Arch Capital (ACGL) 10-Q 8 May 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	5-May-15
Document and Entity Information [Abstract]
Entity Registrant Name	ARCH CAPITAL GROUP LTD.
Entity Central Index Key	947484
Current Fiscal Year End Date	-19
Entity Filer Category	Accelerated Filer
Document Type	10-Q
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Amendment Flag	FALSE
Entity Common Stock, Shares Outstanding		124,030,813

Unaudited_Consolidated_Balance

Unaudited Consolidated Balance Sheets (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Investments:
Fixed maturities available for sale, at fair value (amortized cost: $10,343,330 and $10,701,557)	$10,427,810	$10,750,770
Short-term investments available for sale, at fair value (amortized cost: $859,167 and $801,758)	855,032	797,226
Investment of funds received under securities lending, at fair value (amortized cost: $253,159 and $40,473)	257,059	44,301
Equity securities available for sale, at fair value (cost: $578,520 and $562,534)	687,713	658,182
Other investments available for sale, at fair value (cost: $292,287 and $264,747)	329,677	296,224
Investments accounted for using the fair value option	2,441,986	2,435,532
Investments accounted for using the equity method	412,367	349,014
Total investments	15,411,644	15,331,249
Cash	471,012	485,702
Accrued investment income	73,282	74,316
Investment in joint venture (cost: $100,000)	90,724	90,426
Fixed maturities and short-term investments pledged under securities lending, at fair value	253,115	50,802
Premiums receivable	1,116,389	948,695
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	1,788,619	1,812,845
Contractholder receivables	1,339,433	1,309,192
Prepaid reinsurance premiums	421,908	377,078
Deferred acquisition costs, net	442,775	414,525
Receivable for securities sold	400,113	78,170
Goodwill and intangible assets	106,745	109,539
Other assets	933,921	927,004
Total assets	22,849,680	22,009,543
Liabilities
Reserve for losses and loss adjustment expenses	8,928,950	9,036,448
Unearned premiums	2,400,834	2,231,578
Reinsurance balances payable	200,444	219,312
Contractholder payables	1,339,433	1,309,192
Deposit accounting liabilities	284,828	327,384
Senior notes	800,000	800,000
Revolving credit agreement borrowings	100,000	100,000
Securities lending payable	263,216	50,529
Payable for securities purchased	554,625	128,413
Other liabilities	679,450	688,041
Total liabilities	15,551,780	14,890,897
Commitments and Contingencies
Redeemable noncontrolling interests	219,604	219,512
Shareholders' Equity
Non-cumulative preferred shares	325,000	325,000
Common shares ($0.0033 par, shares issued: 171,832,868 and 171,672,408)	573	572
Additional paid-in capital	399,757	383,073
Retained earnings	7,132,423	6,854,571
Accumulated other comprehensive income, net of deferred income tax	158,023	128,856
Common shares held in treasury, at cost (shares: 47,072,027 and 44,304,474)	-1,727,074	-1,562,019
Total shareholders' equity available to Arch	6,288,702	6,130,053
Non-redeemable noncontrolling interests	789,594	769,081
Total shareholders' equity	7,078,296	6,899,134
Total liabilities, noncontrolling interests and shareholders' equity	$22,849,680	$22,009,543

Unaudited_Consolidated_Balance1

Unaudited Consolidated Balance Sheets (Parentheticals) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, except Share data, unless otherwise specified
Fixed maturities available for sale, at amortized cost	$10,343,330	$10,701,557
Short-term investments available for sale, at amortized cost	859,167	801,758
Investment of funds received under securities lending agreements, at cost	253,159	40,473
Equity securities available for sale, at cost	578,520	562,534
Other investments available for sale, at cost	292,287	264,747
Investment in joint venture, at cost	$100,000	$100,000
Common shares, par value per share	$0.00	$0.00
Common shares issued (shares)	171,832,868	171,672,408
Common shares held in treasury (shares)	47,072,027	44,304,474

Unaudited_Consolidated_Stateme

Unaudited Consolidated Statements of Income (USD $)	3 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Revenues
Net premiums written	$1,066,995	$1,064,990
Change in unearned premiums	-156,731	-205,210
Net premiums earned	910,264	859,780
Net investment income	78,994	66,994
Net realized gains (losses)	83,348	19,697
Other-than-temporary impairment losses	-7,247	-2,971
Less investment impairments recognized in other comprehensive income, before taxes	1,448	0
Net impairment losses recognized in earnings	-5,799	-2,971
Other underwriting income	11,536	1,582
Equity in net income of investment funds accounted for using the equity method	5,889	3,253
Other income (loss)	-1,888	-2,104
Total revenues	1,082,344	946,231
Expenses
Losses and loss adjustment expenses	493,716	436,240
Acquisition expenses	163,076	160,342
Other operating expenses	157,882	145,799
Interest expense	12,736	14,404
Net foreign exchange (gains) losses	-66,501	6,563
Total expenses	760,909	763,348
Income before income taxes	321,435	182,883
Income tax expense	-12,678	-3,738
Net income	308,757	179,145
Amounts attributable to noncontrolling interests	-25,421	3,355
Net income available to Arch	283,336	182,500
Preferred dividends	-5,484	-5,484
Net income available to Arch common shareholders	$277,852	$177,016
Net income per common share
Basic (per share)	$2.24	$1.34
Diluted (per share)	$2.16	$1.30
Weighted average common shares and common share equivalents outstanding
Basic (shares)	124,209,276	131,857,910
Diluted (shares)	128,451,054	136,562,717

Unaudited_Consolidated_Stateme1

Unaudited Consolidated Statements of Comprehensive Income (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Comprehensive Income
Net income	$308,757	$179,145
Unrealized appreciation (decline) in value of available-for-sale investments:
Unrealized holding gains (losses) arising during period	84,304	71,353
Portion of other-than-temporary impairment losses recognized in other comprehensive income, net of deferred income tax	-1,448	0
Reclassification of net realized (gains) losses, net of income taxes, included in net income	-30,932	-21,249
Foreign currency translation adjustments	-22,757	-1,349
Comprehensive income	337,924	227,900
Amounts attributable to noncontrolling interests	-25,421	3,355
Comprehensive income available to Arch	$312,503	$231,255

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	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, 2013		$325,000	$565	$299,517	$6,042,154	$74,964	$80,692	($5,728)	($1,094,704)
Common shares issued, net			2	0
Exercise of stock options				8,054
Amortization of share-based compensation				14,175
Other				-1,243
Net income	179,145				179,145
Amounts attributable to noncontrolling interests	3,355				3,355
Preferred share dividends	-5,484				-5,484
Unrealized holding gains (losses) arising during period, net of reclassification adjustment							50,104
Portion of other-than-temporary impairment losses recognized in other comprehensive income, net of deferred income tax	0						0
Foreign currency translation adjustments	-1,349							-1,349
Shares repurchased for treasury									-2,122
Noncontrolling interest	793,496
Total shareholders' equity at Mar. 31, 2014	6,685,629
Balance at end of period at Mar. 31, 2014	5,892,133	325,000	567	320,503	6,219,170	123,719	130,796	-7,077	-1,096,826
Balance at beginning of year at Dec. 31, 2014	6,130,053	325,000	572	383,073	6,854,571	128,856	161,598	-32,742	-1,562,019
Common shares issued, net			1	0
Exercise of stock options				3,368
Amortization of share-based compensation				13,238
Other				78
Net income	308,757				308,757
Amounts attributable to noncontrolling interests	-25,421				-25,421
Preferred share dividends	-5,484				-5,484
Unrealized holding gains (losses) arising during period, net of reclassification adjustment							53,372
Portion of other-than-temporary impairment losses recognized in other comprehensive income, net of deferred income tax	-1,448						-1,448
Foreign currency translation adjustments	-22,757							-22,757
Shares repurchased for treasury									-165,055
Noncontrolling interest	789,594
Total shareholders' equity at Mar. 31, 2015	7,078,296
Balance at end of period at Mar. 31, 2015	$6,288,702	$325,000	$573	$399,757	$7,132,423	$158,023	$213,522	($55,499)	($1,727,074)

Unaudited_Consolidated_Stateme3

Unaudited Consolidated Statements of Cash Flows (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Operating Activities
Net income	$308,757	$179,145
Adjustments to reconcile net income to net cash provided by operating activities:
Net realized gains	-87,907	-22,367
Net impairment losses recognized in earnings	5,799	2,971
Equity in net income or loss of investment funds accounted for using the equity method and other income or loss	-1,970	9,559
Share-based compensation	13,238	14,175
Changes in:
Reserve for losses and loss adjustment expenses, net of unpaid losses and loss adjustment expenses recoverable	54,327	10,326
Unearned premiums, net of prepaid reinsurance premiums	156,731	205,210
Premiums receivable	-192,247	-242,616
Deferred acquisition costs, net	-36,304	-41,988
Reinsurance balances payable	-16,022	5,428
Other liabilities	-48,856	59,285
Other items	-70,085	19,041
Net Cash Provided By Operating Activities	85,461	198,169
Investing Activities
Purchases of fixed maturity investments	-7,030,731	-7,131,071
Purchases of equity securities	-125,863	-89,227
Purchases of other investments	-685,497	-304,454
Proceeds from the sale of fixed maturity investments	6,857,459	7,014,281
Proceeds from the sale of equity securities	125,906	49,614
Proceeds from sales, redemptions and maturities of other investments	605,263	331,176
Proceeds from redemptions and maturities of fixed maturity investments	272,657	168,484
Net sales (purchases) of short-term investments	66,283	156,262
Change in cash collateral related to securities lending	-5,529	4,669
Purchase of business, net of cash acquired	-2,432	-235,578
Purchases of furniture, equipment and other assets	-32,897	-5,382
Net Cash Provided By (Used For) Investing Activities	44,619	-41,226
Financing Activities
Purchases of common shares under share repurchase program	-162,898	0
Proceeds from common shares issued, net	-412	3,021
Change in cash collateral related to securities lending	5,529	-4,669
Third party investment in non-redeemable noncontrolling interests	0	796,903
Third party investment in redeemable noncontrolling interests	0	186,893
Dividends paid to redeemable noncontrolling interests	-4,816	0
Other	29,779	1,700
Preferred dividends paid	-5,484	-5,484
Net Cash Provided By (Used For) Financing Activities	-138,302	978,364
Effects of exchange rate changes on foreign currency cash	-6,468	241
Increase (decrease) in cash	-14,690	1,135,548
Cash beginning of year	485,702	434,057
Cash end of period	471,012	1,569,605
Income taxes paid	3,569	1,600
Interest paid	$511	$404

General

General	3 Months Ended
	Mar. 31, 2015
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”). The Company’s consolidated financial statements include the results of Watford Holdings Ltd., the parent of Watford Re Ltd. (see Note 3).
	The interim consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). 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, 2014 (“2014 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.

Recent_Accounting_Pronouncemen

Recent Accounting Pronouncements	3 Months Ended
	Mar. 31, 2015
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Recent Accounting Pronouncements	Recent Accounting Pronouncements
	A new accounting standard issued in the 2014 second quarter will change the manner in which most companies recognize revenue. The standard requires that revenue reflect the transfer of goods or services to customers based on the consideration or payment the company expects to be entitled to in exchange for those goods or services; however, the standard does not change the accounting for insurance contracts or financial instruments. The new standard also requires enhanced disclosures about revenue. This accounting guidance is effective in the 2017 first quarter and may be applied on a full retrospective or modified retrospective approach. However, there is currently a proposal to defer the effective date by one year. The Company is assessing the impact the implementation of this standard will have on its consolidated financial statements.
	A new accounting standard was issued in the 2015 first quarter providing targeted improvements to consolidation guidance for limited partnerships and other similarly structured entities. The new standard addresses instances where a reporting entity consolidates another entity when the reporting entity is simply acting on the behalf of others, amongst other related issues. While the standard is targeted, the application is relevant for all companies that are required to assess whether or not to consolidate certain entities. The standard is effective in the 2016 first quarter and early adoption is permitted. The Company is assessing the impact the implementation of this standard will have on its consolidated financial statements.
	In April 2015, new accounting guidance was issued relating to the presentation of debt issuance costs in the balance sheet. The new guidance requires that those costs be presented in the balance sheet as a deduction to the carrying amount of the related debt rather than as a separate asset. The new guidance is effective in the 2016 first quarter and early adoption is permitted. The Company is assessing the impact the implementation of this standard will have on its consolidated financial statements.

Variable_Interest_Entities_and

Variable Interest Entities and Noncontrolling Interests	3 Months Ended
	Mar. 31, 2015
Noncontrolling Interest [Abstract]
Variable Interest Entity and Noncontrolling Interests	Variable Interest Entity and Noncontrolling Interests
	Variable interest entity
	On March 20, 2014, 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. Watford Holdings Ltd. is the parent of Watford Re Ltd., a multi-line Bermuda reinsurance company (together with Watford Holdings Ltd., “Watford Re”). Watford Re is considered a variable interest entity (“VIE”) and the Company concluded that it is the primary beneficiary of Watford Re. As such, the results of Watford Re are included in the Company’s consolidated financial statements.
	The Company concluded that Watford Re represents a separate operating segment and provides the income statement and total investable assets, total assets and total liabilities of Watford Re within Note 5. At March 31, 2015, Watford Re’s liabilities included unearned premiums of $233.0 million and reserves for losses and loss adjustment expenses of $109.4 million, some of which is related to transactions with the Company. During the 2015 first quarter, Watford Re generated $69.9 million of cash provided by operating activities and $22.9 million of cash provided by financing activities, partially offset by $35.3 million of cash used for investing activities.
	Because Watford Re is an independent company, the assets of Watford Re can be used only to settle obligations of Watford Re and Watford Re is solely responsible for its own liabilities and commitments. The Company’s financial exposure to Watford Re is limited to its investment in Watford Re’s common shares and counterparty credit risk (mitigated by collateral) arising from the reinsurance transactions.
	Non-redeemable noncontrolling interests
	The Company accounts for the portion of Watford Re’s common equity attributable to third party investors in the shareholders’ equity section of its consolidated balance sheets. The noncontrolling ownership in Watford Re’s common shares was approximately 89% at March 31, 2015. The portion of Watford Re’s income or loss attributable to third party investors is recorded in the consolidated statements of income in ‘amounts attributable to noncontrolling interests.’ The following table sets forth activity in the non-redeemable noncontrolling interests:
		Three Months Ended
		March 31,
		2015		2014
	Balance, beginning of period	$	769,081	$	—
	Sale of shares to noncontrolling interests	—		796,903
	Amounts attributable to noncontrolling interests	20,513		(3,407		)
	Balance, end of period	$	789,594	$	793,496
	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. Third party investors own 100% of the Watford Preference Shares at March 31, 2015. 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.
	The portion of Watford Re’s income or loss attributable to third party investors is recorded in the consolidated statements of income in ‘amounts attributable to noncontrolling interests.’ During the 2015 first quarter, amounts attributable to noncontrolling interests of $25.4 million reflected the net impact of amounts attributable to non-redeemable noncontrolling interests of $20.5 million and amounts attributable to redeemable noncontrolling interests of $4.9 million, as described above.

Earnings_Per_Common_Share

Earnings Per Common Share	3 Months Ended
	Mar. 31, 2015
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
		March 31,
		2015			2014
	Numerator:
	Net income	$	308,757		$	179,145
	Amounts attributable to noncontrolling interests	(25,421		)	3,355
	Net income available to Arch	283,336			182,500
	Preferred dividends	(5,484		)	(5,484		)
	Net income available to Arch common shareholders	$	277,852		$	177,016
	Denominator:
	Weighted average common shares outstanding — basic	124,209,276			131,857,910
	Effect of dilutive common share equivalents:
	Nonvested restricted shares	1,416,801			1,346,401
	Stock options (1)	2,824,977			3,358,406
	Weighted average common shares and common share equivalents outstanding — diluted	128,451,054			136,562,717
	Earnings per common share:
	Basic	$	2.24		$	1.34
	Diluted	$	2.16		$	1.3
	_________________________________________________
	-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 2015 first quarter and 2014 first quarter, the number of stock options excluded were 703,853 and 759,172, respectively.

