative instruments.
[2]	Represents the absolute notional value of all outstanding contracts, consisting of long and short positions.

Commitments_and_Contingencies_

Commitments and Contingencies (Details) (USD $)	6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014
Unfunded Investment Commitments [Abstract]	'
Investment commitments	$966.10
LOC Facilities [Member]	'
Line of Credit Facility [Abstract]	'
Letters of credit outstanding, amount	424.2
Investments pledged as collateral	484.1
Other secured letter of credit facilities	'
Line of Credit Facility [Abstract]	'
Maximum borrowing capacity	194.8
Credit Agreement	'
Line of Credit Facility [Abstract]	'
Credit facility expiration date	30-Jun-19
Line of credit facility, amount outstanding	100
Unsecured revolving loan and letter of credit facility	'
Line of Credit Facility [Abstract]	'
Maximum borrowing capacity	300
Secured letter of credit facility	'
Line of Credit Facility [Abstract]	'
Maximum borrowing capacity	500
Watford Holdings Ltd | Line of credit	'
Line of Credit Facility [Abstract]	'
Maximum borrowing capacity	200
Credit facility expiration date	19-May-15
Letters of credit outstanding, amount	$1.40

Share_Transactions_Share_repur

Share Transactions - Share repurchases (Details) (Common Shares, USD $)	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Common Shares	'	'	'
Class of Stock [Line Items]	'	'	'
Cumulative number of shares acquired since inception of share repurchase program	'	109,900,000	'
Aggregate purchase price of shares acquired since inception of share repurchase program	'	$2,790,000,000	'
Treasury stock, shares acquired (shares)	307,659	'	1,238,418
Treasury stock, value of shares acquired	15,500,000	'	56,500,000
Remaining authorized repurchase amount	'	$712,100,000	'

Share_Transactions_Sharebased_

Share Transactions - Share-based compensation (Details) (USD $)	3 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Stock appreciation rights and options	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'
Stock grant amount (in shares)	551,836	516,859
Weighted average grant date fair value (in dollars per share)	$15.23	$13.35
Restricted shares and units	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'
Stock grant amount (in shares)	571,108	544,075
Weighted average grant date fair value (in dollars per share)	$57.25	$53.50

Income_Taxes_Details

Income Taxes (Details) (USD $)	6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Income Tax Disclosure [Abstract]	'	'	'
Effective tax rate on income before income taxes (percentage)	2.70%	2.20%	'
Tax effect of unusual or infrequent items (percentage)	0.10%	0.30%	'
Net deferred tax assets	$151.50	'	$128.60
Income taxes paid	$8.30	$4.80	'

Other_Comprehensive_Income_Los2

Other Comprehensive Income (Loss) - Amounts reclassified from accumulated other comprehensive income (Details) (USD $)	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Mar. 31, 2013	Jun. 30, 2014	Jun. 30, 2013
					Reclassification out of accumulated other comprehensive income	Reclassification out of accumulated other comprehensive income	Reclassification out of accumulated other comprehensive income	Reclassification out of accumulated other comprehensive income
					Unrealized appreciation on available-for-sale investments	Unrealized appreciation on available-for-sale investments	Unrealized appreciation on available-for-sale investments	Unrealized appreciation on available-for-sale investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'	'	'	'	'	'	'	'
Net realized gains	'	'	'	'	$23,735	$14,634	$49,773	$59,001
Other-than-temporary impairment losses	-14,749	-724	-17,720	-2,970	-14,749	-724	-17,720	-2,972
Income before income taxes	219,006	182,011	401,889	443,340	8,986	13,910	32,053	56,029
Income tax expense	-7,289	-5,071	-11,027	-9,924	-701	6	-2,519	-3,412
Net of tax	$208,016	$176,940	$390,516	$433,416	$8,285	$13,916	$29,534	$52,617