Segment_Information

Segment Information	3 Months Ended
	Mar. 31, 2015
Segment Reporting [Abstract]
Segment Information	Segment Information
	The Company classifies its businesses into three underwriting segments — insurance, reinsurance and mortgage — and two other operating segments — ‘other’ and corporate (non-underwriting). The Company determined its reportable segments using the management approach described in accounting guidance regarding disclosures about segments of an enterprise and related information. The accounting policies of the segments are the same as those used for the preparation of the Company’s consolidated financial statements. Intersegment business is allocated to the segment accountable for the underwriting results.
	The Company’s insurance, reinsurance and mortgage segments each have managers who are responsible for the overall profitability of their respective segments and who are directly accountable to the Company’s chief operating decision makers, the Chairman, President and Chief Executive Officer of ACGL and the Chief Financial Officer of ACGL. The chief operating decision makers do not assess performance, measure return on equity or make resource allocation decisions on a line of business basis. Management measures segment performance for its three underwriting segments based on underwriting income or loss. The Company does not manage its assets by underwriting segment, with the exception of goodwill and intangible assets, and, accordingly, investment income is not allocated to each underwriting segment.
	The insurance segment consists of the Company’s insurance underwriting units which offer specialty product lines on a worldwide basis. Product lines include: construction and national accounts; excess and surplus casualty; lenders products; professional lines; programs; property, energy, marine and aviation; travel, accident and health; and other (consisting of alternative markets, excess workers' compensation and surety business).
	The reinsurance segment consists of the Company’s reinsurance underwriting units which offer specialty product lines on a worldwide basis. Product lines include: casualty; marine and aviation; other specialty; property catastrophe; property excluding property catastrophe (losses on a single risk, both excess of loss and pro rata); and other (consisting of life reinsurance, casualty clash and other).
	The mortgage segment consists of the Company’s mortgage insurance and reinsurance business and includes the results of Arch Mortgage Insurance Company (“Arch MI U.S.”). Arch MI U.S. is approved as an eligible mortgage insurer by Federal National Mortgage Association and Federal Home Loan Mortgage Corporation (each a government sponsored enterprise, or “GSE”) and provides mortgage insurance to banks, credit unions and other lenders nationwide. The mortgage segment also provides reinsurance on a global basis, direct mortgage insurance in Europe and various risk-sharing products to government agencies and mortgage lenders.
	The 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 Re (see Note 3). Watford Re has its own management and board of directors that is responsible for the overall profitability of the ‘other’ segment. For the ‘other’ segment, performance is measured based on net income or loss.
	The 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
		31-Mar-15
		Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
	Gross premiums written (1)	$	766,153		$	485,112		$	60,541		$	1,311,678		$	128,633		$	1,342,022
	Premiums ceded	(224,150		)	(136,569		)	(8,670		)	(369,261		)	(4,055		)	(275,027		)
	Net premiums written	542,003			348,543			51,871			942,417			124,578			1,066,995
	Change in unearned premiums	(34,089		)	(68,826		)	(1,504		)	(104,419		)	(52,312		)	(156,731		)
	Net premiums earned	507,914			279,717			50,367			837,998			72,266			910,264
	Other underwriting income	427			1,429			7,718			9,574			1,962			11,536
	Losses and loss adjustment expenses	(317,896		)	(112,532		)	(13,809		)	(444,237		)	(49,479		)	(493,716		)
	Acquisition expenses, net	(75,078		)	(56,604		)	(10,418		)	(142,100		)	(20,976		)	(163,076		)
	Other operating expenses	(88,119		)	(38,044		)	(20,369		)	(146,532		)	(2,005		)	(148,537		)
	Underwriting income (loss)	$	27,248		$	73,966		$	13,489		114,703			1,768			116,471
	Net investment income										70,288			8,706			78,994
	Net realized gains (losses)										65,509			17,839			83,348
	Net impairment losses recognized in earnings										(5,799		)	—			(5,799		)
	Equity in net income of investment funds accounted for using the equity method										5,889			—			5,889
	Other income (loss)										(1,888		)	—			(1,888		)
	Other expenses										(9,345		)	—			(9,345		)
	Interest expense										(12,736		)	—			(12,736		)
	Net foreign exchange gains (losses)										66,853			(352		)	66,501
	Income before income taxes										293,474			27,961			321,435
	Income tax expense										(12,678		)	—			(12,678		)
	Net income										280,796			27,961			308,757
	Dividends attributable to redeemable noncontrolling interests										—			(4,908		)	(4,908		)
	Amounts attributable to noncontrolling interests										—			(20,513		)	(20,513		)
	Net income available to Arch										280,796			2,540			283,336
	Preferred dividends										(5,484		)	—			(5,484		)
	Net income available to Arch common shareholders										$	275,312		$	2,540		$	277,852
	Underwriting Ratios
	Loss ratio	62.6		%	40.2		%	27.4		%	53		%	68.5		%	54.2		%
	Acquisition expense ratio	14.8		%	20.2		%	20.7		%	17		%	29		%	17.9		%
	Other operating expense ratio	17.3		%	13.6		%	40.4		%	17.5		%	2.8		%	16.3		%
	Combined ratio	94.7		%	74		%	88.5		%	87.5		%	100.3		%	88.4		%
	Goodwill and intangible assets	$	30,526		$	2,687		$	73,532		$	106,745		$	—		$	106,745
	Total investable assets										$	14,456,612		$	1,267,588		$	15,724,200
	Total assets										21,227,143			1,622,537			22,849,680
	Total liabilities										15,036,245			515,535			15,551,780
	_________________________________________________
	-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.
		Three Months Ended
		31-Mar-14
		Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
	Gross premiums written (1)	$	730,646		$	517,053		$	47,907		$	1,295,136		$	32,194		$	1,295,136
	Premiums ceded	(185,044		)	(73,127		)	(4,639		)	(262,340		)	—			(230,146		)
	Net premiums written	545,602			443,926			43,268			1,032,796			32,194			1,064,990
	Change in unearned premiums	(68,101		)	(102,578		)	(4,503		)	(175,182		)	(30,028		)	(205,210		)
	Net premiums earned	477,501			341,348			38,765			857,614			2,166			859,780
	Other underwriting income	500			316			766			1,582			—			1,582
	Losses and loss adjustment expenses	(286,770		)	(139,636		)	(8,478		)	(434,884		)	(1,356		)	(436,240		)
	Acquisition expenses, net	(76,932		)	(73,433		)	(9,154		)	(159,519		)	(823		)	(160,342		)
	Other operating expenses	(81,144		)	(36,195		)	(13,876		)	(131,215		)	(1,109		)	(132,324		)
	Underwriting income	$	33,155		$	92,400		$	8,023		133,578			(1,122		)	132,456
	Net investment income										66,993			1			66,994
	Net realized gains										19,697			—			19,697
Net impairment losses recognized in earnings										(2,971		)	—			(2,971		)
Equity in net income of investment funds accounted for using the equity method										3,253			—			3,253
Other income (loss)										(2,104		)	—			(2,104		)
Other expenses										(10,799		)	(2,676		)	(13,475		)
Interest expense										(14,404		)	—			(14,404		)
Net foreign exchange losses										(6,656		)	93			(6,563		)
Income before income taxes										186,587			(3,704		)	182,883
Income tax expense										(3,738		)	—			(3,738		)
Net income										182,849			(3,704		)	179,145
Dividends attributable to redeemable noncontrolling interests										—			(52		)	(52		)
Amounts attributable to noncontrolling interests										—			3,407			3,407
Net income available to Arch										182,849			(349		)	182,500
Preferred dividends										(5,484		)	—			(5,484		)
Net income available to Arch common shareholders										$	177,365		$	(349	)	$	177,016
Underwriting Ratios
Loss ratio	60.1		%	40.9		%	21.9		%	50.7		%	62.6		%	50.7		%
Acquisition expense ratio	16.1		%	21.5		%	23.6		%	18.6		%	38		%	18.6		%
Other operating expense ratio	17		%	10.6		%	35.8		%	15.3		%	51.2		%	15.4		%
Combined ratio	93.2		%	73		%	81.3		%	84.6		%	151.8		%	84.7		%
Goodwill and intangible assets	$	20,184		$	5,803		$	94,888		$	120,875		$	—		$	120,875
Total investable assets										$	14,261,106		$	1,083,280		$	15,344,386
Total assets										20,653,897			1,146,461			21,800,358
Total liabilities										14,860,124			35,371			14,895,495
_________________________________________________
-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.

Investment_Information

Investment Information	3 Months Ended
	Mar. 31, 2015
Disclosure Investment Information [Abstract]
Investment Information	Investment Information
	At March 31, 2015, total investable assets of $15.72 billion included $14.46 billion managed by the Company and $1.27 billion attributable to Watford Re.
	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:
		Estimated			Gross			Gross			Cost or			OTTI
		Fair			Unrealized			Unrealized			Amortized			Unrealized
		Value			Gains			Losses			Cost			Losses (2)
	31-Mar-15
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	3,119,437		$	62,922		$	(46,982	)	$	3,103,497		$	(1,143	)
	Mortgage backed securities	821,611			20,996			(2,065		)	802,680			(3,557		)
	Municipal bonds	1,626,580			33,377			(2,013		)	1,595,216			—
	Commercial mortgage backed securities	1,106,332			23,083			(2,497		)	1,085,746			—
	U.S. government and government agencies	1,517,834			16,821			(776		)	1,501,789			—
	Non-U.S. government securities	831,200			23,118			(52,685		)	860,767			—
	Asset backed securities	1,657,931			14,080			(3,563		)	1,647,414			(22		)
	Total	10,680,925			194,397			(110,581		)	10,597,109			(4,722		)
	Equity securities	687,713			121,660			(12,467		)	578,520			—
	Other investments	329,677			37,964			(574		)	292,287			(165		)
	Short-term investments	855,032			16			(4,151		)	859,167			—
	Total	$	12,553,347		$	354,037		$	(127,773	)	$	12,327,083		$	(4,887	)
	31-Dec-14
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	3,108,513		$	37,928		$	(38,974	)	$	3,109,559		$	(317	)
	Mortgage backed securities	943,343			18,843			(3,842		)	928,342			(3,307		)
	Municipal bonds	1,494,122			31,227			(1,044		)	1,463,939			—
	Commercial mortgage backed securities	1,114,528			14,594			(3,822		)	1,103,756			—
	U.S. government and government agencies	1,447,972			8,345			(1,760		)	1,441,387			—
	Non-U.S. government securities	1,015,153			21,311			(37,203		)	1,031,045			—
	Asset backed securities	1,677,941			8,425			(6,089		)	1,675,605			(22		)
	Total	10,801,572			140,673			(92,734		)	10,753,633			(3,646		)
	Equity securities	658,182			109,012			(13,364		)	562,534			—
	Other investments	296,224			31,839			(362		)	264,747			—
	Short-term investments	797,226			738			(5,270		)	801,758			—
	Total	$	12,553,204		$	282,262		$	(111,730	)	$	12,382,672		$	(3,646	)
	_________________________________________________
	-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 March 31, 2015, the net unrealized loss related to securities for which a non-credit OTTI was recognized in AOCI was $0.3 million, compared to a net unrealized gain of $0.9 million at December 31, 2014.
	The following table summarizes, for all available for sale securities in an unrealized loss position, the fair value and gross unrealized loss by length of time the security has been in a continual unrealized loss position:
		Less than 12 Months						12 Months or More						Total
		Estimated			Gross			Estimated			Gross			Estimated			Gross
		Fair			Unrealized			Fair			Unrealized			Fair			Unrealized
		Value			Losses			Value			Losses			Value			Losses
	31-Mar-15
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	810,064		$	(40,337	)	$	60,249		$	(6,645	)	$	870,313		$	(46,982	)
	Mortgage backed securities	142,833			(1,333		)	29,939			(732		)	172,772			(2,065		)
	Municipal bonds	241,211			(1,494		)	13,573			(519		)	254,784			(2,013		)
	Commercial mortgage backed securities	194,085			(2,375		)	30,774			(122		)	224,859			(2,497		)
	U.S. government and government agencies	148,004			(776		)	—			—			148,004			(776		)
	Non-U.S. government securities	376,737			(43,321		)	38,091			(9,364		)	414,828			(52,685		)
	Asset backed securities	331,543			(2,072		)	157,748			(1,491		)	489,291			(3,563		)
	Total	2,244,477			(91,708		)	330,374			(18,873		)	2,574,851			(110,581		)
	Equity securities	226,711			(12,467		)	—			—			226,711			(12,467		)
	Other investments	128,867			(574		)	—			—			128,867			(574		)
	Short-term investments	55,615			(4,151		)	—			—			55,615			(4,151		)
	Total	$	2,655,670		$	(108,900	)	$	330,374		$	(18,873	)	$	2,986,044		$	(127,773	)
	31-Dec-14
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	1,309,637		$	(32,903	)	$	148,963		$	(6,071	)	$	1,458,600		$	(38,974	)
	Mortgage backed securities	293,624			(1,476		)	59,107			(2,366		)	352,731			(3,842		)
	Municipal bonds	210,614			(588		)	13,643			(456		)	224,257			(1,044		)
	Commercial mortgage backed securities	232,147			(770		)	125,894			(3,052		)	358,041			(3,822		)
	U.S. government and government agencies	618,381			(1,626		)	3,438			(134		)	621,819			(1,760		)
Non-U.S. government securities	510,766			(31,172		)	46,910			(6,031		)	557,676			(37,203		)
Asset backed securities	612,950			(2,486		)	243,452			(3,603		)	856,402			(6,089		)
Total	3,788,119			(71,021		)	641,407			(21,713		)	4,429,526			(92,734		)
Equity securities	181,002			(13,364		)	—			—			181,002			(13,364		)
Other investments	59,638			(362		)	—			—			59,638			(362		)
Short-term investments	79,271			(5,270		)	—			—			79,271			(5,270		)
Total	$	4,108,030		$	(90,017	)	$	641,407		$	(21,713	)	$	4,749,437		$	(111,730	)
_________________________________________________
-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 March 31, 2015, on a lot level basis, approximately 1,360 security lots out of a total of approximately 5,130 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 $3.3 million. At December 31, 2014, on a lot level basis, approximately 1,900 security lots out of a total of approximately 4,790 security lots were in an unrealized loss position and the largest single unrealized loss from a single lot in the Company’s fixed maturity portfolio was $2.9 million.
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.
		31-Mar-15						31-Dec-14
Maturity		Estimated			Amortized			Estimated			Amortized
		Fair			Cost			Fair			Cost
		Value						Value
Due in one year or less		$	267,975		$	268,219		$	235,930		$	233,794
Due after one year through five years		3,824,832			3,819,036			4,074,562			4,077,408
Due after five years through 10 years		2,651,327			2,630,130			2,475,726			2,461,356
Due after 10 years		350,917			343,884			279,542			273,372
		7,095,051			7,061,269			7,065,760			7,045,930
Mortgage backed securities		821,611			802,680			943,343			928,342
Commercial mortgage backed securities		1,106,332			1,085,746			1,114,528			1,103,756
Asset backed securities		1,657,931			1,647,414			1,677,941			1,675,605
Total		$	10,680,925		$	10,597,109		$	10,801,572		$	10,753,633
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 $253.1 million and $253.8 million, respectively, at March 31, 2015, compared to $50.8 million and $52.1 million, respectively, at December 31, 2014. At March 31, 2015, the fair value of the cash collateral received on securities lending was $49.9 million and the fair value of non-cash collateral received was $207.2 million. At December 31, 2014, the fair value of the cash collateral received on securities lending was $44.3 million and the fair value of non-cash collateral received was nil. Such amounts included approximately $5.7 million of sub-prime securities at March 31, 2015, compared to $5.8 million at December 31, 2014. 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:
	March 31,			December 31,
	2015			2014
Available for sale:
Asian and emerging markets	$	270,087		$	236,586
Investment grade fixed income	59,590			59,638
Total available for sale	329,677			296,224
Fair value option:
Term loan investments (par value: $1,055,264 and $1,094,337)	1,044,762			1,073,649
Mezzanine debt funds	121,570			121,341
Credit related funds	138,576			114,436
Investment grade fixed income	63,162			69,108
Asian and emerging markets	26,846			25,800
Other (1)	162,908			147,573
Total fair value option	1,557,824			1,551,907
Total	$	1,887,501		$	1,848,131
_________________________________________________
-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:
	March 31,			December 31,
	2015			2014
Fixed maturities	$	757,467		$	632,024
Other investments	1,557,824			1,551,907
Short-term investments	125,788			251,601
Equity securities	907			—
Investments accounted for using the fair value option	$	2,441,986		$	2,435,532
Net Investment Income
The components of net investment income were derived from the following sources:
	Three Months Ended
	March 31,
	2015			2014
Fixed maturities	$	68,596		$	62,449
Term loan investments	14,744			5,669
Equity securities (dividends)	2,679			2,921
Short-term investments	196			405
Other (1)	12,747			4,719
Gross investment income	98,962			76,163
Investment expenses	(19,968		)	(9,169		)
Net investment income	$	78,994		$	66,994
_________________________________________________
-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
	March 31,
	2015			2014
Available for sale securities:
Gross gains on investment sales	$	97,591		$	66,051
Gross losses on investment sales	(55,160		)	(40,013		)
Change in fair value of assets and liabilities accounted for using the fair value option:
Fixed maturities	(3,302		)	(1,643		)
Other investments	6,287			10,776
Equity securities	(2		)	—
Short-term investments	5,846			—
Derivative instruments (1)	36,676			(12,768		)
Other (2)	(4,588		)	(2,706		)
Net realized gains (losses)	$	83,348		$	19,697
_________________________________________________
-1	See Note 8 for information on the Company’s derivative instruments.
-2	Includes accretion of contingent consideration liability amounts related to the 2014 acquisition of CMG Mortgage Insurance Company and its affiliated mortgage insurance companies.
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
	March 31,
	2015			2014
Fixed maturities:
Mortgage backed securities	$	(1,072	)	$	—
Corporate bonds	(1,976		)	—
Asset backed securities	—			(6		)
Total	(3,048		)	(6		)
Short-term investments	(2,341		)	—
Equity securities	(129		)	—
Other investments	(281		)	(2,965		)
Net impairment losses recognized in earnings	$	(5,799	)	$	(2,971	)
A description of the methodology and significant inputs used to measure the amount of net impairment losses recognized in earnings 2015 first quarter is as follows:
•	Short-term investments — net impairment losses for the 2015 first quarter related to a reduction in the carrying value of a Euro-denominated investment in an unrealized loss position due to the decline of the Euro against the U.S. Dollar. Due to the short time to maturity, the Company recorded an OTTI on such security;
•	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;
•	Mortgage backed securities — the Company utilized underlying data provided by asset managers, cash flow projections and additional information from credit agencies in order to determine an expected recovery value for each security. The analysis includes expected cash flow projections under base case and stress case scenarios which modify the expected default expectations and loss severities and slow down prepayment assumptions. The significant inputs in the models include the expected default rates, delinquency rates and foreclosure costs. Net impairment losses for the 2015 first quarter primarily resulted from a revised analysis of prepayment assumptions on one interest only mortgage backed security and small adjustments in other holdings. The amortized cost basis of the mortgage backed securities were adjusted down, if required, to the expected recovery value calculated in the OTTI review process.
The Company believes that the $4.9 million of OTTI included in accumulated other comprehensive income at March 31, 2015 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 March 31, 2015, 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
	March 31,
	2015			2014
Balance at start of period	$	20,196		$	60,062
Credit loss impairments recognized on securities not previously impaired	4,489			—
Credit loss impairments recognized on securities previously impaired	79			6
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	(420		)	(12,812		)
Balance at end of period	$	24,344		$	47,256
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 9 for further details. The following table details the value of the Company’s restricted assets:
	March 31,			December 31,
	2015			2014
Assets used for collateral or guarantees:
Affiliated transactions	$	4,054,848		$	4,138,527
Third party agreements	1,010,214			970,120
Deposits with U.S. regulatory authorities	407,190			337,981
Trust funds	64,673			72,461
Total restricted assets	$	5,536,925		$	5,519,089