Other_Comprehensive_Income_Los3

Other Comprehensive Income (Loss) - Components of other comprehensive income (Details) (USD $)	3 Months Ended			6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Before tax amount:	'	'	'	'	'
Unrealized holding gains arising during period, before tax	'	$115,150	($281,388)	$190,550	($271,041)
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), before tax	'	0	0	0	-2
Less reclassification of net realized gains included in net income, before tax	'	8,986	13,910	32,053	56,029
Foreign currency translation adjustments, before tax	'	10,021	-5,407	8,672	-33,629
Other comprehensive income (loss), before tax	'	116,185	-300,705	167,169	-360,701
Tax expense (benefit):	'	'	'	'	'
Unrealized holding gains arising during period, tax	'	6,722	-21,826	10,769	-20,950
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), tax	'	0	0	0	0
Less reclassification of net realized gains included in net income, tax	'	701	-6	2,519	3,412
Foreign currency translation adjustments, tax	'	0	0	0	0
Other comprehensive income (loss), tax	'	6,021	-21,820	8,250	-24,362
Net of tax amount:	'	'	'	'	'
Unrealized holding gains arising during period, net of tax	108,428	108,428	-259,562	179,781	-250,091
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), net of tax	'	0	0	0	-2
Less reclassification of net realized gains included in net income, net of tax	'	8,285	13,916	29,534	52,617
Foreign currency translation adjustments, net of tax	'	10,021	-5,407	8,672	-33,629
Net current period other comprehensive income (loss)	'	$110,164	($278,885)	$158,919	($336,339)