Fair_Value

Fair Value	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015.
	The independent pricing sources obtain market quotations and actual transaction prices for securities that have quoted prices in active markets. Each source has its own proprietary method for determining the fair value of securities that are not actively traded. In general, these methods involve the use of “matrix pricing” in which the independent pricing source uses observable market inputs including, but not limited to, investment yields, credit risks and spreads, benchmarking of like securities, broker-dealer quotes, reported trades and sector groupings to determine a reasonable fair value. In addition, pricing vendors use model processes, such as an Option Adjusted Spread model, to develop prepayment and interest rate scenarios. The Option Adjusted Spread model is commonly used to estimate fair value for securities such as mortgage backed and asset backed securities. In certain circumstances, when fair values are unavailable from these independent pricing sources, quotes are obtained directly from broker-dealers who are active in the corresponding markets. Such quotes are subject to the validation procedures noted above. Of the $15.06 billion of financial assets and liabilities measured at fair value at March 31, 2015, approximately $241.0 million, or 1.6%, were priced using non-binding broker-dealer quotes. Of the $15.05 billion of financial assets and liabilities measured at fair value at December 31, 2014, approximately $260.8 million, or 1.7%, were priced using non-binding broker-dealer quotes.
	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. The Company determined that certain corporate bonds and asset backed securities for which there is a low level of transparency around inputs to the valuation process should be classified within Level 3 of the valuation hierarchy.
	Equity securities. The Company determined that exchange-traded equity securities would be included in Level 1 as their fair values are based on quoted market prices in active markets. Other equity securities are included in Level 2 of the valuation hierarchy.
	Other investments. The fair values for certain of the Company’s other investments are determined using net asset values (“NAV”) as advised by external fund managers. The NAV is based on the fund manager’s valuation of the underlying holdings in accordance with the fund’s governing documents. Periodically, the Company performs a number of monitoring procedures in order to assess the quality of the NAVs, including regular review and discussion of each fund’s performance, regular evaluation of fund performance against applicable benchmarks and the backtesting of the NAVs against audited and interim financial statements. Other investments with liquidity terms allowing the Company to substantially redeem its holdings in a short time frame at the applicable NAV are reflected in Level 2. Other investments with redemption restrictions that prevent the Company from redeeming in the near term are classified in Level 3 of the valuation hierarchy. 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 CMG Mortgage Insurance Company and its affiliated mortgage insurance companies 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 March 31, 2015:
					Estimated Fair Value Measurement Using:
		Estimated			Quoted Prices in			Significant			Significant
		Fair			Active Markets			Other			Unobservable
		Value			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	$	3,119,437		$	—		$	3,119,437		$	—
	Mortgage backed securities	821,611			—			821,611			—
	Municipal bonds	1,626,580			—			1,626,580			—
	Commercial mortgage backed securities	1,106,332			—			1,106,332			—
	U.S. government and government agencies	1,517,834			1,517,834			—			—
	Non-U.S. government securities	831,200			—			831,200			—
	Asset backed securities	1,657,931			—			1,600,431			57,500
	Total	10,680,925			1,517,834			9,105,591			57,500
	Equity securities	687,713			685,714			1,999			—
	Other investments	329,677			—			201,943			127,734
	Short-term investments	855,032			835,065			19,967			—
	Fair value option:
	Corporate bonds	636,762			—			636,762			—
	Non-U.S. government bonds	77,903			—			77,903			—
	Mortgage backed securities	18,010			—			18,010			—
	Asset backed securities	24,792			—			24,792			—
	Other investments	1,557,824			—			1,088,930			468,894
	Short-term investments	125,788			125,788			—			—
	Equity securities	907			907			—			—
	Total	2,441,986			126,695			1,846,397			468,894
	Total assets measured at fair value	$	14,995,333		$	3,165,308		$	11,175,897		$	654,128
	Liabilities measured at fair value:
	Contingent consideration liability	$	(66,461	)	$	—		$	—		$	(66,461	)
	Total liabilities measured at fair value	$	(66,461	)	$	—		$	—		$	(66,461	)
	_________________________________________________
	-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, 2014:
					Estimated Fair Value Measurement Using:
		Estimated			Quoted Prices in			Significant			Significant
		Fair			Active Markets			Other			Unobservable
		Value			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	$	3,108,513		$	—		$	3,108,513		$	—
	Mortgage backed securities	943,343			—			943,343			—
Municipal bonds	1,494,122			—			1,494,122			—
Commercial mortgage backed securities	1,114,528			—			1,114,528			—
U.S. government and government agencies	1,447,972			1,447,972			—			—
Non-U.S. government securities	1,015,153			—			1,015,153			—
Asset backed securities	1,677,941			—			1,620,441			57,500
Total	10,801,572			1,447,972			9,296,100			57,500
Equity securities	658,182			658,182			—			—
Other investments	296,224			—			192,687			103,537
Short-term investments	797,226			759,621			37,605			—
Fair value option:
Corporate bonds	497,101			—			497,101			—
Non-U.S. government bonds	88,411			—			88,411			—
Mortgage backed securities	22,190			—			22,190			—
Asset backed securities	24,322			—			24,322			—
Other investments	1,551,907			—			1,085,557			466,350
Short-term investments	251,601			250,580			1,021			—
Total	2,435,532			250,580			1,718,602			466,350
Total assets measured at fair value	$	14,988,736		$	3,116,355		$	11,244,994		$	627,387
Liabilities measured at fair value:
Fair value option:
Contingent consideration liability	$	(61,845	)	$	—		$	—		$	(61,845	)
Total liabilities measured at fair value	$	(61,845	)	$	—		$	—		$	(61,845	)
_________________________________________________
-1	In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged. For purposes of this table, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged.
The following 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:
	Estimated Fair Value Measurements Using:
	Significant Unobservable Inputs (Level 3)
				Assets												Liabilities
s	Available-For-Sale									Fair Value Option
	Asset Backed Securities			Corporate			Other			Other			Total			Contingent Consideration Liability
				Bonds			Investments			Investments
Three Months Ended March 31, 2015
Balance at beginning of period	$	57,500		$	—		$	103,537		$	466,350		$	627,387		$	(61,845	)
Total gains or (losses) (realized/unrealized)
Included in earnings (1)	—			—			(281		)	(851		)	(1,132		)	(3,548		)
Included in other comprehensive income	—			—			(522		)	—			(522		)	—
Purchases, issuances, sales and settlements
Purchases	—			—			25,000			22,925			47,925			—
Issuances	—			—			—			—			—			(1,068		)
Sales	—			—			—			—			—			—
Settlements	—			—			—			(19,530		)	(19,530		)	—
Transfers in and/or out of Level 3	—			—			—			—			—			—
Balance at end of period	$	57,500		$	—		$	127,734		$	468,894		$	654,128		$	(66,461	)
Three Months Ended March 31, 2014
Balance at beginning of period	$	—		$	2,045		$	170,420		$	377,525		$	549,990		$	—
Total gains or (losses) (realized/unrealized)
Included in earnings (1)	—			—			2,275			6,310			8,585			(1,394		)
Included in other comprehensive income	—			—			(2,196		)	—			(2,196		)	—
Purchases, issuances, sales and settlements
Purchases	—			—			—			40,110			40,110			—
Issuances	—			—			—			—			—			(41,762		)
Sales	—			(2,045		)	(66,164		)	—			(68,209		)	—
Settlements	—			—			—			(4,799		)	(4,799		)	—
Transfers in and/or out of Level 3	—			—			—			—			—			—
Balance at end of period	$	—		$	—		$	104,335		$	419,146		$	523,481		$	(43,156	)
_________________________________________________
-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 losses for the 2015 first quarter included in earnings attributable to the change in unrealized gains or losses relating to assets still held at March 31, 2015 was $1.1 million. The amount of total gains for the 2014 first quarter included in earnings attributable to the change in unrealized gains or losses relating to assets still held at March 31, 2014 was $3.3 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 March 31, 2015, due to their respective short maturities. As these financial instruments are not actively traded, their respective fair values are classified within Level 2.
At March 31, 2015, the senior notes of ACGL were carried at their cost of $300.0 million and had a fair value of $410.5 million while the senior notes of Arch-U.S. were carried at their cost of $500.0 million and had a fair value of $559.1 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	3 Months Ended
	Mar. 31, 2015
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 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 investments are managed in portfolios which incorporate the use of foreign currency forward contracts which are intended to provide an economic hedge against foreign currency movements. In addition, the Company routinely utilizes 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 fair value of TBAs is included in ‘fixed maturities available for sale, at fair value.’ The Company did not hold any derivatives which were designated as hedging instruments at March 31, 2015 or December 31, 2014.
	The following table summarizes information on the fair values and notional values of the Company’s derivative instruments.
		Asset			Liability Derivatives				Net
		Derivatives							Derivatives
		Estimated Fair Value			Estimated Fair Value				Estimated Fair Value			Notional
												Value (1)
	31-Mar-15
	Futures contracts	$	2,534		$	(3,148	)		$	(614	)	$	2,398,085
	Foreign currency forward contracts	23,327			(6,751		)		16,576			1,848,503
	TBAs	350,576			(245,150		)		105,426			562,031
	Other	8,810			(4,327		)		4,483			1,252,273
	Total	$	385,247		$	(259,376	)		$	125,871
	31-Dec-14
	Futures contracts	$	2,156		$	(1,907	)		$	249		$	2,549,027
	Foreign currency forward contracts	10,511			(1,145		)		9,366			397,106
	TBAs	10,592			—				10,592			10,056
	Other	3,209			(2,345		)		864			735,684
	Total	$	26,468		$	(5,397	)		$	21,071
	_________________________________________________
	-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 March 31, 2015, asset derivatives and liability derivatives of $160.5 million and $131.1 million, respectively, were subject to a master netting agreement, compared to $25.3 million and $5.4 million, respectively, at December 31, 2014. 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
	Derivatives not designated as		March 31,
	hedging instruments:		2015			2014
	Futures contracts		$	19,326		$	(9,862	)
	Foreign currency forward contracts		16,819			(2,257		)
	TBAs		486			(37		)
	Other		45			(612		)
	Total		$	36,676		$	(12,768	)

Commitments_and_Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies
	Letter of Credit and Revolving Credit Facilities
	As of March 31, 2015, 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 $180.0 million as of March 31, 2015, 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 March 31, 2015, the Company had $415.9 million in outstanding letters of credit under the LOC Facilities, which were secured by investments with a fair value of $488.4 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 March 31, 2015. As of March 31, 2015, Watford Re had a $200 million line of credit facility that expires on May 19, 2015. At March 31, 2015, Watford Re had $32.1 million in outstanding letters of credit under that credit facility. Watford Re was in compliance with all covenants contained in its credit facility at March 31, 2015.
	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 $1.22 billion at March 31, 2015.

Share_Transactions

Share Transactions	3 Months Ended
	Mar. 31, 2015
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. Since the inception of the share repurchase program, ACGL has repurchased approximately 120.9 million common shares for an aggregate purchase price of $3.40 billion. For the 2015 first quarter, ACGL repurchased 2.7 million common shares for an aggregate purchase price of $162.9 million. For the 2014 first quarter, ACGL did not repurchase any common shares. At March 31, 2015, $724.2 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.

Income_Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2015
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 4.3% for the 2015 first quarter, compared to an expense of 2.0% for the 2014 first quarter. 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 $124.7 million at March 31, 2015, compared to $131.0 million at December 31, 2014. In addition, the Company paid $3.6 million in income taxes for the 2015 first quarter, while the Company paid $1.6 million for the 2014 first quarter.

Other_Comprehensive_Income_Los

Other Comprehensive Income (Loss)	3 Months Ended
	Mar. 31, 2015
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 Reclassified from AOCI
			Consolidated Statement of Income		Three Months Ended
	Details About		Line Item That Includes		March 31,
	AOCI Components		Reclassification		2015				2014
	Unrealized appreciation on available-for-sale investments
			Net realized gains (losses)		$	42,431			$	26,038
			Other-than-temporary impairment losses		(7,247		)		(2,971		)
			Total before tax		35,184				23,067
			Income tax (expense) benefit		(4,252		)		(1,818		)
			Net of tax		$	30,932			$	21,249
	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 March 31, 2015
	Unrealized appreciation (decline) in value of investments:
	Unrealized holding gains (losses) arising during period	$	94,387			$	10,083			$	84,304
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	(1,448		)		—				(1,448		)
	Less reclassification of net realized gains included in net income	35,184				4,252				30,932
	Foreign currency translation adjustments	(23,626		)		(869		)		(22,757		)
	Other comprehensive income (loss)	$	34,129			$	4,962			$	29,167
	Three Months Ended March 31, 2014
	Unrealized appreciation (decline) in value of investments:
	Unrealized holding gains (losses) arising during period	$	75,400			$	4,047			$	71,353
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	—				—				—
	Less reclassification of net realized gains (losses) included in net income	23,067				1,818				21,249
	Foreign currency translation adjustments	(1,349		)		—				(1,349		)
	Other comprehensive income (loss)	$	50,984			$	2,229			$	48,755

Guarantor_Financial_Informatio

Guarantor Financial Information	3 Months Ended
	Mar. 31, 2015
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.
			31-Mar-15
	Condensed Consolidating Balance Sheet		ACGL (Parent Guarantor)				Arch-U.S. (Subsidiary Issuer)				Other ACGL Subsidiaries				Consolidating Adjustments and Eliminations				ACGL Consolidated
	Assets
	Total investments		$	24			$	53,890			$	15,357,730			$	—			$	15,411,644
	Cash		7,992				2,822				460,198				—				471,012
	Investments in subsidiaries		6,688,930				1,710,358				—				(8,399,288		)		—
	Due from subsidiaries and affiliates		31				15,144				380,577				(395,752		)		—
	Premiums receivable		—				—				1,575,230				(458,841		)		1,116,389
	Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses		—				—				5,545,517				(3,756,898		)		1,788,619
	Contractholder receivables		—				—				1,339,433				—				1,339,433
	Prepaid reinsurance premiums		—				—				1,545,190				(1,123,282		)		421,908
	Deferred acquisition costs, net		—				—				442,775				—				442,775
	Other assets		6,998				50,869				2,138,959				(338,926		)		1,857,900
		Total assets	$	6,703,975			$	1,833,083			$	28,785,609			$	(14,472,987	)		$	22,849,680
	Liabilities
	Reserve for losses and loss adjustment expenses		$	—			$	—			$	12,656,904			$	(3,727,954	)		$	8,928,950
	Unearned premiums		—				—				3,524,116				(1,123,282		)		2,400,834
	Reinsurance balances payable		—				—				645,723				(445,279		)		200,444
	Contractholder payables		—				—				1,339,433				—				1,339,433
	Deposit accounting liabilities		—				—				508,592				(223,764		)		284,828
	Senior notes		300,000				500,000				—				—				800,000
	Revolving credit agreement borrowings		100,000				—				—				—				100,000
	Due to subsidiaries and affiliates		1,305				15,022				379,425				(395,752		)		—
	Other liabilities		13,968				47,944				1,593,048				(157,669		)		1,497,291
		Total liabilities	415,273				562,966				20,647,241				(6,073,700		)		15,551,780
	Redeemable noncontrolling interests		—				—				219,604				—				219,604
	Shareholders’ Equity
	Total shareholders’ equity available to Arch		6,288,702				1,270,117				7,129,170				(8,399,287		)		6,288,702
	Non-redeemable noncontrolling interests		—				—				789,594				—				789,594
		Total shareholders’ equity	6,288,702				1,270,117				7,918,764				(8,399,287		)		7,078,296
		Total liabilities, noncontrolling interests and shareholders’ equity	$	6,703,975			$	1,833,083			$	28,785,609			$	(14,472,987	)		$	22,849,680
			31-Dec-14
	Condensed Consolidating Balance Sheet		ACGL (Parent Guarantor)				Arch-U.S. (Subsidiary Issuer)				Other ACGL Subsidiaries				Consolidating Adjustments and Eliminations				ACGL Consolidated
	Assets
	Total investments		$	107			$	62,867			$	15,268,275			$	—			$	15,331,249
	Cash		3,218				2,787				479,697				—				485,702
	Investments in subsidiaries		6,536,644				1,685,185				—				(8,221,829		)		—
	Due from subsidiaries and affiliates		48				7,517				370,429				(377,994		)		—
	Premiums receivable		—				—				1,331,511				(382,816		)		948,695
	Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses		—				—				5,584,973				(3,772,128		)		1,812,845
	Contractholder receivables		—				—				1,309,192				—				1,309,192
	Prepaid reinsurance premiums		—				—				1,373,008				(995,930		)		377,078
	Deferred acquisition costs, net		—				—				414,525				—				414,525
	Other assets		7,590				49,576				1,689,670				(416,579		)		1,330,257
		Total assets	$	6,547,607			$	1,807,932			$	27,821,280			$	(14,167,276	)		$	22,009,543
	Liabilities
	Reserve for losses and loss adjustment expenses		$	—			$	—			$	12,784,030			$	(3,747,582	)		$	9,036,448
	Unearned premiums		—				—				3,227,508				(995,930		)		2,231,578
	Reinsurance balances payable		—				—				589,289				(369,977		)		219,312
	Contractholder payables		—				—				1,309,192				—				1,309,192
	Deposit accounting liabilities		—				—				587,050				(259,666		)		327,384
	Senior notes		300,000				500,000				—				—				800,000
	Revolving credit agreement borrowings		100,000				—				—				—				100,000
	Due to subsidiaries and affiliates		417				7,505				370,072				(377,994		)		—
	Other liabilities		17,137				49,403				994,741				(194,298		)		866,983
		Total liabilities	417,554				556,908				19,861,882				(5,945,447		)		14,890,897
	Redeemable noncontrolling interests		—				—				219,512				—				219,512
	Shareholders’ Equity
	Total shareholders’ equity available to Arch		6,130,053				1,251,024				6,970,805				(8,221,829		)		6,130,053
	Non-redeemable noncontrolling interests		—				—				769,081				—				769,081
		Total shareholders’ equity	6,130,053				1,251,024				7,739,886				(8,221,829		)		6,899,134
		Total liabilities, noncontrolling interests and shareholders’ equity	$	6,547,607			$	1,807,932			$	27,821,280			$	(14,167,276	)		$	22,009,543
		Three Months Ended March 31, 2015
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		$	—			$	—			$	910,264			$	—			$	910,264
Net investment income		—				5				86,010				(7,021		)		78,994
Net realized gains		—				—				83,348				—				83,348
Net impairment losses recognized in earnings		—				—				(5,799		)		—				(5,799		)
Other underwriting income		—				—				11,536				—				11,536
Equity in net income of investment funds accounted for using the equity method		—				—				5,889				—				5,889
Other income (loss)		—				—				(1,888		)		—				(1,888		)
	Total revenues	—				5				1,089,360				(7,021		)		1,082,344
Expenses
Losses and loss adjustment expenses		—				—				493,716				—				493,716
Acquisition expenses		—				—				163,076				—				163,076
Other operating expenses		8,632				1,259				147,991				—				157,882
Interest expense		5,856				6,366				7,535				(7,021		)		12,736
Net foreign exchange gains		—				—				(39,630		)		(26,871		)		(66,501		)
	Total expenses	14,488				7,625				772,688				(33,892		)		760,909
Income (loss) before income taxes		(14,488		)		(7,620		)		316,672				26,871				321,435
Income tax benefit (expense)		—				1,413				(14,091		)		—				(12,678		)
Income (loss) before equity in net income of subsidiaries		(14,488		)		(6,207		)		302,581				26,871				308,757
Equity in net income of subsidiaries		297,824				14,495				—				(312,319		)		—
Net income		283,336				8,288				302,581				(285,448		)		308,757
Amounts attributable to noncontrolling interests		—				—				(25,421		)		—				(25,421		)
Net income available to Arch		283,336				8,288				277,160				(285,448		)		283,336
Preferred dividends		(5,484		)		—				—				—				(5,484		)
Net income available to Arch common shareholders		$	277,852			$	8,288			$	277,160			$	(285,448	)		$	277,852
Comprehensive income available to Arch		$	312,503			$	11,750			$	333,193			$	(344,943	)		$	312,503
		Three Months Ended March 31, 2014
Condensed Consolidating Statement of Income and Comprehensive Income		ACGL (Parent Guarantor)				Arch-U.S. (Subsidiary Issuer)				Other ACGL Subsidiaries				Consolidating Adjustments and Eliminations				ACGL Consolidated
Revenues
Net premiums earned		$	—			$	—			$	859,780			$	—			$	859,780
Net investment income		—				—				77,001				(10,007		)		66,994
Net realized gains		—				—				19,697				—				19,697
Net impairment losses recognized in earnings		—				—				(2,971		)		—				(2,971		)
Other underwriting income		—				—				1,582				—				1,582
Equity in net income of investment funds accounted for using the equity method		—				—				3,253				—				3,253
Other income (loss)		—				—				(2,104		)		—				(2,104		)
	Total revenues	—				—				956,238				(10,007		)		946,231
Expenses
Losses and loss adjustment expenses		—				—				436,240				—				436,240
Acquisition expenses		—				—				160,342				—				160,342
Other operating expenses		10,307				977				134,515				—				145,799
Interest expense		5,853				6,514				12,044				(10,007		)		14,404
Net foreign exchange losses		—				—				5,741				822				6,563
	Total expenses	16,160				7,491				748,882				(9,185		)		763,348
Income (loss) before income taxes		(16,160		)		(7,491		)		207,356				(822		)		182,883
Income tax expense		—				2,788				(6,526		)		—				(3,738		)
Income (loss) before equity in net income of subsidiaries		(16,160		)		(4,703		)		200,830				(822		)		179,145
Equity in net income of subsidiaries		198,660				22,152				—				(220,812		)		—
Net income		182,500				17,449				200,830				(221,634		)		179,145
Amounts attributable to noncontrolling interests		—				—				3,355				—				3,355
Net income available to Arch		182,500				17,449				204,185				(221,634		)		182,500
Preferred dividends		(5,484		)		—				—				—				(5,484		)
Net income available to Arch common shareholders		$	177,016			$	17,449			$	204,185			$	(221,634	)		$	177,016
Comprehensive income available to Arch		$	231,255			$	17,220			$	252,119			$	(269,339	)		$	231,255
				Three Months Ended March 31, 2015
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			$	173,508			$	(8,950	)		$	106,903			$	(186,000	)		$	85,461
Investing Activities
Purchases of fixed maturity investments				—				—				(7,030,731		)		—				(7,030,731		)
Purchases of equity securities				—				—				(125,863		)		—				(125,863		)
Purchases of other investments				—				—				(685,497		)		—				(685,497		)
Proceeds from the sales of fixed maturity investments				—				9,001				6,848,458				—				6,857,459
Proceeds from the sales of equity securities				—				—				125,906				—				125,906
Proceeds from the sales of other investments				—				—				605,263				—				605,263
Proceeds from redemptions and maturities of fixed maturity investments				—				—				272,657				—				272,657
Net purchases (sales) of short-term investments				83				(16		)		66,216				—				66,283
Change in cash collateral related to securities lending				—				—				(5,529		)		—				(5,529		)
Contributions to subsidiaries				—				—				(5,500		)		5,500				—
Intercompany loans issued				—				(7,500		)		(7,500		)		15,000				—
Purchase of business, net of cash acquired				—				—				(2,432		)		—				(2,432		)
Purchases of furniture, equipment and other assets				(23		)		—				(32,874		)		—				(32,897		)
	Net Cash Provided By (Used For) Investing Activities			60				1,485				22,574				20,500				44,619
Financing Activities
Purchases of common shares under share repurchase program				(162,898		)		—				—				—				(162,898		)
Proceeds from common shares issued, net				(412		)		—				5,500				(5,500		)		(412		)
Proceeds from intercompany borrowings				—				7,500				7,500				(15,000		)		—
Change in cash collateral related to securities lending				—				—				5,529				—				5,529
Dividends paid to redeemable noncontrolling interests				—				—				(4,816		)		—				(4,816		)
Dividends paid to parent				—				—				(186,000		)		186,000				—
Other				—				—				29,779				—				29,779
Preferred dividends paid				(5,484		)		—				—				—				(5,484		)
	Net Cash Provided By (Used For) Financing Activities			(168,794		)		7,500				(142,508		)		165,500				(138,302		)
Effects of exchange rates changes on foreign currency cash				—				—				(6,468		)		—				(6,468		)
Increase (decrease) in cash				4,774				35				(19,499		)		—				(14,690		)
Cash beginning of year				3,218				2,787				479,697				—				485,702
Cash end of period				$	7,992			$	2,822			$	460,198			$	—			$	471,012
				Three Months Ended March 31, 2014
Condensed Consolidating Statement				ACGL (Parent Guarantor)				Arch-U.S. (Subsidiary Issuer)				Other ACGL Subsidiaries				Consolidating Adjustments and Eliminations				ACGL Consolidated
of Cash Flows
Operating Activities
	Net Cash Provided By (Used For) Operating Activities			$	11,685			$	7,151			$	194,833			$	(15,500	)		$	198,169
Investing Activities
Purchases of fixed maturity investments				—				—				(7,131,071		)		—				(7,131,071		)
Purchases of equity securities				—				—				(89,227		)		—				(89,227		)
Purchases of other investments				—				—				(304,454		)		—				(304,454		)
Proceeds from the sales of fixed maturity investments				—				—				7,014,281				—				7,014,281
Proceeds from the sales of equity securities				—				—				49,614				—				49,614
Proceeds from the sales of other investments				—				—				331,176				—				331,176
Proceeds from redemptions and maturities of fixed maturity investments				—				—				168,484				—				168,484
Net (purchases) sales of short-term investments				(16		)		360,269				(203,991		)		—				156,262
Change in cash collateral related to securities lending				—				—				4,669				—				4,669
Contributions to subsidiaries				—				(312,207		)		(100,000		)		412,207				—
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				(76		)		—				(5,306		)		—				(5,382		)
	Net Cash Provided By (Used For) Investing Activities			(92		)		48,062				(491,153		)		401,957				(41,226		)
Financing Activities
Proceeds from common shares issued, net				3,021				—				412,207				(412,207		)		3,021
Repayments of intercompany borrowings				—				(10,250		)		—				10,250				—
Change in cash collateral related to securities lending				—				—				(4,669		)		—				(4,669		)
Third party investment in non-redeemable noncontrolling interests				—				—				796,903				—				796,903
Third party investment in redeemable noncontrolling interests				—				—				186,893				—				186,893
Dividends paid to parent				—				—				(15,500		)		15,500				—
Other				—				—				1,700				—				1,700
Preferred dividends paid				(5,484		)		—				—				—				(5,484		)
	Net Cash Provided By (Used For) Financing Activities			(2,463		)		(10,250		)		1,377,534				(386,457		)		978,364
Effects of exchange rates changes on foreign currency cash				—				—				241				—				241
Increase in cash				9,130				44,963				1,081,455				—				1,135,548
Cash beginning of year				3,223				509				430,325				—				434,057
Cash end of period				$	12,353			$	45,472			$	1,511,780			$	—			$	1,569,605