Guarantor_Financial_Informatio2

Guarantor Financial Information - Condensed consolidating balance sheet (Details) (USD $)	Jun. 30, 2014		Dec. 31, 2013	Jun. 30, 2013		Dec. 31, 2012
In Thousands, unless otherwise specified
Assets	'		'	'		'
Total investments	$15,163,135		$13,611,734	'		'
Cash	926,443		434,057	375,119		371,041
Investments in subsidiaries	0		0	'		'
Due from subsidiaries and affiliates	0		0	'		'
Premiums receivable	1,098,692		753,924	'		'
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	1,796,403		1,804,330	'		'
Contractholder receivables	1,234,392		1,064,246	'		'
Prepaid reinsurance premiums	430,214		328,343	'		'
Deferred acquisition costs, net	399,385		342,314	'		'
Other assets	1,524,851		1,227,146	'		'
Total assets	22,573,515		19,566,094	18,917,683		'
Liabilities	'		'	'		'
Reserve for losses and loss adjustment expenses	9,018,989		8,824,696	'		'
Unearned premiums	2,299,692		1,896,365	'		'
Reinsurance balances payable	263,347		196,167	'		'
Contractholder payables	1,234,392		1,064,246	'		'
Deposit accounting liabilities	397,337		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	1,218,693		615,827	'		'
Total liabilities	15,332,450		13,918,598	13,683,365		'
Redeemable noncontrolling interests	219,326		0	'		'
Shareholders' Equity	'		'	'		'
Total shareholdersb equity available to Arch	6,229,399		5,647,496	5,234,318		'
Non-redeemable noncontrolling interests (1)	792,340	[1]	0	0	[1]	'
Total shareholdersb equity	7,021,739		5,647,496	5,234,318		'
Total liabilities, noncontrolling interests and shareholdersb equity	22,573,515		19,566,094	'		'
Reportable Legal Entities | ACGL (Parent Guarantor)	'		'	'		'
Assets	'		'	'		'
Total investments	2,561		2,530	'		'
Cash	4,715		3,223	4,330		6,417
Investments in subsidiaries	6,629,043		6,046,060	'		'
Due from subsidiaries and affiliates	636		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	8,025		6,598	'		'
Total assets	6,644,980		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	4,954		18	'		'
Other liabilities	10,627		13,148	'		'
Total liabilities	415,581		413,166	'		'
Redeemable noncontrolling interests	0		0	'		'
Shareholders' Equity	'		'	'		'
Total shareholdersb equity available to Arch	6,229,399		5,647,496	'		'
Non-redeemable noncontrolling interests (1)	0		0	'		'
Total shareholdersb equity	6,229,399		5,647,496	'		'
Total liabilities, noncontrolling interests and shareholdersb equity	6,644,980		6,060,662	'		'
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)	'		'	'		'
Assets	'		'	'		'
Total investments	78,702		408,957	'		'
Cash	7,746		509	1,074		612
Investments in subsidiaries	1,639,914		1,258,889	'		'
Due from subsidiaries and affiliates	0		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	64,629		60,342	'		'
Total assets	1,790,991		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	1,189		10,250	'		'
Other liabilities	45,057		33,206	'		'
Total liabilities	546,246		543,456	'		'
Redeemable noncontrolling interests	0		0	'		'
Shareholders' Equity	'		'	'		'
Total shareholdersb equity available to Arch	1,244,745		1,185,241	'		'
Non-redeemable noncontrolling interests (1)	0		0	'		'
Total shareholdersb equity	1,244,745		1,185,241	'		'
Total liabilities, noncontrolling interests and shareholdersb equity	1,790,991		1,728,697	'		'
Reportable Legal Entities | Other ACGL Subsidiaries	'		'	'		'
Assets	'		'	'		'
Total investments	15,081,872		13,200,247	'		'
Cash	913,982		430,325	369,715		364,012
Investments in subsidiaries	0		0	'		'
Due from subsidiaries and affiliates	411,687		405,110	'		'
Premiums receivable	1,477,354		1,085,369	'		'
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	5,634,614		5,645,156	'		'
Contractholder receivables	1,234,392		1,064,246	'		'
Prepaid reinsurance premiums	1,407,938		1,109,312	'		'
Deferred acquisition costs, net	399,385		342,314	'		'
Other assets	2,022,687		1,714,651	'		'
Total assets	28,583,911		24,996,730	'		'
Liabilities	'		'	'		'
Reserve for losses and loss adjustment expenses	12,848,119		12,625,766	'		'
Unearned premiums	3,277,416		2,677,334	'		'
Reinsurance balances payable	804,239		662,394	'		'
Contractholder payables	1,234,392		1,064,246	'		'
Deposit accounting liabilities	715,369		758,490	'		'
Senior notes	0		0	'		'
Revolving credit agreement borrowings	0		0	'		'
Due to subsidiaries and affiliates	406,180		397,093	'		'
Other liabilities	1,262,318		691,699	'		'
Total liabilities	20,548,033		18,877,022	'		'
Redeemable noncontrolling interests	219,326		0	'		'
Shareholders' Equity	'		'	'		'
Total shareholdersb equity available to Arch	7,024,212		6,119,708	'		'
Non-redeemable noncontrolling interests (1)	792,340		0	'		'
Total shareholdersb equity	7,816,552		6,119,708	'		'
Total liabilities, noncontrolling interests and shareholdersb equity	28,583,911		24,996,730	'		'
Consolidating Adjustments and Eliminations	'		'	'		'
Assets	'		'	'		'
Total investments	0		0	'		'
Cash	0		0	0		0
Investments in subsidiaries	-8,268,957		-7,304,949	'		'
Due from subsidiaries and affiliates	-412,323		-407,361	'		'
Premiums receivable	-378,662		-331,445	'		'
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	-3,838,211		-3,840,826	'		'
Contractholder receivables	0		0	'		'
Prepaid reinsurance premiums	-977,724		-780,969	'		'
Deferred acquisition costs, net	0		0	'		'
Other assets	-570,490		-554,445	'		'
Total assets	-14,446,367		-13,219,995	'		'
Liabilities	'		'	'		'
Reserve for losses and loss adjustment expenses	-3,829,130		-3,801,070	'		'
Unearned premiums	-977,724		-780,969	'		'
Reinsurance balances payable	-540,892		-466,227	'		'
Contractholder payables	0		0	'		'
Deposit accounting liabilities	-318,032		-337,193	'		'
Senior notes	0		0	'		'
Revolving credit agreement borrowings	0		0	'		'
Due to subsidiaries and affiliates	-412,323		-407,361	'		'
Other liabilities	-99,309		-122,226	'		'
Total liabilities	-6,177,410		-5,915,046	'		'
Redeemable noncontrolling interests	0		0	'		'
Shareholders' Equity	'		'	'		'
Total shareholdersb equity available to Arch	-8,268,957		-7,304,949	'		'
Non-redeemable noncontrolling interests (1)	0		0	'		'
Total shareholdersb equity	-8,268,957		-7,304,949	'		'
Total liabilities, noncontrolling interests and shareholdersb equity	($14,446,367)		($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				6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Revenues	'		'		'		'
Premiums Earned, Net	$907,152		$758,816		$1,766,932		$1,511,586
Net investment income	72,990		68,369		139,984		134,041
Net realized gains	54,144		12,652		73,841		70,992
Net impairment losses recognized in earnings	-14,749		-724		-17,720		-2,970
Other underwriting income	2,033		902		3,615		1,440
Equity in net income of investment funds accounted for using the equity method	9,240		10,941		12,493		24,764
Other income (loss)	4,850		834		2,746		2,078
Total revenues	1,035,660		851,790		1,981,891		1,741,931
Expenses	'		'		'		'
Losses and loss adjustment expenses	485,518		418,653		921,758		818,056
Acquisition expenses	158,158		131,677		318,500		259,269
Other operating expenses	156,350		127,408		302,149		247,591
Interest expense	14,334		5,852		28,738		11,750
Net foreign exchange losses (gains)	2,294		-13,811		8,857		-38,075
Total expenses	816,654		669,779		1,580,002		1,298,591
Income (loss) before income taxes	219,006		182,011		401,889		443,340
Income tax expense (benefit)	-7,289		-5,071		-11,027		-9,924
Income Loss Net Of Income Taxes And Before Equity In Net Income Of Subsidiaries	211,717		176,940		390,862		433,416
Income (Loss) from Subsidiaries, Net of Tax	0		0		0		0
Net income	211,717		176,940		390,862		433,416
Amounts attributable to noncontrolling interests	-3,701	[1]	0	[1]	-346	[1]	0	[1]
Net income available to Arch	208,016		176,940		390,516		433,416
Preferred dividends	-5,485		-5,485		-10,969		-10,969
Net income available to Arch common shareholders	202,531		171,455		379,547		422,447
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	318,180		-101,945		549,435		97,077
Reportable Legal Entities | ACGL (Parent Guarantor)	'		'		'		'
Revenues	'		'		'		'
Premiums Earned, Net	0		0		0		0
Net investment income	0		0		0		0
Net realized gains	0		0		0		0
Net impairment losses recognized in earnings	0		0		0		0
Other underwriting income	0		0		0		0
Equity in net income of investment funds accounted for using the equity method	0		0		0		0
Other income (loss)	0		0		0		0
Total revenues	0		0		0		0
Expenses	'		'		'		'
Losses and loss adjustment expenses	0		0		0		0
Acquisition expenses	0		0		0		0
Other operating expenses	15,250		13,691		25,557		23,633
Interest expense	5,854		5,843		11,707		11,663
Net foreign exchange losses (gains)	0		0		0		0
Total expenses	21,104		19,534		37,264		35,296
Income (loss) before income taxes	-21,104		-19,534		-37,264		-35,296
Income tax expense (benefit)	0		0		0		0
Income Loss Net Of Income Taxes And Before Equity In Net Income Of Subsidiaries	-21,104		-19,534		-37,264		-35,296
Income (Loss) from Subsidiaries, Net of Tax	229,120		196,474		427,780		468,712
Net income	208,016		176,940		390,516		433,416
Amounts attributable to noncontrolling interests	0	[1]	0		0	[1]	0
Net income available to Arch	208,016		176,940		390,516		433,416
Preferred dividends	-5,485		-5,485		-10,969		-10,969
Net income available to Arch common shareholders	202,531		171,455		379,547		422,447