Legal_Proceedings

Legal Proceedings	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015, 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.

Transactions_with_Related_Part

Transactions with Related Parties (Notes)	3 Months Ended
	Mar. 31, 2015
Related Party Transactions [Abstract]
Transactions with Related Parties	Transactions with Related Parties
	As part of its investment philosophy, the Company invests a portion of its investment portfolio in alternative investment funds. As of March 31, 2015, the Company had aggregate commitments of $545.9 million to funds managed by The Carlyle Group (“Carlyle”). Of such amount, $393.8 million was unfunded as of March 31, 2015. The Company may make additional commitments to funds managed by Carlyle from time to time. During the 2015 first quarter and 2014 first quarter, the Company made aggregate capital contributions to funds managed by Carlyle of $23.4 million and $19.1 million, respectively, and received aggregate cash distributions from funds managed by Carlyle of $7.1 million and $35.1 million, respectively. Kewsong Lee, a director of ACGL, joined Carlyle as Managing Director and Deputy Chief Investment Officer for Corporate Private Equity on December 31, 2013.

Subsequent_Events

Subsequent Events	3 Months Ended
	Mar. 31, 2015
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events
	In May 2008, the Company provided $100.0 million of funding to Gulf Reinsurance Limited (“Gulf Re”), a newly formed reinsurer based in the Dubai International Financial Centre, pursuant to the joint venture agreement with Gulf Investment Corporation GSC (“GIC”). Under the agreement, Arch Re Bermuda and GIC each own 50% of Gulf Re, which commenced underwriting activities in June 2008. Gulf Re provides property and casualty reinsurance primarily in the member states of the Gulf Cooperation Council, which include Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the United Arab Emirates. The initial capital of the joint venture consisted of $200.0 million. The Company accounts for its investment in Gulf Re, shown as “Investment in joint venture,” using the equity method and records its equity in the operating results of Gulf Re in “Other income (loss).” The Company entered into a number of strategic initiatives related to Gulf Re in the 2014 fourth quarter, including an agreement to acquire complete ownership of Gulf Re. Such agreement was approved by the Dubai Financial Services Authority in April 2015.

General_Policies

General (Policies)	3 Months Ended
	Mar. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [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”). 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.

Variable_Interest_Entities_and1

Variable Interest Entities and Noncontrolling Interests (Tables)	3 Months Ended
	Mar. 31, 2015
Noncontrolling Interest [Abstract]
Activity in non-redeemable noncontrolling interests	The following table sets forth activity in the non-redeemable noncontrolling interests:
		Three Months Ended
		March 31,
		2015		2014
	Balance, beginning of period	$	769,081	$	—
	Sale of shares to noncontrolling interests	—		796,903
	Amounts attributable to noncontrolling interests	20,513		(3,407		)
	Balance, end of period	$	789,594	$	793,496

Earnings_Per_Common_Share_Tabl

Earnings Per Common Share (Tables)	3 Months Ended
	Mar. 31, 2015
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
		March 31,
		2015			2014
	Numerator:
	Net income	$	308,757		$	179,145
	Amounts attributable to noncontrolling interests	(25,421		)	3,355
	Net income available to Arch	283,336			182,500
	Preferred dividends	(5,484		)	(5,484		)
	Net income available to Arch common shareholders	$	277,852		$	177,016
	Denominator:
	Weighted average common shares outstanding — basic	124,209,276			131,857,910
	Effect of dilutive common share equivalents:
	Nonvested restricted shares	1,416,801			1,346,401
	Stock options (1)	2,824,977			3,358,406
	Weighted average common shares and common share equivalents outstanding — diluted	128,451,054			136,562,717
	Earnings per common share:
	Basic	$	2.24		$	1.34
	Diluted	$	2.16		$	1.3
	_________________________________________________
	-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 2015 first quarter and 2014 first quarter, the number of stock options excluded were 703,853 and 759,172, respectively.

Segment_Information_Tables

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2015
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
		31-Mar-15
		Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
	Gross premiums written (1)	$	766,153		$	485,112		$	60,541		$	1,311,678		$	128,633		$	1,342,022
	Premiums ceded	(224,150		)	(136,569		)	(8,670		)	(369,261		)	(4,055		)	(275,027		)
	Net premiums written	542,003			348,543			51,871			942,417			124,578			1,066,995
	Change in unearned premiums	(34,089		)	(68,826		)	(1,504		)	(104,419		)	(52,312		)	(156,731		)
	Net premiums earned	507,914			279,717			50,367			837,998			72,266			910,264
	Other underwriting income	427			1,429			7,718			9,574			1,962			11,536
	Losses and loss adjustment expenses	(317,896		)	(112,532		)	(13,809		)	(444,237		)	(49,479		)	(493,716		)
	Acquisition expenses, net	(75,078		)	(56,604		)	(10,418		)	(142,100		)	(20,976		)	(163,076		)
	Other operating expenses	(88,119		)	(38,044		)	(20,369		)	(146,532		)	(2,005		)	(148,537		)
	Underwriting income (loss)	$	27,248		$	73,966		$	13,489		114,703			1,768			116,471
	Net investment income										70,288			8,706			78,994
	Net realized gains (losses)										65,509			17,839			83,348
	Net impairment losses recognized in earnings										(5,799		)	—			(5,799		)
	Equity in net income of investment funds accounted for using the equity method										5,889			—			5,889
	Other income (loss)										(1,888		)	—			(1,888		)
	Other expenses										(9,345		)	—			(9,345		)
	Interest expense										(12,736		)	—			(12,736		)
	Net foreign exchange gains (losses)										66,853			(352		)	66,501
	Income before income taxes										293,474			27,961			321,435
	Income tax expense										(12,678		)	—			(12,678		)
	Net income										280,796			27,961			308,757
	Dividends attributable to redeemable noncontrolling interests										—			(4,908		)	(4,908		)
	Amounts attributable to noncontrolling interests										—			(20,513		)	(20,513		)
	Net income available to Arch										280,796			2,540			283,336
	Preferred dividends										(5,484		)	—			(5,484		)
	Net income available to Arch common shareholders										$	275,312		$	2,540		$	277,852
	Underwriting Ratios
	Loss ratio	62.6		%	40.2		%	27.4		%	53		%	68.5		%	54.2		%
	Acquisition expense ratio	14.8		%	20.2		%	20.7		%	17		%	29		%	17.9		%
	Other operating expense ratio	17.3		%	13.6		%	40.4		%	17.5		%	2.8		%	16.3		%
	Combined ratio	94.7		%	74		%	88.5		%	87.5		%	100.3		%	88.4		%
	Goodwill and intangible assets	$	30,526		$	2,687		$	73,532		$	106,745		$	—		$	106,745
	Total investable assets										$	14,456,612		$	1,267,588		$	15,724,200
	Total assets										21,227,143			1,622,537			22,849,680
	Total liabilities										15,036,245			515,535			15,551,780
	_________________________________________________
	-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.
		Three Months Ended
		31-Mar-14
		Insurance			Reinsurance			Mortgage			Sub-Total			Other			Total
	Gross premiums written (1)	$	730,646		$	517,053		$	47,907		$	1,295,136		$	32,194		$	1,295,136
	Premiums ceded	(185,044		)	(73,127		)	(4,639		)	(262,340		)	—			(230,146		)
	Net premiums written	545,602			443,926			43,268			1,032,796			32,194			1,064,990
	Change in unearned premiums	(68,101		)	(102,578		)	(4,503		)	(175,182		)	(30,028		)	(205,210		)
	Net premiums earned	477,501			341,348			38,765			857,614			2,166			859,780
	Other underwriting income	500			316			766			1,582			—			1,582
	Losses and loss adjustment expenses	(286,770		)	(139,636		)	(8,478		)	(434,884		)	(1,356		)	(436,240		)
	Acquisition expenses, net	(76,932		)	(73,433		)	(9,154		)	(159,519		)	(823		)	(160,342		)
	Other operating expenses	(81,144		)	(36,195		)	(13,876		)	(131,215		)	(1,109		)	(132,324		)
	Underwriting income	$	33,155		$	92,400		$	8,023		133,578			(1,122		)	132,456
	Net investment income										66,993			1			66,994
	Net realized gains										19,697			—			19,697
	Net impairment losses recognized in earnings										(2,971		)	—			(2,971		)
	Equity in net income of investment funds accounted for using the equity method										3,253			—			3,253
	Other income (loss)										(2,104		)	—			(2,104		)
	Other expenses										(10,799		)	(2,676		)	(13,475		)
	Interest expense										(14,404		)	—			(14,404		)
	Net foreign exchange losses										(6,656		)	93			(6,563		)
	Income before income taxes										186,587			(3,704		)	182,883
	Income tax expense										(3,738		)	—			(3,738		)
Net income										182,849			(3,704		)	179,145
Dividends attributable to redeemable noncontrolling interests										—			(52		)	(52		)
Amounts attributable to noncontrolling interests										—			3,407			3,407
Net income available to Arch										182,849			(349		)	182,500
Preferred dividends										(5,484		)	—			(5,484		)
Net income available to Arch common shareholders										$	177,365		$	(349	)	$	177,016
Underwriting Ratios
Loss ratio	60.1		%	40.9		%	21.9		%	50.7		%	62.6		%	50.7		%
Acquisition expense ratio	16.1		%	21.5		%	23.6		%	18.6		%	38		%	18.6		%
Other operating expense ratio	17		%	10.6		%	35.8		%	15.3		%	51.2		%	15.4		%
Combined ratio	93.2		%	73		%	81.3		%	84.6		%	151.8		%	84.7		%
Goodwill and intangible assets	$	20,184		$	5,803		$	94,888		$	120,875		$	—		$	120,875
Total investable assets										$	14,261,106		$	1,083,280		$	15,344,386
Total assets										20,653,897			1,146,461			21,800,358
Total liabilities										14,860,124			35,371			14,895,495
_________________________________________________
-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.