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	318,180		-101,945		549,435		97,077
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)	'		'		'		'
Revenues	'		'		'		'
Premiums Earned, Net	0		0		0		0
Net investment income	0		0		0		0
Net realized gains	0		0		0		0
Net impairment losses recognized in earnings	0		0		0		0
Other underwriting income	0		0		0		0
Equity in net income of investment funds accounted for using the equity method	0		0		0		0
Other income (loss)	0		0		0		0
Total revenues	0		0		0		0
Expenses	'		'		'		'
Losses and loss adjustment expenses	0		0		0		0
Acquisition expenses	0		0		0		0
Other operating expenses	494		773		1,471		1,550
Interest expense	6,448		5		12,962		8
Net foreign exchange losses (gains)	0		0		0		0
Total expenses	6,942		778		14,433		1,558
Income (loss) before income taxes	-6,942		-778		-14,433		-1,558
Income tax expense (benefit)	3,294		2,750		6,082		2,985
Income Loss Net Of Income Taxes And Before Equity In Net Income Of Subsidiaries	-3,648		1,972		-8,351		1,427
Income (Loss) from Subsidiaries, Net of Tax	9,334		10,703		31,486		29,185
Net income	5,686		12,675		23,135		30,612
Amounts attributable to noncontrolling interests	0	[1]	0		0	[1]	0
Net income available to Arch	5,686		12,675		23,135		30,612
Preferred dividends	0		0		0		0
Net income available to Arch common shareholders	5,686		12,675		23,135		30,612
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	23,186		-28,981		40,406		-22,829
Reportable Legal Entities | Other ACGL Subsidiaries	'		'		'		'
Revenues	'		'		'		'
Premiums Earned, Net	907,152		758,816		1,766,932		1,511,586
Net investment income	80,380		74,980		157,381		147,302
Net realized gains	54,144		12,652		73,841		70,992
Net impairment losses recognized in earnings	-14,749		-724		-17,720		-2,970
Other underwriting income	2,033		902		3,615		1,440
Equity in net income of investment funds accounted for using the equity method	9,240		10,941		12,493		24,764
Other income (loss)	4,850		834		2,746		2,078
Total revenues	1,043,050		858,401		1,999,288		1,755,192
Expenses	'		'		'		'
Losses and loss adjustment expenses	485,518		418,653		921,758		818,056
Acquisition expenses	158,158		131,677		318,500		259,269
Other operating expenses	140,606		112,944		275,121		222,408
Interest expense	9,422		6,615		21,466		13,340
Net foreign exchange losses (gains)	1,054		-15,882		6,795		-28,654
Total expenses	794,758		654,007		1,543,640		1,284,419
Income (loss) before income taxes	248,292		204,394		455,648		470,773
Income tax expense (benefit)	-10,583		-7,821		-17,109		-12,909
Income Loss Net Of Income Taxes And Before Equity In Net Income Of Subsidiaries	237,709		196,573		438,539		457,864
Income (Loss) from Subsidiaries, Net of Tax	0		0		0		0
Net income	237,709		196,573		438,539		457,864
Amounts attributable to noncontrolling interests	-3,701	[1]	0		-346	[1]	0
Net income available to Arch	234,008		196,573		438,193		457,864
Preferred dividends	0		0		0		0
Net income available to Arch common shareholders	234,008		196,573		438,193		457,864
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	335,348		-84,383		587,467		130,946
Consolidating Adjustments and Eliminations	'		'		'		'
Revenues	'		'		'		'
Premiums Earned, Net	0		0		0		0
Net investment income	-7,390		-6,611		-17,397		-13,261
Net realized gains	0		0		0		0
Net impairment losses recognized in earnings	0		0		0		0
Other underwriting income	0		0		0		0
Equity in net income of investment funds accounted for using the equity method	0		0		0		0
Other income (loss)	0		0		0		0
Total revenues	-7,390		-6,611		-17,397		-13,261
Expenses	'		'		'		'
Losses and loss adjustment expenses	0		0		0		0
Acquisition expenses	0		0		0		0
Other operating expenses	0		0		0		0
Interest expense	-7,390		-6,611		-17,397		-13,261
Net foreign exchange losses (gains)	1,240		2,071		2,062		-9,421
Total expenses	-6,150		-4,540		-15,335		-22,682
Income (loss) before income taxes	-1,240		-2,071		-2,062		9,421
Income tax expense (benefit)	0		0		0		0
Income Loss Net Of Income Taxes And Before Equity In Net Income Of Subsidiaries	-1,240		-2,071		-2,062		9,421
Income (Loss) from Subsidiaries, Net of Tax	-238,454		-207,177		-459,266		-497,897
Net income	-239,694		-209,248		-461,328		-488,476
Amounts attributable to noncontrolling interests	0	[1]	0		0	[1]	0
Net income available to Arch	-239,694		-209,248		-461,328		-488,476
Preferred dividends	0		0		0		0
Net income available to Arch common shareholders	-239,694		-209,248		-461,328		-488,476
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	($358,534)		$113,364		($627,873)		($108,117)
[1]	See Note 4.

Guarantor_Financial_Informatio4

Guarantor Financial Information - Condensed consolidating statement of cash flows (Details) (USD $)	6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013
Condensed Cash Flow Statements, Captions [Line Items]	'		'
Net Cash Provided by (Used in) Operating Activities	$453,063		$388,354
Purchases of fixed maturity investments	-14,311,748		-8,599,697
Purchases of equity securities	-174,687		-272,323
Purchases of other investments	-1,022,987		-648,915
Proceeds from the sale of fixed maturity investments	13,204,854		8,468,641
Proceeds from the sale of equity securities	98,687		194,212
Proceeds from the sale of other investments	618,707		506,434
Proceeds from redemptions and maturities of fixed maturity investments	432,040		424,953
Net sales (purchases) of short-term investments	430,304		-375,146
Change in investment of securities lending collateral	18,701		3,221
Contributions to subsidiaries	0		0
Intercompany loans issued	0		0
Purchase of business, net of cash acquired	-235,578	[1]	0	[1]
Purchases of furniture, equipment and other assets	-10,360		-7,092
Net Cash Used For Investing Activities	-952,067		-305,712
Purchases of common shares under share repurchase program	0		-56,463
Proceeds from common shares issued, net	2,521		-517
Proceeds from intercompany borrowings	'		0
Repayments of intercompany borrowings	0		'
Change in securities lending collateral	-18,701		-3,221
Third party investment in non-redeemable noncontrolling interests	796,903	[2]	0	[2]
Third party investment in redeemable noncontrolling interests	219,233	[2]	0	[2]
Dividends paid to redeemable noncontrolling interests	-4,816	[2]	0	[2]
Dividends paid to parent	0		0
Other	4,706		5,042
Preferred dividends paid	-10,969		-10,969
Net Cash Provided By (Used For) Financing Activities	988,877		-66,128
Effects of exchange rate changes on foreign currency cash	2,513		-12,436
Cash and Cash Equivalents, Period Increase (Decrease)	492,386		4,078
Cash	926,443		375,119
Reportable Legal Entities | ACGL (Parent Guarantor)	'		'
Condensed Cash Flow Statements, Captions [Line Items]	'		'
Net Cash Provided by (Used in) Operating Activities	10,099		69,021
Purchases of fixed maturity investments	0		0
Purchases of equity securities	0		0
Purchases of other investments	0		0
Proceeds from the sale of fixed maturity investments	0		0
Proceeds from the sale of equity securities	0		0
Proceeds from the sale of other investments	0		0
Proceeds from redemptions and maturities of fixed maturity investments	0		0
Net sales (purchases) of short-term investments	-31		-3,028
Change in investment of securities lending collateral	0		0
Contributions to subsidiaries	0		0
Intercompany loans issued	0		0
Purchase of business, net of cash acquired	0		'
Purchases of furniture, equipment and other assets	-128		-131
Net Cash Used For Investing Activities	-159		-3,159
Purchases of common shares under share repurchase program	0		-56,463
Proceeds from common shares issued, net	2,521		-517
Proceeds from intercompany borrowings	'		0
Repayments of intercompany borrowings	0		'
Change in securities lending collateral	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		0
Other	0		0
Preferred dividends paid	-10,969		-10,969
Net Cash Provided By (Used For) Financing Activities	-8,448		-67,949
Effects of exchange rate changes on foreign currency cash	0		0
Cash and Cash Equivalents, Period Increase (Decrease)	1,492		-2,087
Cash	4,715		4,330
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)	'		'
Condensed Cash Flow Statements, Captions [Line Items]	'		'
Net Cash Provided by (Used in) Operating Activities	424		-534
Purchases of fixed maturity investments	-78,509		0
Purchases of equity securities	0		0
Purchases of other investments	0		0
Proceeds from the sale of fixed maturity investments	0		0
Proceeds from the sale of equity securities	0		0
Proceeds from the sale of other investments	0		0
Proceeds from redemptions and maturities of fixed maturity investments	0		0
Net sales (purchases) of short-term investments	408,779		996