Investment_Information_Tables

Investment Information (Tables)	3 Months Ended
	Mar. 31, 2015
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:
		Estimated			Gross			Gross			Cost or			OTTI
		Fair			Unrealized			Unrealized			Amortized			Unrealized
		Value			Gains			Losses			Cost			Losses (2)
	31-Mar-15
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	3,119,437		$	62,922		$	(46,982	)	$	3,103,497		$	(1,143	)
	Mortgage backed securities	821,611			20,996			(2,065		)	802,680			(3,557		)
	Municipal bonds	1,626,580			33,377			(2,013		)	1,595,216			—
	Commercial mortgage backed securities	1,106,332			23,083			(2,497		)	1,085,746			—
	U.S. government and government agencies	1,517,834			16,821			(776		)	1,501,789			—
	Non-U.S. government securities	831,200			23,118			(52,685		)	860,767			—
	Asset backed securities	1,657,931			14,080			(3,563		)	1,647,414			(22		)
	Total	10,680,925			194,397			(110,581		)	10,597,109			(4,722		)
	Equity securities	687,713			121,660			(12,467		)	578,520			—
	Other investments	329,677			37,964			(574		)	292,287			(165		)
	Short-term investments	855,032			16			(4,151		)	859,167			—
	Total	$	12,553,347		$	354,037		$	(127,773	)	$	12,327,083		$	(4,887	)
	31-Dec-14
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	3,108,513		$	37,928		$	(38,974	)	$	3,109,559		$	(317	)
	Mortgage backed securities	943,343			18,843			(3,842		)	928,342			(3,307		)
	Municipal bonds	1,494,122			31,227			(1,044		)	1,463,939			—
	Commercial mortgage backed securities	1,114,528			14,594			(3,822		)	1,103,756			—
	U.S. government and government agencies	1,447,972			8,345			(1,760		)	1,441,387			—
	Non-U.S. government securities	1,015,153			21,311			(37,203		)	1,031,045			—
	Asset backed securities	1,677,941			8,425			(6,089		)	1,675,605			(22		)
	Total	10,801,572			140,673			(92,734		)	10,753,633			(3,646		)
	Equity securities	658,182			109,012			(13,364		)	562,534			—
	Other investments	296,224			31,839			(362		)	264,747			—
	Short-term investments	797,226			738			(5,270		)	801,758			—
	Total	$	12,553,204		$	282,262		$	(111,730	)	$	12,382,672		$	(3,646	)
	_________________________________________________
	-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 March 31, 2015, the net unrealized loss related to securities for which a non-credit OTTI was recognized in AOCI was $0.3 million, compared to a net unrealized gain of $0.9 million at December 31, 2014.
Summary of available for sale securities in a continual unrealized loss position	The following table summarizes, for all available for sale securities in an unrealized loss position, the fair value and gross unrealized loss by length of time the security has been in a continual unrealized loss position:
		Less than 12 Months						12 Months or More						Total
		Estimated			Gross			Estimated			Gross			Estimated			Gross
		Fair			Unrealized			Fair			Unrealized			Fair			Unrealized
		Value			Losses			Value			Losses			Value			Losses
	31-Mar-15
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	810,064		$	(40,337	)	$	60,249		$	(6,645	)	$	870,313		$	(46,982	)
	Mortgage backed securities	142,833			(1,333		)	29,939			(732		)	172,772			(2,065		)
	Municipal bonds	241,211			(1,494		)	13,573			(519		)	254,784			(2,013		)
	Commercial mortgage backed securities	194,085			(2,375		)	30,774			(122		)	224,859			(2,497		)
	U.S. government and government agencies	148,004			(776		)	—			—			148,004			(776		)
	Non-U.S. government securities	376,737			(43,321		)	38,091			(9,364		)	414,828			(52,685		)
	Asset backed securities	331,543			(2,072		)	157,748			(1,491		)	489,291			(3,563		)
	Total	2,244,477			(91,708		)	330,374			(18,873		)	2,574,851			(110,581		)
	Equity securities	226,711			(12,467		)	—			—			226,711			(12,467		)
	Other investments	128,867			(574		)	—			—			128,867			(574		)
	Short-term investments	55,615			(4,151		)	—			—			55,615			(4,151		)
	Total	$	2,655,670		$	(108,900	)	$	330,374		$	(18,873	)	$	2,986,044		$	(127,773	)
	31-Dec-14
	Fixed maturities and fixed maturities pledged under securities lending agreements (1):
	Corporate bonds	$	1,309,637		$	(32,903	)	$	148,963		$	(6,071	)	$	1,458,600		$	(38,974	)
	Mortgage backed securities	293,624			(1,476		)	59,107			(2,366		)	352,731			(3,842		)
	Municipal bonds	210,614			(588		)	13,643			(456		)	224,257			(1,044		)
	Commercial mortgage backed securities	232,147			(770		)	125,894			(3,052		)	358,041			(3,822		)
	U.S. government and government agencies	618,381			(1,626		)	3,438			(134		)	621,819			(1,760		)
	Non-U.S. government securities	510,766			(31,172		)	46,910			(6,031		)	557,676			(37,203		)
	Asset backed securities	612,950			(2,486		)	243,452			(3,603		)	856,402			(6,089		)
	Total	3,788,119			(71,021		)	641,407			(21,713		)	4,429,526			(92,734		)
	Equity securities	181,002			(13,364		)	—			—			181,002			(13,364		)
	Other investments	59,638			(362		)	—			—			59,638			(362		)
	Short-term investments	79,271			(5,270		)	—			—			79,271			(5,270		)
	Total	$	4,108,030		$	(90,017	)	$	641,407		$	(21,713	)	$	4,749,437		$	(111,730	)
	_________________________________________________
	-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.
			31-Mar-15						31-Dec-14
	Maturity		Estimated			Amortized			Estimated			Amortized
			Fair			Cost			Fair			Cost
			Value						Value
	Due in one year or less		$	267,975		$	268,219		$	235,930		$	233,794
	Due after one year through five years		3,824,832			3,819,036			4,074,562			4,077,408
	Due after five years through 10 years		2,651,327			2,630,130			2,475,726			2,461,356
	Due after 10 years		350,917			343,884			279,542			273,372
			7,095,051			7,061,269			7,065,760			7,045,930
	Mortgage backed securities		821,611			802,680			943,343			928,342
	Commercial mortgage backed securities		1,106,332			1,085,746			1,114,528			1,103,756
	Asset backed securities		1,657,931			1,647,414			1,677,941			1,675,605
	Total		$	10,680,925		$	10,597,109		$	10,801,572		$	10,753,633
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:
		March 31,			December 31,
		2015			2014
	Available for sale:
	Asian and emerging markets	$	270,087		$	236,586
	Investment grade fixed income	59,590			59,638
	Total available for sale	329,677			296,224
	Fair value option:
	Term loan investments (par value: $1,055,264 and $1,094,337)	1,044,762			1,073,649
	Mezzanine debt funds	121,570			121,341
	Credit related funds	138,576			114,436
	Investment grade fixed income	63,162			69,108
	Asian and emerging markets	26,846			25,800
	Other (1)	162,908			147,573
	Total fair value option	1,557,824			1,551,907
	Total	$	1,887,501		$	1,848,131
	_________________________________________________
	-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:
		March 31,			December 31,
		2015			2014
	Fixed maturities	$	757,467		$	632,024
	Other investments	1,557,824			1,551,907
	Short-term investments	125,788			251,601
	Equity securities	907			—
	Investments accounted for using the fair value option	$	2,441,986		$	2,435,532
Components of net investment income	The components of net investment income were derived from the following sources:
		Three Months Ended
		March 31,
		2015			2014
	Fixed maturities	$	68,596		$	62,449
	Term loan investments	14,744			5,669
	Equity securities (dividends)	2,679			2,921
	Short-term investments	196			405
	Other (1)	12,747			4,719
	Gross investment income	98,962			76,163
	Investment expenses	(19,968		)	(9,169		)
	Net investment income	$	78,994		$	66,994
	_________________________________________________
	-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
		March 31,
		2015			2014
	Available for sale securities:
	Gross gains on investment sales	$	97,591		$	66,051
	Gross losses on investment sales	(55,160		)	(40,013		)
	Change in fair value of assets and liabilities accounted for using the fair value option:
	Fixed maturities	(3,302		)	(1,643		)
	Other investments	6,287			10,776
	Equity securities	(2		)	—
	Short-term investments	5,846			—
	Derivative instruments (1)	36,676			(12,768		)
	Other (2)	(4,588		)	(2,706		)
	Net realized gains (losses)	$	83,348		$	19,697
	_________________________________________________
	-1	See Note 8 for information on the Company’s derivative instruments.
	-2	Includes accretion of contingent consideration liability amounts related to the 2014 acquisition of CMG Mortgage Insurance Company and its affiliated mortgage insurance companies.
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
		March 31,
		2015			2014
	Fixed maturities:
	Mortgage backed securities	$	(1,072	)	$	—
	Corporate bonds	(1,976		)	—
	Asset backed securities	—			(6		)
	Total	(3,048		)	(6		)
	Short-term investments	(2,341		)	—
	Equity securities	(129		)	—
	Other investments	(281		)	(2,965		)
	Net impairment losses recognized in earnings	$	(5,799	)	$	(2,971	)
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
		March 31,
		2015			2014
	Balance at start of period	$	20,196		$	60,062
	Credit loss impairments recognized on securities not previously impaired	4,489			—
	Credit loss impairments recognized on securities previously impaired	79			6
	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	(420		)	(12,812		)
	Balance at end of period	$	24,344		$	47,256
Summary of restricted assets	The following table details the value of the Company’s restricted assets:
		March 31,			December 31,
		2015			2014
	Assets used for collateral or guarantees:
	Affiliated transactions	$	4,054,848		$	4,138,527
	Third party agreements	1,010,214			970,120
	Deposits with U.S. regulatory authorities	407,190			337,981
	Trust funds	64,673			72,461
	Total restricted assets	$	5,536,925		$	5,519,089

Fair_Value_Tables

Fair Value (Tables)	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015:
					Estimated Fair Value Measurement Using:
		Estimated			Quoted Prices in			Significant			Significant
		Fair			Active Markets			Other			Unobservable
		Value			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	$	3,119,437		$	—		$	3,119,437		$	—
	Mortgage backed securities	821,611			—			821,611			—
	Municipal bonds	1,626,580			—			1,626,580			—
	Commercial mortgage backed securities	1,106,332			—			1,106,332			—
	U.S. government and government agencies	1,517,834			1,517,834			—			—
	Non-U.S. government securities	831,200			—			831,200			—
	Asset backed securities	1,657,931			—			1,600,431			57,500
	Total	10,680,925			1,517,834			9,105,591			57,500
	Equity securities	687,713			685,714			1,999			—
	Other investments	329,677			—			201,943			127,734
	Short-term investments	855,032			835,065			19,967			—
	Fair value option:
	Corporate bonds	636,762			—			636,762			—
	Non-U.S. government bonds	77,903			—			77,903			—
	Mortgage backed securities	18,010			—			18,010			—
	Asset backed securities	24,792			—			24,792			—
	Other investments	1,557,824			—			1,088,930			468,894
	Short-term investments	125,788			125,788			—			—
	Equity securities	907			907			—			—
	Total	2,441,986			126,695			1,846,397			468,894
	Total assets measured at fair value	$	14,995,333		$	3,165,308		$	11,175,897		$	654,128
	Liabilities measured at fair value:
	Contingent consideration liability	$	(66,461	)	$	—		$	—		$	(66,461	)
	Total liabilities measured at fair value	$	(66,461	)	$	—		$	—		$	(66,461	)
	_________________________________________________
	-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, 2014:
					Estimated Fair Value Measurement Using:
		Estimated			Quoted Prices in			Significant			Significant
		Fair			Active Markets			Other			Unobservable
		Value			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	$	3,108,513		$	—		$	3,108,513		$	—
	Mortgage backed securities	943,343			—			943,343			—
	Municipal bonds	1,494,122			—			1,494,122			—
	Commercial mortgage backed securities	1,114,528			—			1,114,528			—
	U.S. government and government agencies	1,447,972			1,447,972			—			—
	Non-U.S. government securities	1,015,153			—			1,015,153			—
	Asset backed securities	1,677,941			—			1,620,441			57,500
	Total	10,801,572			1,447,972			9,296,100			57,500
	Equity securities	658,182			658,182			—			—
	Other investments	296,224			—			192,687			103,537
	Short-term investments	797,226			759,621			37,605			—
	Fair value option:
	Corporate bonds	497,101			—			497,101			—
	Non-U.S. government bonds	88,411			—			88,411			—
	Mortgage backed securities	22,190			—			22,190			—
	Asset backed securities	24,322			—			24,322			—
	Other investments	1,551,907			—			1,085,557			466,350
	Short-term investments	251,601			250,580			1,021			—
Total	2,435,532			250,580			1,718,602			466,350
Total assets measured at fair value	$	14,988,736		$	3,116,355		$	11,244,994		$	627,387
Liabilities measured at fair value:
Fair value option:
Contingent consideration liability	$	(61,845	)	$	—		$	—		$	(61,845	)
Total liabilities measured at fair value	$	(61,845	)	$	—		$	—		$	(61,845	)
_________________________________________________
-1	In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged. For purposes of this table, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged.
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:
		Estimated Fair Value Measurements Using:
		Significant Unobservable Inputs (Level 3)
					Assets												Liabilities
	s	Available-For-Sale									Fair Value Option
		Asset Backed Securities			Corporate			Other			Other			Total			Contingent Consideration Liability
					Bonds			Investments			Investments
	Three Months Ended March 31, 2015
	Balance at beginning of period	$	57,500		$	—		$	103,537		$	466,350		$	627,387		$	(61,845	)
	Total gains or (losses) (realized/unrealized)
	Included in earnings (1)	—			—			(281		)	(851		)	(1,132		)	(3,548		)
	Included in other comprehensive income	—			—			(522		)	—			(522		)	—
	Purchases, issuances, sales and settlements
	Purchases	—			—			25,000			22,925			47,925			—
	Issuances	—			—			—			—			—			(1,068		)
	Sales	—			—			—			—			—			—
	Settlements	—			—			—			(19,530		)	(19,530		)	—
	Transfers in and/or out of Level 3	—			—			—			—			—			—
	Balance at end of period	$	57,500		$	—		$	127,734		$	468,894		$	654,128		$	(66,461	)
	Three Months Ended March 31, 2014
	Balance at beginning of period	$	—		$	2,045		$	170,420		$	377,525		$	549,990		$	—
	Total gains or (losses) (realized/unrealized)
	Included in earnings (1)	—			—			2,275			6,310			8,585			(1,394		)
	Included in other comprehensive income	—			—			(2,196		)	—			(2,196		)	—
	Purchases, issuances, sales and settlements
	Purchases	—			—			—			40,110			40,110			—
	Issuances	—			—			—			—			—			(41,762		)
	Sales	—			(2,045		)	(66,164		)	—			(68,209		)	—
	Settlements	—			—			—			(4,799		)	(4,799		)	—
	Transfers in and/or out of Level 3	—			—			—			—			—			—
	Balance at end of period	$	—		$	—		$	104,335		$	419,146		$	523,481		$	(43,156	)
	_________________________________________________
	-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)	3 Months Ended
	Mar. 31, 2015
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.
		Asset			Liability Derivatives				Net
		Derivatives							Derivatives
		Estimated Fair Value			Estimated Fair Value				Estimated Fair Value			Notional
												Value (1)
	31-Mar-15
	Futures contracts	$	2,534		$	(3,148	)		$	(614	)	$	2,398,085
	Foreign currency forward contracts	23,327			(6,751		)		16,576			1,848,503
	TBAs	350,576			(245,150		)		105,426			562,031
	Other	8,810			(4,327		)		4,483			1,252,273
	Total	$	385,247		$	(259,376	)		$	125,871
	31-Dec-14
	Futures contracts	$	2,156		$	(1,907	)		$	249		$	2,549,027
	Foreign currency forward contracts	10,511			(1,145		)		9,366			397,106
	TBAs	10,592			—				10,592			10,056
	Other	3,209			(2,345		)		864			735,684
	Total	$	26,468		$	(5,397	)		$	21,071
	_________________________________________________
	-1	Represents the absolute notional value of all outstanding contracts, consisting of long and short positions.
Summary of net realized gains (losses) recorded in the consolidated statements of income	The following table summarizes net realized gains (losses) recorded on the Company’s derivative instruments in the consolidated statements of income:
			Three Months Ended
	Derivatives not designated as		March 31,
	hedging instruments:		2015			2014
	Futures contracts		$	19,326		$	(9,862	)
	Foreign currency forward contracts		16,819			(2,257		)
	TBAs		486			(37		)
	Other		45			(612		)
	Total		$	36,676		$	(12,768	)

Other_Comprehensive_Income_Los1

Other Comprehensive Income (Loss) (Tables)	3 Months Ended
	Mar. 31, 2015
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 Reclassified from AOCI
			Consolidated Statement of Income		Three Months Ended
	Details About		Line Item That Includes		March 31,
	AOCI Components		Reclassification		2015				2014
	Unrealized appreciation on available-for-sale investments
			Net realized gains (losses)		$	42,431			$	26,038
			Other-than-temporary impairment losses		(7,247		)		(2,971		)
			Total before tax		35,184				23,067
			Income tax (expense) benefit		(4,252		)		(1,818		)
			Net of tax		$	30,932			$	21,249
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 March 31, 2015
	Unrealized appreciation (decline) in value of investments:
	Unrealized holding gains (losses) arising during period	$	94,387			$	10,083			$	84,304
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	(1,448		)		—				(1,448		)
	Less reclassification of net realized gains included in net income	35,184				4,252				30,932
	Foreign currency translation adjustments	(23,626		)		(869		)		(22,757		)
	Other comprehensive income (loss)	$	34,129			$	4,962			$	29,167
	Three Months Ended March 31, 2014
	Unrealized appreciation (decline) in value of investments:
	Unrealized holding gains (losses) arising during period	$	75,400			$	4,047			$	71,353
	Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss)	—				—				—
	Less reclassification of net realized gains (losses) included in net income	23,067				1,818				21,249
	Foreign currency translation adjustments	(1,349		)		—				(1,349		)
	Other comprehensive income (loss)	$	50,984			$	2,229			$	48,755