Change in investment of securities lending collateral	0		0
Contributions to subsidiaries	-313,207		-10,250
Intercompany loans issued	0		0
Purchase of business, net of cash acquired	0		'
Purchases of furniture, equipment and other assets	0		0
Net Cash Used For Investing Activities	17,063		-9,254
Purchases of common shares under share repurchase program	0		0
Proceeds from common shares issued, net	0		0
Proceeds from intercompany borrowings	'		10,250
Repayments of intercompany borrowings	-10,250		'
Change in securities lending collateral	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		0
Other	0		0
Preferred dividends paid	0		0
Net Cash Provided By (Used For) Financing Activities	-10,250		10,250
Effects of exchange rate changes on foreign currency cash	0		0
Cash and Cash Equivalents, Period Increase (Decrease)	7,237		462
Cash	7,746		1,074
Reportable Legal Entities | Other ACGL Subsidiaries	'		'
Condensed Cash Flow Statements, Captions [Line Items]	'		'
Net Cash Provided by (Used in) Operating Activities	474,565		412,867
Purchases of fixed maturity investments	-14,233,239		-8,599,697
Purchases of equity securities	-174,687		-272,323
Purchases of other investments	-1,022,987		-648,915
Proceeds from the sale of fixed maturity investments	13,204,854		8,468,641
Proceeds from the sale of equity securities	98,687		194,212
Proceeds from the sale of other investments	618,707		506,434
Proceeds from redemptions and maturities of fixed maturity investments	432,040		424,953
Net sales (purchases) of short-term investments	21,556		-373,114
Change in investment of securities lending collateral	18,701		3,221
Contributions to subsidiaries	-100,000		-250
Intercompany loans issued	10,250		-10,250
Purchase of business, net of cash acquired	-235,578		'
Purchases of furniture, equipment and other assets	-10,232		-6,961
Net Cash Used For Investing Activities	-1,371,928		-314,049
Purchases of common shares under share repurchase program	0		0
Proceeds from common shares issued, net	413,207		10,500
Proceeds from intercompany borrowings	'		0
Repayments of intercompany borrowings	0		'
Change in securities lending collateral	-18,701		-3,221
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	-4,816	[2]	'
Dividends paid to parent	-32,025		-93,000
Other	4,706		5,042
Preferred dividends paid	0		0
Net Cash Provided By (Used For) Financing Activities	1,378,507		-80,679
Effects of exchange rate changes on foreign currency cash	2,513		-12,436
Cash and Cash Equivalents, Period Increase (Decrease)	483,657		5,703
Cash	913,982		369,715
Consolidating Adjustments and Eliminations	'		'
Condensed Cash Flow Statements, Captions [Line Items]	'		'
Net Cash Provided by (Used in) Operating Activities	-32,025		-93,000
Purchases of fixed maturity investments	0		0
Purchases of equity securities	0		0
Purchases of other investments	0		0
Proceeds from the sale of fixed maturity investments	0		0
Proceeds from the sale of equity securities	0		0
Proceeds from the sale of other investments	0		0
Proceeds from redemptions and maturities of fixed maturity investments	0		0
Net sales (purchases) of short-term investments	0		0
Change in investment of securities lending collateral	0		0
Contributions to subsidiaries	413,207		10,500
Intercompany loans issued	-10,250		10,250
Purchase of business, net of cash acquired	0		'
Purchases of furniture, equipment and other assets	0		0
Net Cash Used For Investing Activities	402,957		20,750
Purchases of common shares under share repurchase program	0		0
Proceeds from common shares issued, net	-413,207		-10,500
Proceeds from intercompany borrowings	'		-10,250
Repayments of intercompany borrowings	10,250		'
Change in securities lending collateral	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	32,025		93,000
Other	0		0
Preferred dividends paid	0		0
Net Cash Provided By (Used For) Financing Activities	-370,932		72,250
Effects of exchange rate changes on foreign currency cash	0		0
Cash and Cash Equivalents, Period Increase (Decrease)	0		0
Cash	$0		$0
[1]	See Note 2.
[2]	See Note 4.