Guarantor_Financial_Informatio1

Guarantor Financial Information (Tables)	3 Months Ended
	Mar. 31, 2015
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Condensed consolidating balance sheet
			31-Mar-15
	Condensed Consolidating Balance Sheet		ACGL (Parent Guarantor)				Arch-U.S. (Subsidiary Issuer)				Other ACGL Subsidiaries				Consolidating Adjustments and Eliminations				ACGL Consolidated
	Assets
	Total investments		$	24			$	53,890			$	15,357,730			$	—			$	15,411,644
	Cash		7,992				2,822				460,198				—				471,012
	Investments in subsidiaries		6,688,930				1,710,358				—				(8,399,288		)		—
	Due from subsidiaries and affiliates		31				15,144				380,577				(395,752		)		—
	Premiums receivable		—				—				1,575,230				(458,841		)		1,116,389
	Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses		—				—				5,545,517				(3,756,898		)		1,788,619
	Contractholder receivables		—				—				1,339,433				—				1,339,433
	Prepaid reinsurance premiums		—				—				1,545,190				(1,123,282		)		421,908
	Deferred acquisition costs, net		—				—				442,775				—				442,775
	Other assets		6,998				50,869				2,138,959				(338,926		)		1,857,900
		Total assets	$	6,703,975			$	1,833,083			$	28,785,609			$	(14,472,987	)		$	22,849,680
	Liabilities
	Reserve for losses and loss adjustment expenses		$	—			$	—			$	12,656,904			$	(3,727,954	)		$	8,928,950
	Unearned premiums		—				—				3,524,116				(1,123,282		)		2,400,834
	Reinsurance balances payable		—				—				645,723				(445,279		)		200,444
	Contractholder payables		—				—				1,339,433				—				1,339,433
	Deposit accounting liabilities		—				—				508,592				(223,764		)		284,828
	Senior notes		300,000				500,000				—				—				800,000
	Revolving credit agreement borrowings		100,000				—				—				—				100,000
	Due to subsidiaries and affiliates		1,305				15,022				379,425				(395,752		)		—
	Other liabilities		13,968				47,944				1,593,048				(157,669		)		1,497,291
		Total liabilities	415,273				562,966				20,647,241				(6,073,700		)		15,551,780
	Redeemable noncontrolling interests		—				—				219,604				—				219,604
	Shareholders’ Equity
	Total shareholders’ equity available to Arch		6,288,702				1,270,117				7,129,170				(8,399,287		)		6,288,702
	Non-redeemable noncontrolling interests		—				—				789,594				—				789,594
		Total shareholders’ equity	6,288,702				1,270,117				7,918,764				(8,399,287		)		7,078,296
		Total liabilities, noncontrolling interests and shareholders’ equity	$	6,703,975			$	1,833,083			$	28,785,609			$	(14,472,987	)		$	22,849,680
			31-Dec-14
	Condensed Consolidating Balance Sheet		ACGL (Parent Guarantor)				Arch-U.S. (Subsidiary Issuer)				Other ACGL Subsidiaries				Consolidating Adjustments and Eliminations				ACGL Consolidated
	Assets
	Total investments		$	107			$	62,867			$	15,268,275			$	—			$	15,331,249
	Cash		3,218				2,787				479,697				—				485,702
	Investments in subsidiaries		6,536,644				1,685,185				—				(8,221,829		)		—
	Due from subsidiaries and affiliates		48				7,517				370,429				(377,994		)		—
	Premiums receivable		—				—				1,331,511				(382,816		)		948,695
	Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses		—				—				5,584,973				(3,772,128		)		1,812,845
	Contractholder receivables		—				—				1,309,192				—				1,309,192
	Prepaid reinsurance premiums		—				—				1,373,008				(995,930		)		377,078
	Deferred acquisition costs, net		—				—				414,525				—				414,525
	Other assets		7,590				49,576				1,689,670				(416,579		)		1,330,257
		Total assets	$	6,547,607			$	1,807,932			$	27,821,280			$	(14,167,276	)		$	22,009,543
	Liabilities
	Reserve for losses and loss adjustment expenses		$	—			$	—			$	12,784,030			$	(3,747,582	)		$	9,036,448
	Unearned premiums		—				—				3,227,508				(995,930		)		2,231,578
	Reinsurance balances payable		—				—				589,289				(369,977		)		219,312
	Contractholder payables		—				—				1,309,192				—				1,309,192
	Deposit accounting liabilities		—				—				587,050				(259,666		)		327,384
	Senior notes		300,000				500,000				—				—				800,000
	Revolving credit agreement borrowings		100,000				—				—				—				100,000
	Due to subsidiaries and affiliates		417				7,505				370,072				(377,994		)		—
	Other liabilities		17,137				49,403				994,741				(194,298		)		866,983
		Total liabilities	417,554				556,908				19,861,882				(5,945,447		)		14,890,897
	Redeemable noncontrolling interests		—				—				219,512				—				219,512
	Shareholders’ Equity
	Total shareholders’ equity available to Arch		6,130,053				1,251,024				6,970,805				(8,221,829		)		6,130,053
	Non-redeemable noncontrolling interests		—				—				769,081				—				769,081
		Total shareholders’ equity	6,130,053				1,251,024				7,739,886				(8,221,829		)		6,899,134
		Total liabilities, noncontrolling interests and shareholders’ equity	$	6,547,607			$	1,807,932			$	27,821,280			$	(14,167,276	)		$	22,009,543
Condensed consolidating statement of income and comprehensive income
			Three Months Ended March 31, 2015
	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		$	—			$	—			$	910,264			$	—			$	910,264
	Net investment income		—				5				86,010				(7,021		)		78,994
	Net realized gains		—				—				83,348				—				83,348
	Net impairment losses recognized in earnings		—				—				(5,799		)		—				(5,799		)
	Other underwriting income		—				—				11,536				—				11,536
	Equity in net income of investment funds accounted for using the equity method		—				—				5,889				—				5,889
	Other income (loss)		—				—				(1,888		)		—				(1,888		)
		Total revenues	—				5				1,089,360				(7,021		)		1,082,344
	Expenses
	Losses and loss adjustment expenses		—				—				493,716				—				493,716
	Acquisition expenses		—				—				163,076				—				163,076
	Other operating expenses		8,632				1,259				147,991				—				157,882
	Interest expense		5,856				6,366				7,535				(7,021		)		12,736
	Net foreign exchange gains		—				—				(39,630		)		(26,871		)		(66,501		)
		Total expenses	14,488				7,625				772,688				(33,892		)		760,909
	Income (loss) before income taxes		(14,488		)		(7,620		)		316,672				26,871				321,435
	Income tax benefit (expense)		—				1,413				(14,091		)		—				(12,678		)
	Income (loss) before equity in net income of subsidiaries		(14,488		)		(6,207		)		302,581				26,871				308,757
	Equity in net income of subsidiaries		297,824				14,495				—				(312,319		)		—
	Net income		283,336				8,288				302,581				(285,448		)		308,757
	Amounts attributable to noncontrolling interests		—				—				(25,421		)		—				(25,421		)
	Net income available to Arch		283,336				8,288				277,160				(285,448		)		283,336
	Preferred dividends		(5,484		)		—				—				—				(5,484		)
	Net income available to Arch common shareholders		$	277,852			$	8,288			$	277,160			$	(285,448	)		$	277,852
	Comprehensive income available to Arch		$	312,503			$	11,750			$	333,193			$	(344,943	)		$	312,503
			Three Months Ended March 31, 2014
	Condensed Consolidating Statement of Income and Comprehensive Income		ACGL (Parent Guarantor)				Arch-U.S. (Subsidiary Issuer)				Other ACGL Subsidiaries				Consolidating Adjustments and Eliminations				ACGL Consolidated
	Revenues
	Net premiums earned		$	—			$	—			$	859,780			$	—			$	859,780
	Net investment income		—				—				77,001				(10,007		)		66,994
	Net realized gains		—				—				19,697				—				19,697
	Net impairment losses recognized in earnings		—				—				(2,971		)		—				(2,971		)
	Other underwriting income		—				—				1,582				—				1,582
	Equity in net income of investment funds accounted for using the equity method		—				—				3,253				—				3,253
	Other income (loss)		—				—				(2,104		)		—				(2,104		)
		Total revenues	—				—				956,238				(10,007		)		946,231
	Expenses
	Losses and loss adjustment expenses		—				—				436,240				—				436,240
	Acquisition expenses		—				—				160,342				—				160,342
	Other operating expenses		10,307				977				134,515				—				145,799
	Interest expense		5,853				6,514				12,044				(10,007		)		14,404
	Net foreign exchange losses		—				—				5,741				822				6,563
		Total expenses	16,160				7,491				748,882				(9,185		)		763,348
	Income (loss) before income taxes		(16,160		)		(7,491		)		207,356				(822		)		182,883
	Income tax expense		—				2,788				(6,526		)		—				(3,738		)
	Income (loss) before equity in net income of subsidiaries		(16,160		)		(4,703		)		200,830				(822		)		179,145
	Equity in net income of subsidiaries		198,660				22,152				—				(220,812		)		—
	Net income		182,500				17,449				200,830				(221,634		)		179,145
	Amounts attributable to noncontrolling interests		—				—				3,355				—				3,355
	Net income available to Arch		182,500				17,449				204,185				(221,634		)		182,500
	Preferred dividends		(5,484		)		—				—				—				(5,484		)
	Net income available to Arch common shareholders		$	177,016			$	17,449			$	204,185			$	(221,634	)		$	177,016
	Comprehensive income available to Arch		$	231,255			$	17,220			$	252,119			$	(269,339	)		$	231,255
Condensed consolidating statement of cash flows
					Three Months Ended March 31, 2015
	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			$	173,508			$	(8,950	)		$	106,903			$	(186,000	)		$	85,461
	Investing Activities
	Purchases of fixed maturity investments				—				—				(7,030,731		)		—				(7,030,731		)
	Purchases of equity securities				—				—				(125,863		)		—				(125,863		)
	Purchases of other investments				—				—				(685,497		)		—				(685,497		)
	Proceeds from the sales of fixed maturity investments				—				9,001				6,848,458				—				6,857,459
	Proceeds from the sales of equity securities				—				—				125,906				—				125,906
	Proceeds from the sales of other investments				—				—				605,263				—				605,263
	Proceeds from redemptions and maturities of fixed maturity investments				—				—				272,657				—				272,657
	Net purchases (sales) of short-term investments				83				(16		)		66,216				—				66,283
	Change in cash collateral related to securities lending				—				—				(5,529		)		—				(5,529		)
	Contributions to subsidiaries				—				—				(5,500		)		5,500				—
	Intercompany loans issued				—				(7,500		)		(7,500		)		15,000				—
	Purchase of business, net of cash acquired				—				—				(2,432		)		—				(2,432		)
	Purchases of furniture, equipment and other assets				(23		)		—				(32,874		)		—				(32,897		)
		Net Cash Provided By (Used For) Investing Activities			60				1,485				22,574				20,500				44,619
	Financing Activities
	Purchases of common shares under share repurchase program				(162,898		)		—				—				—				(162,898		)
	Proceeds from common shares issued, net				(412		)		—				5,500				(5,500		)		(412		)
	Proceeds from intercompany borrowings				—				7,500				7,500				(15,000		)		—
	Change in cash collateral related to securities lending				—				—				5,529				—				5,529
	Dividends paid to redeemable noncontrolling interests				—				—				(4,816		)		—				(4,816		)
	Dividends paid to parent				—				—				(186,000		)		186,000				—
	Other				—				—				29,779				—				29,779
	Preferred dividends paid				(5,484		)		—				—				—				(5,484		)
		Net Cash Provided By (Used For) Financing Activities			(168,794		)		7,500				(142,508		)		165,500				(138,302		)
	Effects of exchange rates changes on foreign currency cash				—				—				(6,468		)		—				(6,468		)
	Increase (decrease) in cash				4,774				35				(19,499		)		—				(14,690		)
	Cash beginning of year				3,218				2,787				479,697				—				485,702
	Cash end of period				$	7,992			$	2,822			$	460,198			$	—			$	471,012
					Three Months Ended March 31, 2014
	Condensed Consolidating Statement				ACGL (Parent Guarantor)				Arch-U.S. (Subsidiary Issuer)				Other ACGL Subsidiaries				Consolidating Adjustments and Eliminations				ACGL Consolidated
	of Cash Flows
	Operating Activities
		Net Cash Provided By (Used For) Operating Activities			$	11,685			$	7,151			$	194,833			$	(15,500	)		$	198,169
	Investing Activities
	Purchases of fixed maturity investments				—				—				(7,131,071		)		—				(7,131,071		)
	Purchases of equity securities				—				—				(89,227		)		—				(89,227		)
	Purchases of other investments				—				—				(304,454		)		—				(304,454		)
	Proceeds from the sales of fixed maturity investments				—				—				7,014,281				—				7,014,281
	Proceeds from the sales of equity securities				—				—				49,614				—				49,614
	Proceeds from the sales of other investments				—				—				331,176				—				331,176
	Proceeds from redemptions and maturities of fixed maturity investments				—				—				168,484				—				168,484
	Net (purchases) sales of short-term investments				(16		)		360,269				(203,991		)		—				156,262
	Change in cash collateral related to securities lending				—				—				4,669				—				4,669
	Contributions to subsidiaries				—				(312,207		)		(100,000		)		412,207				—
	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				(76		)		—				(5,306		)		—				(5,382		)
		Net Cash Provided By (Used For) Investing Activities			(92		)		48,062				(491,153		)		401,957				(41,226		)
	Financing Activities
	Proceeds from common shares issued, net				3,021				—				412,207				(412,207		)		3,021
	Repayments of intercompany borrowings				—				(10,250		)		—				10,250				—
	Change in cash collateral related to securities lending				—				—				(4,669		)		—				(4,669		)
	Third party investment in non-redeemable noncontrolling interests				—				—				796,903				—				796,903
	Third party investment in redeemable noncontrolling interests				—				—				186,893				—				186,893
	Dividends paid to parent				—				—				(15,500		)		15,500				—
	Other				—				—				1,700				—				1,700
	Preferred dividends paid				(5,484		)		—				—				—				(5,484		)
	Net Cash Provided By (Used For) Financing Activities			(2,463		)		(10,250		)		1,377,534				(386,457		)		978,364
Effects of exchange rates changes on foreign currency cash				—				—				241				—				241
Increase in cash				9,130				44,963				1,081,455				—				1,135,548
Cash beginning of year				3,223				509				430,325				—				434,057
Cash end of period				$	12,353			$	45,472			$	1,511,780			$	—			$	1,569,605

Variable_Interest_Entities_and2

Variable Interest Entities and Noncontrolling Interests - Variable interest entity (Details) (USD $)	3 Months Ended		1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2014	Dec. 31, 2014
Variable Interest Entity [Line Items]
Unearned premiums	$2,400,834,000			$2,231,578,000
Reserve for losses and loss adjustment expenses	8,928,950,000			9,036,448,000
Net Cash Provided by (Used in) Operating Activities	85,461,000	198,169,000
Net Cash Provided by (Used in) Financing Activities	-138,302,000	978,364,000
Net Cash Provided by (Used in) Investing Activities	44,619,000	-41,226,000
Variable Interest Entity, Primary Beneficiary | Watford Holdings Ltd
Variable Interest Entity [Line Items]
Initial investment contribution amount			100,000,000
Ownership percentage			11.00%
Variable Interest Entity, Primary Beneficiary | Watford Re
Variable Interest Entity [Line Items]
Unearned premiums	233,000,000
Reserve for losses and loss adjustment expenses	109,400,000
Net Cash Provided by (Used in) Operating Activities	69,900,000
Net Cash Provided by (Used in) Financing Activities	22,900,000
Net Cash Provided by (Used in) Investing Activities	($35,300,000)

Variable_Interest_Entities_and3

Variable Interest Entities and Noncontrolling Interests - Non-redeemable noncontrolling interests (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Noncontrolling Interest [Line Items]
Non-redeemable noncontrolling interests, beginning of period	$769,081
Amounts attributable to noncontrolling interests	20,513	-3,407
Non-redeemable noncontrolling interests, end of period	789,594	793,496
Watford Re
Noncontrolling Interest [Line Items]
Non-redeemable noncontrolling interests, beginning of period	769,081	0
Sale of shares to noncontrolling interests	0	796,903
Amounts attributable to noncontrolling interests	20,513	-3,407
Non-redeemable noncontrolling interests, end of period	$789,594
Common shares | Watford Re
Noncontrolling Interest [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners	89.00%

Variable_Interest_Entities_and4

Variable Interest Entities and Noncontrolling Interests - Redeemable noncontrolling interests (Details) (USD $)	3 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Redeemable Noncontrolling Interest [Line Items]
Amounts attributable to noncontrolling interests	$25,421	($3,355)
Amounts attributable to noncontrolling interests	20,513	-3,407
Dividends attributable to redeemable noncontrolling interests	4,908	52
Watford Re
Redeemable Noncontrolling Interest [Line Items]
Preference shares, number of shares issued	9,065,200
Par value per share	$0.01
Liquidation preference per share	$25
Amounts attributable to noncontrolling interests	$20,513	($3,407)
Cumulative redeemable preference shares | Watford Re
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
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Numerator:
Net income	$308,757		$179,145
Amounts attributable to noncontrolling interests	-25,421		3,355
Net income available to Arch	283,336		182,500
Preferred dividends	-5,484		-5,484
Net income available to Arch common shareholders	$277,852		$177,016
Denominator:
Weighted average common shares outstanding b basic	124,209,276		131,857,910
Effect of dilutive common share equivalents:
Nonvested restricted shares	1,416,801		1,346,401
Stock options	2,824,977	[1]	3,358,406	[1]
Weighted average common shares and common share equivalents outstanding b diluted	128,451,054		136,562,717
Earnings per common share:
Basic (per share)	$2.24		$1.34
Diluted (per share)	$2.16		$1.30
Stock Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per common share (shares)	703,853		759,172
[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 2015 first quarter and 2014 first quarter, the number of stock options excluded were 703,853 and 759,172, respectively.

Segment_Information_Summary_of

Segment Information - Summary of underwriting income or loss by segment (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014
Segment Reporting Information [Line Items]
Gross premiums written	$1,342,022		$1,295,136
Premiums ceded	-275,027		-230,146
Net premiums written	1,066,995		1,064,990
Change in unearned premiums	-156,731		-205,210
Net premiums earned	910,264		859,780
Other underwriting income	11,536		1,582
Losses and loss adjustment expenses	-493,716		-436,240
Acquisition expenses, net	-163,076		-160,342
Other operating expenses	-148,537		-132,324
Underwriting income (loss)	116,471		132,456
Net investment income	78,994		66,994
Net realized gains (losses)	83,348		19,697
Net impairment losses recognized in earnings	-5,799		-2,971
Equity in net income (loss) of investment funds accounted for using the equity method	5,889		3,253
Other income (loss)	-1,888		-2,104
Other expenses	-9,345		-13,475
Interest expense	-12,736		-14,404
Net foreign exchange gains (losses)	66,501		-6,563
Income before income taxes	321,435		182,883
Income tax expense	-12,678		-3,738
Net income	308,757		179,145
Dividends attributable to redeemable noncontrolling interests	-4,908		-52
Amounts attributable to noncontrolling interests	-20,513		3,407
Net income available to Arch	283,336		182,500
Preferred dividends	-5,484		-5,484
Net income available to Arch common shareholders	277,852		177,016
Underwriting Ratios
Loss ratio	54.20%		50.70%
Acquisition expense ratio	17.90%		18.60%
Other operating expense ratio	16.30%		15.40%
Combined ratio	88.40%		84.70%
Goodwill and intangible assets	106,745		120,875		109,539
Total investable assets	15,724,200		15,344,386
Total assets	22,849,680		21,800,358		22,009,543
Total liabilities	15,551,780		14,895,495		14,890,897
Sub-Total
Segment Reporting Information [Line Items]
Gross premiums written	1,311,678	[1]	1,295,136	[1]
Premiums ceded	-369,261		-262,340
Net premiums written	942,417		1,032,796
Change in unearned premiums	-104,419		-175,182
Net premiums earned	837,998		857,614
Other underwriting income	9,574		1,582
Losses and loss adjustment expenses	-444,237		-434,884
Acquisition expenses, net	-142,100		-159,519
Other operating expenses	-146,532		-131,215
Underwriting income (loss)	114,703		133,578
Net investment income	70,288		66,993
Net realized gains (losses)	65,509		19,697
Net impairment losses recognized in earnings	-5,799		-2,971
Equity in net income (loss) of investment funds accounted for using the equity method	5,889		3,253
Other income (loss)	-1,888		-2,104
Other expenses	-9,345		-10,799
Interest expense	-12,736		-14,404
Net foreign exchange gains (losses)	66,853		-6,656
Income before income taxes	293,474		186,587
Income tax expense	-12,678		-3,738
Net income	280,796		182,849
Dividends attributable to redeemable noncontrolling interests	0		0
Amounts attributable to noncontrolling interests	0		0
Net income available to Arch	280,796		182,849
Preferred dividends	-5,484		-5,484
Net income available to Arch common shareholders	275,312		177,365
Underwriting Ratios
Loss ratio	53.00%		50.70%
Acquisition expense ratio	17.00%		18.60%
Other operating expense ratio	17.50%		15.30%
Combined ratio	87.50%		84.60%
Goodwill and intangible assets	106,745		120,875
Total investable assets	14,456,612		14,261,106
Total assets	21,227,143		20,653,897
Total liabilities	15,036,245		14,860,124
Sub-Total | Insurance
Segment Reporting Information [Line Items]
Gross premiums written	766,153	[1]	730,646	[1]
Premiums ceded	-224,150		-185,044
Net premiums written	542,003		545,602
Change in unearned premiums	-34,089		-68,101
Net premiums earned	507,914		477,501
Other underwriting income	427		500
Losses and loss adjustment expenses	-317,896		-286,770
Acquisition expenses, net	-75,078		-76,932
Other operating expenses	-88,119		-81,144
Underwriting income (loss)	27,248		33,155
Underwriting Ratios
Loss ratio	62.60%		60.10%
Acquisition expense ratio	14.80%		16.10%
Other operating expense ratio	17.30%		17.00%
Combined ratio	94.70%		93.20%
Goodwill and intangible assets	30,526		20,184
Sub-Total | Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	485,112	[1]	517,053	[1]
Premiums ceded	-136,569		-73,127
Net premiums written	348,543		443,926
Change in unearned premiums	-68,826		-102,578
Net premiums earned	279,717		341,348
Other underwriting income	1,429		316
Losses and loss adjustment expenses	-112,532		-139,636
Acquisition expenses, net	-56,604		-73,433
Other operating expenses	-38,044		-36,195
Underwriting income (loss)	73,966		92,400
Underwriting Ratios
Loss ratio	40.20%		40.90%
Acquisition expense ratio	20.20%		21.50%
Other operating expense ratio	13.60%		10.60%
Combined ratio	74.00%		73.00%
Goodwill and intangible assets	2,687		5,803
Sub-Total | Mortgage
Segment Reporting Information [Line Items]
Gross premiums written	60,541	[1]	47,907	[1]
Premiums ceded	-8,670		-4,639
Net premiums written	51,871		43,268
Change in unearned premiums	-1,504		-4,503
Net premiums earned	50,367		38,765
Other underwriting income	7,718		766
Losses and loss adjustment expenses	-13,809		-8,478
Acquisition expenses, net	-10,418		-9,154
Other operating expenses	-20,369		-13,876
Underwriting income (loss)	13,489		8,023
Underwriting Ratios
Loss ratio	27.40%		21.90%
Acquisition expense ratio	20.70%		23.60%
Other operating expense ratio	40.40%		35.80%
Combined ratio	88.50%		81.30%
Goodwill and intangible assets	73,532		94,888
Other
Segment Reporting Information [Line Items]
Gross premiums written	128,633	[1]	32,194	[1]
Premiums ceded	-4,055		0
Net premiums written	124,578		32,194
Change in unearned premiums	-52,312		-30,028
Net premiums earned	72,266		2,166
Other underwriting income	1,962		0
Losses and loss adjustment expenses	-49,479		-1,356
Acquisition expenses, net	-20,976		-823
Other operating expenses	-2,005		-1,109
Underwriting income (loss)	1,768		-1,122
Net investment income	8,706		1
Net realized gains (losses)	17,839		0
Net impairment losses recognized in earnings	0		0
Equity in net income (loss) of investment funds accounted for using the equity method	0		0
Other income (loss)	0		0
Other expenses	0		-2,676
Interest expense	0		0
Net foreign exchange gains (losses)	-352		93
Income before income taxes	27,961		-3,704
Income tax expense	0		0
Net income	27,961		-3,704
Dividends attributable to redeemable noncontrolling interests	-4,908		-52
Amounts attributable to noncontrolling interests	-20,513		3,407
Net income available to Arch	2,540		-349
Preferred dividends	0		0
Net income available to Arch common shareholders	2,540		-349
Underwriting Ratios
Loss ratio	68.50%		62.60%
Acquisition expense ratio	29.00%		38.00%
Other operating expense ratio	2.80%		51.20%
Combined ratio	100.30%		151.80%
Goodwill and intangible assets	0		0
Total investable assets	1,267,588		1,083,280
Total assets	1,622,537		1,146,461
Total liabilities	$515,535		$35,371
[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.

Segment_Information_Narrative_

Segment Information - Narrative (Details)	3 Months Ended
	Mar. 31, 2015
	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 $)	3 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Available for sale securities:
Estimated Fair Value	$12,553,347		$12,553,204
Gross Unrealized Gains	354,037		282,262
Gross Unrealized Losses	-127,773		-111,730
Cost or Amortized Cost	12,327,083		12,382,672
OTTI Unrealized Losses	-4,887	[1]	-3,646	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements
Available for sale securities:
Estimated Fair Value	10,680,925		10,801,572
Gross Unrealized Gains	194,397	[2]	140,673	[2]
Gross Unrealized Losses	-110,581	[2]	-92,734	[2]
Cost or Amortized Cost	10,597,109	[2]	10,753,633	[2]
OTTI Unrealized Losses	-4,722	[1],[2]	-3,646	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Available for sale securities:
Estimated Fair Value	3,119,437		3,108,513
Gross Unrealized Gains	62,922	[2]	37,928	[2]
Gross Unrealized Losses	-46,982	[2]	-38,974	[2]
Cost or Amortized Cost	3,103,497	[2]	3,109,559	[2]
OTTI Unrealized Losses	-1,143	[1],[2]	-317	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Available for sale securities:
Estimated Fair Value	821,611		943,343
Gross Unrealized Gains	20,996	[2]	18,843	[2]
Gross Unrealized Losses	-2,065	[2]	-3,842	[2]
Cost or Amortized Cost	802,680	[2]	928,342	[2]
OTTI Unrealized Losses	-3,557	[1],[2]	-3,307	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds
Available for sale securities:
Estimated Fair Value	1,626,580		1,494,122
Gross Unrealized Gains	33,377	[2]	31,227	[2]
Gross Unrealized Losses	-2,013	[2]	-1,044	[2]
Cost or Amortized Cost	1,595,216	[2]	1,463,939	[2]
OTTI Unrealized Losses	0	[1],[2]	0	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Available for sale securities:
Estimated Fair Value	1,106,332		1,114,528
Gross Unrealized Gains	23,083	[2]	14,594	[2]
Gross Unrealized Losses	-2,497	[2]	-3,822	[2]
Cost or Amortized Cost	1,085,746	[2]	1,103,756	[2]
OTTI Unrealized Losses	0	[1],[2]	0	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies
Available for sale securities:
Estimated Fair Value	1,517,834		1,447,972
Gross Unrealized Gains	16,821	[2]	8,345	[2]
Gross Unrealized Losses	-776	[2]	-1,760	[2]
Cost or Amortized Cost	1,501,789	[2]	1,441,387	[2]
OTTI Unrealized Losses	0	[1],[2]	0	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities
Available for sale securities:
Estimated Fair Value	831,200		1,015,153
Gross Unrealized Gains	23,118	[2]	21,311	[2]
Gross Unrealized Losses	-52,685	[2]	-37,203	[2]
Cost or Amortized Cost	860,767	[2]	1,031,045	[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	1,657,931		1,677,941
Gross Unrealized Gains	14,080	[2]	8,425	[2]
Gross Unrealized Losses	-3,563	[2]	-6,089	[2]
Cost or Amortized Cost	1,647,414	[2]	1,675,605	[2]
OTTI Unrealized Losses	-22	[1],[2]	-22	[1],[2]
Equity securities
Available for sale securities:
Estimated Fair Value	687,713		658,182
Gross Unrealized Gains	121,660		109,012
Gross Unrealized Losses	-12,467		-13,364
Cost or Amortized Cost	578,520		562,534
OTTI Unrealized Losses	0	[1]	0	[1]
Other investments
Available for sale securities:
Estimated Fair Value	329,677		296,224
Gross Unrealized Gains	37,964		31,839
Gross Unrealized Losses	-574		-362
Cost or Amortized Cost	292,287		264,747
OTTI Unrealized Losses	-165	[1]	0	[1]
Short-term investments
Available for sale securities:
Estimated Fair Value	855,032		797,226
Gross Unrealized Gains	16		738
Gross Unrealized Losses	-4,151		-5,270
Cost or Amortized Cost	859,167		801,758
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 March 31, 2015, the net unrealized loss related to securities for which a non-credit OTTI was recognized in AOCI was $0.3 million, compared to a net unrealized gain of $0.9 million at December 31, 2014.
[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 $)	Mar. 31, 2015		Dec. 31, 2014
In Thousands, unless otherwise specified
Available for sale securities:
Estimated Fair Value - Less than 12 Months	$2,655,670		$4,108,030
Gross Unrealized Losses - Less than 12 Months	-108,900		-90,017
Estimated Fair Value - 12 Months or More	330,374		641,407
Gross Unrealized Losses - 12 Months or More	-18,873		-21,713
Estimated Fair Value - Total	2,986,044		4,749,437
Gross Unrealized Losses - Total	-127,773		-111,730
Fixed maturities and fixed maturities pledged under securities lending agreements
Available for sale securities:
Estimated Fair Value - Less than 12 Months	2,244,477	[1]	3,788,119	[1]
Gross Unrealized Losses - Less than 12 Months	-91,708	[1]	-71,021	[1]
Estimated Fair Value - 12 Months or More	330,374	[1]	641,407	[1]
Gross Unrealized Losses - 12 Months or More	-18,873	[1]	-21,713	[1]
Estimated Fair Value - Total	2,574,851	[1]	4,429,526	[1]
Gross Unrealized Losses - Total	-110,581	[1]	-92,734	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Available for sale securities:
Estimated Fair Value - Less than 12 Months	810,064	[1]	1,309,637	[1]
Gross Unrealized Losses - Less than 12 Months	-40,337	[1]	-32,903	[1]
Estimated Fair Value - 12 Months or More	60,249	[1]	148,963	[1]
Gross Unrealized Losses - 12 Months or More	-6,645	[1]	-6,071	[1]
Estimated Fair Value - Total	870,313	[1]	1,458,600	[1]
Gross Unrealized Losses - Total	-46,982	[1]	-38,974	[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	142,833	[1]	293,624	[1]
Gross Unrealized Losses - Less than 12 Months	-1,333	[1]	-1,476	[1]
Estimated Fair Value - 12 Months or More	29,939	[1]	59,107	[1]
Gross Unrealized Losses - 12 Months or More	-732	[1]	-2,366	[1]
Estimated Fair Value - Total	172,772	[1]	352,731	[1]
Gross Unrealized Losses - Total	-2,065	[1]	-3,842	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds
Available for sale securities:
Estimated Fair Value - Less than 12 Months	241,211	[1]	210,614	[1]
Gross Unrealized Losses - Less than 12 Months	-1,494	[1]	-588	[1]
Estimated Fair Value - 12 Months or More	13,573	[1]	13,643	[1]
Gross Unrealized Losses - 12 Months or More	-519	[1]	-456	[1]
Estimated Fair Value - Total	254,784	[1]	224,257	[1]
Gross Unrealized Losses - Total	-2,013	[1]	-1,044	[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	194,085	[1]	232,147	[1]
Gross Unrealized Losses - Less than 12 Months	-2,375	[1]	-770	[1]
Estimated Fair Value - 12 Months or More	30,774	[1]	125,894	[1]
Gross Unrealized Losses - 12 Months or More	-122	[1]	-3,052	[1]
Estimated Fair Value - Total	224,859	[1]	358,041	[1]
Gross Unrealized Losses - Total	-2,497	[1]	-3,822	[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	148,004	[1]	618,381	[1]
Gross Unrealized Losses - Less than 12 Months	-776	[1]	-1,626	[1]
Estimated Fair Value - 12 Months or More	0	[1]	3,438	[1]
Gross Unrealized Losses - 12 Months or More	0	[1]	-134	[1]
Estimated Fair Value - Total	148,004	[1]	621,819	[1]
Gross Unrealized Losses - Total	-776	[1]	-1,760	[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	376,737	[1]	510,766	[1]
Gross Unrealized Losses - Less than 12 Months	-43,321	[1]	-31,172	[1]
Estimated Fair Value - 12 Months or More	38,091	[1]	46,910	[1]
Gross Unrealized Losses - 12 Months or More	-9,364	[1]	-6,031	[1]
Estimated Fair Value - Total	414,828	[1]	557,676	[1]
Gross Unrealized Losses - Total	-52,685	[1]	-37,203	[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	331,543	[1]	612,950	[1]
Gross Unrealized Losses - Less than 12 Months	-2,072	[1]	-2,486	[1]
Estimated Fair Value - 12 Months or More	157,748	[1]	243,452	[1]
Gross Unrealized Losses - 12 Months or More	-1,491	[1]	-3,603	[1]
Estimated Fair Value - Total	489,291	[1]	856,402	[1]
Gross Unrealized Losses - Total	-3,563	[1]	-6,089	[1]
Equity securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	226,711		181,002
Gross Unrealized Losses - Less than 12 Months	-12,467		-13,364
Estimated Fair Value - 12 Months or More	0		0
Gross Unrealized Losses - 12 Months or More	0		0
Estimated Fair Value - Total	226,711		181,002
Gross Unrealized Losses - Total	-12,467		-13,364
Other investments
Available for sale securities:
Estimated Fair Value - Less than 12 Months	128,867		59,638
Gross Unrealized Losses - Less than 12 Months	-574		-362
Estimated Fair Value - 12 Months or More	0		0
Gross Unrealized Losses - 12 Months or More	0		0
Estimated Fair Value - Total	128,867		59,638
Gross Unrealized Losses - Total	-574		-362
Short-term investments
Available for sale securities:
Estimated Fair Value - Less than 12 Months	55,615		79,271
Gross Unrealized Losses - Less than 12 Months	-4,151		-5,270
Estimated Fair Value - 12 Months or More	0		0
Gross Unrealized Losses - 12 Months or More	0		0
Estimated Fair Value - Total	55,615		79,271
Gross Unrealized Losses - Total	($4,151)		($5,270)
[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 $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Estimated Fair Value:
Estimated fair value	$10,427,810	$10,750,770
Amortized Cost:
Amortized cost	10,343,330	10,701,557
Fixed maturities and fixed maturities pledged under securities lending agreements
Estimated Fair Value:
Due in one year or less	267,975	235,930
Due after one year through five years	3,824,832	4,074,562
Due after five years through 10 years	2,651,327	2,475,726
Due after 10 years	350,917	279,542
Single maturity date	7,095,051	7,065,760
Estimated fair value	10,680,925	10,801,572
Amortized Cost:
Due in one year or less	268,219	233,794
Due after one year through five years	3,819,036	4,077,408
Due after five years through 10 years	2,630,130	2,461,356
Due after 10 years	343,884	273,372
Single maturity date	7,061,269	7,045,930
Amortized cost	10,597,109	10,753,633
Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Estimated Fair Value:
Securities without single maturity date	821,611	943,343
Amortized Cost:
Securities without single maturity date	802,680	928,342
Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Estimated Fair Value:
Securities without single maturity date	1,106,332	1,114,528
Amortized Cost:
Securities without single maturity date	1,085,746	1,103,756
Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Estimated Fair Value:
Securities without single maturity date	1,657,931	1,677,941
Amortized Cost:
Securities without single maturity date	$1,647,414	$1,675,605

Investment_Information_Other_i

Investment Information - Other investments (Details) (USD $)	Mar. 31, 2015		Dec. 31, 2014
In Thousands, unless otherwise specified
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	$1,887,501		$1,848,131
Available for sale
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	329,677		296,224
Available for sale | Investment grade fixed income
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	59,590		59,638
Available for sale | Asian and emerging markets
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	270,087		236,586
Fair value option
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	1,557,824		1,551,907
Fair value option | Term loan investments
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	1,044,762		1,073,649
Other investments par	1,055,264		1,094,337
Fair value option | Mezzanine debt funds
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	121,570		121,341
Fair value option | Credit related funds
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	138,576		114,436
Fair value option | Investment grade fixed income
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	63,162		69,108
Fair value option | Asian and emerging markets
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	26,846		25,800
Fair value option | Other
Schedule Of Other Investments [Line Items]
Other investments and fair value option investments	$162,908	[1]	$147,573	[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 $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	$2,441,986	$2,435,532
Fixed Maturities
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	757,467	632,024
Other investments
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	1,557,824	1,551,907
Short-term investments
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	125,788	251,601
Equity securities
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	$907	$0

Investment_Information_Net_inv

Investment Information - Net investment income (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Net investment income:
Gross investment income	$98,962		$76,163
Investment expenses	-19,968		-9,169
Net investment income	78,994		66,994
Fixed Maturities
Net investment income:
Gross investment income	68,596		62,449
Term loan investments
Net investment income:
Gross investment income	14,744		5,669
Equity securities (dividends)
Net investment income:
Gross investment income	2,679		2,921
Short-term investments
Net investment income:
Gross investment income	196		405
Other
Net investment income:
Gross investment income	$12,747	[1]	$4,719	[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
In Thousands, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Net realized gains and losses:
Available for sale securities, gross gains on investment sales	$97,591		$66,051
Available for sale securities, gross losses on investment sales	-55,160		-40,013
Derivative instruments	36,676	[1]	-12,768	[1]
Other	-4,588	[2]	-2,706	[2]
Net realized gains (losses)	83,348		19,697
Fixed Maturities
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option	-3,302		-1,643
Other investments
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option	6,287		10,776
Equity securities
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option	-2		0
Short-term investments
Net realized gains and losses:
Change in fair value of assets and liabilities accounted for using the fair value option	$5,846		$0
[1]	See Note 8 for information on the Companybs derivative instruments.
[2]	Includes accretion of contingent consideration liability amounts related to the 2014 acquisition of CMG Mortgage Insurance Company and its affiliated mortgage insurance companies.

Investment_Information_Other_t

Investment Information - Other than temporary impairments recognized in earnings (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	($5,799)	($2,971)
Fixed Maturities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	-3,048	-6
Fixed Maturities | Mortgage backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	-1,072	0
Fixed Maturities | Corporate bonds
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	-1,976	0
Fixed Maturities | Asset backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	0	-6
Short-term investments
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	-2,341	0
Equity securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	-129	0
Other investments
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses recognized in earnings	($281)	($2,965)

Investment_Information_Other_t1

Investment Information - Other than temporary impairments rollforward (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Rollforward:
Balance at start of period	$20,196	$60,062
Credit loss impairments recognized on securities not previously impaired	4,489	0
Credit loss impairments recognized on securities previously impaired	79	6
Reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security	0	0
Reductions for securities sold during the period	-420	-12,812
Balance at end of period	$24,344	$47,256

Investment_Information_Restric

Investment Information - Restricted assets (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Restricted Assets [Line Items]
Restricted assets	$5,536,925	$5,519,089
Collateral or guarantees - affiliated transactions
Restricted Assets [Line Items]
Restricted assets	4,054,848	4,138,527
Collateral or guarantees - third party agreements
Restricted Assets [Line Items]
Restricted assets	1,010,214	970,120
Deposits with US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	407,190	337,981
Trust funds
Restricted Assets [Line Items]
Restricted assets	$64,673	$72,461

Investment_Information_Narrati

Investment Information - Narrative (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014
	lots		lots
Narrative items:
Investable assets	$15,724,200,000				$15,344,386,000
OTTI unrealized losses at current fair value	-300,000		900,000
Continuous unrealized loss, qualitative disclosures:
Number of positions in an unrealized loss position (lots)	1,360		1,900
Total number of positions (lots)	5,130		4,790
Largest single loss	3,300,000		2,900,000
Securities lending, qualitative disclosures:
Fair value of fixed maturities and short-term investments pledged under securities lending agreements	253,100,000		50,800,000
Amortized cost of fixed maturities and short-term investments pledged under securities lending agreements	253,800,000		52,100,000
Fair value of cash collateral received on securities lending	49,900,000		44,300,000
Fair value of non-cash collateral received on securities lending	207,200,000		0
Approximate fair value of sub-prime securities backing the securities lending program	5,700,000		5,800,000
OTTI unrealized losses (gains)	4,887,000	[1]	3,646,000	[1]
Underwriting segments
Narrative items:
Investable assets	14,456,612,000				14,261,106,000
Other
Narrative items:
Investable assets	$1,267,588,000				$1,083,280,000
[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 March 31, 2015, the net unrealized loss related to securities for which a non-credit OTTI was recognized in AOCI was $0.3 million, compared to a net unrealized gain of $0.9 million at December 31, 2014.

Fair_Value_Fair_Value_Hierarch

Fair Value - Fair Value Hierarchy (Details) (Recurring, USD $)	Mar. 31, 2015		Dec. 31, 2014
In Thousands, unless otherwise specified
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	$3,165,308		$3,116,355
Liabilities measured at fair value	0		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		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	1,517,834	[1]	1,447,972	[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	1,517,834	[1]	1,447,972	[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	685,714		658,182
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	835,065		759,621
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	126,695		250,580
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		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	125,788		250,580
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	907
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	11,175,897		11,244,994
Liabilities measured at fair value	0		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		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	9,105,591	[1]	9,296,100	[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	3,119,437	[1]	3,108,513	[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	821,611	[1]	943,343	[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	1,626,580	[1]	1,494,122	[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	1,106,332	[1]	1,114,528	[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	831,200	[1]	1,015,153	[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	1,600,431	[1]	1,620,441	[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	1,999		0
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	201,943		192,687
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	19,967		37,605
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	1,846,397		1,718,602
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	636,762		497,101
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	18,010		22,190
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	77,903		88,411
Significant Other Observable Inputs (Level 2) | Fair value option | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	24,792		24,322
Significant Other Observable Inputs (Level 2) | Fair value option | Other investments fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,088,930		1,085,557
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		1,021
Significant Other Observable Inputs (Level 2) | Fair value option | Equity 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, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	654,128		627,387
Liabilities measured at fair value	-66,461		-61,845
Significant Unobservable Inputs (Level 3) | Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	-66,461		-61,845
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	57,500	[1]	57,500	[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]	0	[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	57,500	[1]	57,500	[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	127,734		103,537
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	468,894		466,350
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		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	468,894		466,350
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		0
Significant Unobservable Inputs (Level 3) | Fair value option | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	14,995,333		14,988,736
Liabilities measured at fair value	-66,461		-61,845
Estimated Fair Value | Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	-66,461		-61,845
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	10,680,925		10,801,572
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	3,119,437		3,108,513
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	821,611		943,343
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	1,626,580		1,494,122
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	1,106,332		1,114,528
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	1,517,834		1,447,972
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	831,200		1,015,153
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	1,657,931		1,677,941
Estimated Fair Value | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	687,713		658,182
Estimated Fair Value | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	329,677		296,224
Estimated Fair Value | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	855,032		797,226
Estimated Fair Value | Fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	2,441,986		2,435,532
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	636,762		497,101
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	18,010		22,190
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	77,903		88,411
Estimated Fair Value | Fair value option | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	24,792		24,322
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	1,557,824		1,551,907
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	125,788		251,601
Estimated Fair Value | Fair value option | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	$907
[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
In Thousands, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$627,387		$549,990
Total gains or (losses) (realized/unrealized) - included in earnings	-1,132	[1]	8,585	[1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	-522		-2,196
Purchases, issuances, sales and settlements
Purchases	47,925		40,110
Issuances	0		0
Sales	0		-68,209
Settlements	-19,530		-4,799
Transfers in and/or out of Level 3	0		0
Balance at end of period	654,128		523,481
Contingent consideration liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	-61,845		0
Total gains or (losses) (realized/unrealized) - included in earnings	-3,548	[1]	-1,394	[1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	0		0
Purchases, issuances, sales and settlements
Purchases	0		0
Issuances	-1,068		-41,762
Sales	0		0
Settlements	0		0
Transfers in and/or out of Level 3	0		0
Balance at end of period	-66,461		-43,156
Available for sale | Asset backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	57,500		0
Total gains or (losses) (realized/unrealized) - included in earnings	0	[1]	0	[1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	0		0
Purchases, issuances, sales and settlements
Purchases	0		0
Issuances	0		0
Sales	0		0
Settlements	0		0
Transfers in and/or out of Level 3	0		0
Balance at end of period	57,500		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		2,045
Total gains or (losses) (realized/unrealized) - included in earnings	0	[1]	0	[1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	0		0
Purchases, issuances, sales and settlements
Purchases	0		0
Issuances	0		0
Sales	0		-2,045
Settlements	0		0
Transfers in and/or out of Level 3	0		0
Balance at end of period	0		0
Available for sale | Other investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	103,537		170,420
Total gains or (losses) (realized/unrealized) - included in earnings	-281	[1]	2,275	[1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	-522		-2,196
Purchases, issuances, sales and settlements
Purchases	25,000		0
Issuances	0		0
Sales	0		-66,164
Settlements	0		0
Transfers in and/or out of Level 3	0		0
Balance at end of period	127,734		104,335
Fair value option | Other investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	466,350		377,525
Total gains or (losses) (realized/unrealized) - included in earnings	-851	[1]	6,310	[1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	0		0
Purchases, issuances, sales and settlements
Purchases	22,925		40,110
Issuances	0		0
Sales	0		0
Settlements	-19,530		-4,799
Transfers in and/or out of Level 3	0		0
Balance at end of period	$468,894		$419,146
[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_Narrative_Details

Fair Value - Narrative (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Total assets and liabilities measured at fair value	$15,060,000,000		$15,050,000,000
Total assets and liabilities measured at fair value priced using non-binding broker quotes	241,000,000		260,800,000
Total assets and liabilities measured at fair value priced using non-binding broker quotes (percentage)	1.60%		1.70%
Gains (losses) including in earnings attributable to the change in unrealized gains or losses related to assets still held at balance sheet date	-1,100,000	3,300,000
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	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
Estimated fair value of senior notes	410,500,000
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
Estimated fair value of senior notes	$559,100,000

Derivative_Instruments_Details

Derivative Instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014
Derivative offsetting [Abstract]
Derivative assets subject to master netting agreements	$160,500,000				$25,300,000
Derivative liabilities subject to master netting agreements	131,100,000				5,400,000
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	36,676,000	[1]	-12,768,000	[1]
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Asset derivatives - fair value	385,247,000				26,468,000
Liability derivatives - fair value	-259,376,000				-5,397,000
Net derivatives - fair value	125,871,000				21,071,000
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	36,676,000		-12,768,000
Not Designated as Hedging Instrument [Member] | Futures contracts
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	19,326,000		-9,862,000
Not Designated as Hedging Instrument [Member] | Futures contracts | Other investments available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	2,534,000				2,156,000
Liability derivatives - fair value	-3,148,000				-1,907,000
Net derivatives - fair value	-614,000				249,000
Net derivatives - notional value	2,398,085,000	[2]			2,549,027,000	[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	16,819,000		-2,257,000
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	23,327,000				10,511,000
Liability derivatives - fair value	-6,751,000				-1,145,000
Net derivatives - fair value	16,576,000				9,366,000
Net derivatives - notional value	1,848,503,000	[2]			397,106,000	[2]
Not Designated as Hedging Instrument [Member] | TBAs
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	486,000		-37,000
Not Designated as Hedging Instrument [Member] | TBAs | Fixed maturities available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	350,576,000				10,592,000
Liability derivatives - fair value	-245,150,000				0
Net derivatives - fair value	105,426,000				10,592,000
Net derivatives - notional value	562,031,000	[2]			10,056,000	[2]
Not Designated as Hedging Instrument [Member] | Other
Net realized gains (losses) on derivative instruments
Net realized gains (losses) on derivative instruments	45,000		-612,000
Not Designated as Hedging Instrument [Member] | Other | Other investments available for sale, at fair value
Derivative [Line Items]
Asset derivatives - fair value	8,810,000				3,209,000
Liability derivatives - fair value	-4,327,000				-2,345,000
Net derivatives - fair value	4,483,000				864,000
Net derivatives - notional value	$1,252,273,000	[2]			$735,684,000	[2]
[1]	See Note 8 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 $)	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Abstract]
Revolving credit agreement borrowings	$100,000,000	$100,000,000
Unfunded Investment Commitments [Abstract]
Investment commitments	1,220,000,000
LOC Facilities [Member]
Line of Credit Facility [Abstract]
Letters of credit outstanding, amount	415,900,000
Investments pledged as collateral	488,400,000
Other secured letter of credit facilities
Line of Credit Facility [Abstract]
Maximum borrowing capacity	180,000,000
Credit Agreement
Line of Credit Facility [Abstract]
Credit facility expiration date	30-Jun-19
Revolving credit agreement borrowings	100,000,000
Unsecured revolving loan and letter of credit facility
Line of Credit Facility [Abstract]
Maximum borrowing capacity	300,000,000
Secured letter of credit facility
Line of Credit Facility [Abstract]
Maximum borrowing capacity	500,000,000
Watford Holdings Ltd | Line of credit
Line of Credit Facility [Abstract]
Maximum borrowing capacity	200,000,000
Credit facility expiration date	19-May-15
Letters of credit outstanding, amount	$32,100,000

Share_Transactions_Share_repur

Share Transactions - Share repurchases (Details) (Common shares, USD $)	3 Months Ended
Share data in Millions, unless otherwise specified	Mar. 31, 2015
Common shares
Class of Stock [Line Items]
Cumulative number of shares acquired since inception of share repurchase program	120.9
Aggregate purchase price of shares acquired since inception of share repurchase program	$3,400,000,000
Treasury stock, shares acquired (shares)	2.7
Treasury stock, value of shares acquired	162,900,000
Remaining authorized repurchase amount	$724,200,000

Income_Taxes_Details

Income Taxes (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Effective tax rate on income before income taxes (percentage)	4.30%	2.00%
Net deferred tax assets	$124.70		$131
Income taxes paid	$3.60	$1.60

Other_Comprehensive_Income_Los2

Other Comprehensive Income (Loss) - Amounts reclassified from accumulated other comprehensive income (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other-than-temporary impairment losses	($5,799)	($2,971)
Income before income taxes	321,435	182,883
Income tax expense	-12,678	-3,738
Net of tax	283,336	182,500
Reclassification out of accumulated other comprehensive income | Unrealized appreciation on available-for-sale investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains	42,431	26,038
Other-than-temporary impairment losses	-7,247	-2,971
Income before income taxes	35,184	23,067
Income tax expense	-4,252	-1,818
Net of tax	$30,932	$21,249

Other_Comprehensive_Income_Los3

Other Comprehensive Income (Loss) - Components of other comprehensive income (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Before tax amount:
Unrealized holding gains arising during period, before tax	$94,387	$75,400
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), before tax	-1,448	0
Less reclassification of net realized gains included in net income, before tax	35,184	23,067
Foreign currency translation adjustments, before tax	-23,626	-1,349
Other comprehensive income (loss), before tax	34,129	50,984
Tax expense (benefit):
Unrealized holding gains arising during period, tax	10,083	4,047
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), tax	0	0
Less reclassification of net realized gains included in net income, tax	4,252	1,818
Foreign currency translation adjustments, tax	-869	0
Other comprehensive income (loss), tax	4,962	2,229
Net of tax amount:
Unrealized holding gains arising during period, net of tax	84,304	71,353
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), net of tax	-1,448	0
Less reclassification of net realized gains included in net income, net of tax	30,932	21,249
Foreign currency translation adjustments, net of tax	-22,757	-1,349
Net current period other comprehensive income (loss)	$29,167	$48,755

Guarantor_Financial_Informatio2

Guarantor Financial Information - Condensed consolidating balance sheet (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Assets
Total investments	$15,411,644	$15,331,249
Cash	471,012	485,702	1,569,605	434,057
Investments in subsidiaries	0	0
Due from subsidiaries and affiliates	0	0
Premiums receivable	1,116,389	948,695
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	1,788,619	1,812,845
Contractholder receivables	1,339,433	1,309,192
Prepaid reinsurance premiums	421,908	377,078
Deferred acquisition costs, net	442,775	414,525
Other assets	1,857,900	1,330,257
Total assets	22,849,680	22,009,543	21,800,358
Liabilities
Reserve for losses and loss adjustment expenses	8,928,950	9,036,448
Unearned premiums	2,400,834	2,231,578
Reinsurance balances payable	200,444	219,312
Contractholder payables	1,339,433	1,309,192
Deposit accounting liabilities	284,828	327,384
Senior notes	800,000	800,000
Revolving credit agreement borrowings	100,000	100,000
Due to subsidiaries and affiliates	0	0
Other liabilities	1,497,291	866,983
Total liabilities	15,551,780	14,890,897	14,895,495
Redeemable noncontrolling interests	219,604	219,512
Shareholders' Equity
Total shareholdersb equity available to Arch	6,288,702	6,130,053	5,892,133
Non-redeemable noncontrolling interests	789,594	769,081	793,496
Total shareholdersb equity	7,078,296	6,899,134	6,685,629
Total liabilities, noncontrolling interests and shareholdersb equity	22,849,680	22,009,543
Reportable Legal Entities | ACGL (Parent Guarantor)
Assets
Total investments	24	107
Cash	7,992	3,218	12,353	3,223
Investments in subsidiaries	6,688,930	6,536,644
Due from subsidiaries and affiliates	31	48
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	6,998	7,590
Total assets	6,703,975	6,547,607
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	1,305	417
Other liabilities	13,968	17,137
Total liabilities	415,273	417,554
Redeemable noncontrolling interests	0	0
Shareholders' Equity
Total shareholdersb equity available to Arch	6,288,702	6,130,053
Non-redeemable noncontrolling interests	0	0
Total shareholdersb equity	6,288,702	6,130,053
Total liabilities, noncontrolling interests and shareholdersb equity	6,703,975	6,547,607
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)
Assets
Total investments	53,890	62,867
Cash	2,822	2,787	45,472	509
Investments in subsidiaries	1,710,358	1,685,185
Due from subsidiaries and affiliates	15,144	7,517
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	50,869	49,576
Total assets	1,833,083	1,807,932
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	15,022	7,505
Other liabilities	47,944	49,403
Total liabilities	562,966	556,908
Redeemable noncontrolling interests	0	0
Shareholders' Equity
Total shareholdersb equity available to Arch	1,270,117	1,251,024
Non-redeemable noncontrolling interests	0	0
Total shareholdersb equity	1,270,117	1,251,024
Total liabilities, noncontrolling interests and shareholdersb equity	1,833,083	1,807,932
Reportable Legal Entities | Other ACGL Subsidiaries
Assets
Total investments	15,357,730	15,268,275
Cash	460,198	479,697	1,511,780	430,325
Investments in subsidiaries	0	0
Due from subsidiaries and affiliates	380,577	370,429
Premiums receivable	1,575,230	1,331,511
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	5,545,517	5,584,973
Contractholder receivables	1,339,433	1,309,192
Prepaid reinsurance premiums	1,545,190	1,373,008
Deferred acquisition costs, net	442,775	414,525
Other assets	2,138,959	1,689,670
Total assets	28,785,609	27,821,280
Liabilities
Reserve for losses and loss adjustment expenses	12,656,904	12,784,030
Unearned premiums	3,524,116	3,227,508
Reinsurance balances payable	645,723	589,289
Contractholder payables	1,339,433	1,309,192
Deposit accounting liabilities	508,592	587,050
Senior notes	0	0
Revolving credit agreement borrowings	0	0
Due to subsidiaries and affiliates	379,425	370,072
Other liabilities	1,593,048	994,741
Total liabilities	20,647,241	19,861,882
Redeemable noncontrolling interests	219,604	219,512
Shareholders' Equity
Total shareholdersb equity available to Arch	7,129,170	6,970,805
Non-redeemable noncontrolling interests	789,594	769,081
Total shareholdersb equity	7,918,764	7,739,886
Total liabilities, noncontrolling interests and shareholdersb equity	28,785,609	27,821,280
Consolidating Adjustments and Eliminations
Assets
Total investments	0	0
Cash	0	0	0	0
Investments in subsidiaries	-8,399,288	-8,221,829
Due from subsidiaries and affiliates	-395,752	-377,994
Premiums receivable	-458,841	-382,816
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	-3,756,898	-3,772,128
Contractholder receivables	0	0
Prepaid reinsurance premiums	-1,123,282	-995,930
Deferred acquisition costs, net	0	0
Other assets	-338,926	-416,579
Total assets	-14,472,987	-14,167,276
Liabilities
Reserve for losses and loss adjustment expenses	-3,727,954	-3,747,582
Unearned premiums	-1,123,282	-995,930
Reinsurance balances payable	-445,279	-369,977
Contractholder payables	0	0
Deposit accounting liabilities	-223,764	-259,666
Senior notes	0	0
Revolving credit agreement borrowings	0	0
Due to subsidiaries and affiliates	-395,752	-377,994
Other liabilities	-157,669	-194,298
Total liabilities	-6,073,700	-5,945,447
Redeemable noncontrolling interests	0	0
Shareholders' Equity
Total shareholdersb equity available to Arch	-8,399,287	-8,221,829
Non-redeemable noncontrolling interests	0	0
Total shareholdersb equity	-8,399,287	-8,221,829
Total liabilities, noncontrolling interests and shareholdersb equity	($14,472,987)	($14,167,276)

Guarantor_Financial_Informatio3

Guarantor Financial Information - Condensed consolidating statement of income and comprehensive income (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Revenues
Net premiums earned	$910,264	$859,780
Net investment income	78,994	66,994
Net realized gains (losses)	83,348	19,697
Net impairment losses recognized in earnings	-5,799	-2,971
Other underwriting income	11,536	1,582
Equity in net income of investment funds accounted for using the equity method	5,889	3,253
Other income (loss)	-1,888	-2,104
Total revenues	1,082,344	946,231
Expenses
Losses and loss adjustment expenses	493,716	436,240
Acquisition expenses	163,076	160,342
Other operating expenses	157,882	145,799
Interest expense	12,736	14,404
Net foreign exchange (gains) losses	-66,501	6,563
Total expenses	760,909	763,348
Income (loss) before income taxes	321,435	182,883
Income tax expense (benefit)	-12,678	-3,738
Income (loss) before equity in net income of subsidiaries	308,757	179,145
Equity in net income of subsidiaries	0	0
Net income	308,757	179,145
Amounts attributable to noncontrolling interests	-25,421	3,355
Net income available to Arch	283,336	182,500
Preferred dividends	-5,484	-5,484
Net income available to Arch common shareholders	277,852	177,016
Comprehensive income available to Arch	312,503	231,255
Reportable Legal Entities | ACGL (Parent Guarantor)
Revenues
Net premiums earned	0	0
Net investment income	0	0
Net realized gains (losses)	0	0
Net impairment losses recognized in earnings	0	0
Other underwriting income	0	0
Equity in net income of investment funds accounted for using the equity method	0	0
Other income (loss)	0	0
Total revenues	0	0
Expenses
Losses and loss adjustment expenses	0	0
Acquisition expenses	0	0
Other operating expenses	8,632	10,307
Interest expense	5,856	5,853
Net foreign exchange (gains) losses	0	0
Total expenses	14,488	16,160
Income (loss) before income taxes	-14,488	-16,160
Income tax expense (benefit)	0	0
Income (loss) before equity in net income of subsidiaries	-14,488	-16,160
Equity in net income of subsidiaries	297,824	198,660
Net income	283,336	182,500
Amounts attributable to noncontrolling interests	0	0
Net income available to Arch	283,336	182,500
Preferred dividends	-5,484	-5,484
Net income available to Arch common shareholders	277,852	177,016
Comprehensive income available to Arch	312,503	231,255
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)
Revenues
Net premiums earned	0	0
Net investment income	5	0
Net realized gains (losses)	0	0
Net impairment losses recognized in earnings	0	0
Other underwriting income	0	0
Equity in net income of investment funds accounted for using the equity method	0	0
Other income (loss)	0	0
Total revenues	5	0
Expenses
Losses and loss adjustment expenses	0	0
Acquisition expenses	0	0
Other operating expenses	1,259	977
Interest expense	6,366	6,514
Net foreign exchange (gains) losses	0	0
Total expenses	7,625	7,491
Income (loss) before income taxes	-7,620	-7,491
Income tax expense (benefit)	1,413	2,788
Income (loss) before equity in net income of subsidiaries	-6,207	-4,703
Equity in net income of subsidiaries	14,495	22,152
Net income	8,288	17,449
Amounts attributable to noncontrolling interests	0	0
Net income available to Arch	8,288	17,449
Preferred dividends	0	0
Net income available to Arch common shareholders	8,288	17,449
Comprehensive income available to Arch	11,750	17,220
Reportable Legal Entities | Other ACGL Subsidiaries
Revenues
Net premiums earned	910,264	859,780
Net investment income	86,010	77,001
Net realized gains (losses)	83,348	19,697
Net impairment losses recognized in earnings	-5,799	-2,971
Other underwriting income	11,536	1,582
Equity in net income of investment funds accounted for using the equity method	5,889	3,253
Other income (loss)	-1,888	-2,104
Total revenues	1,089,360	956,238
Expenses
Losses and loss adjustment expenses	493,716	436,240
Acquisition expenses	163,076	160,342
Other operating expenses	147,991	134,515
Interest expense	7,535	12,044
Net foreign exchange (gains) losses	-39,630	5,741
Total expenses	772,688	748,882
Income (loss) before income taxes	316,672	207,356
Income tax expense (benefit)	-14,091	-6,526
Income (loss) before equity in net income of subsidiaries	302,581	200,830
Equity in net income of subsidiaries	0	0
Net income	302,581	200,830
Amounts attributable to noncontrolling interests	-25,421	3,355
Net income available to Arch	277,160	204,185
Preferred dividends	0	0
Net income available to Arch common shareholders	277,160	204,185
Comprehensive income available to Arch	333,193	252,119
Consolidating Adjustments and Eliminations
Revenues
Net premiums earned	0	0
Net investment income	-7,021	-10,007
Net realized gains (losses)	0	0
Net impairment losses recognized in earnings	0	0
Other underwriting income	0	0
Equity in net income of investment funds accounted for using the equity method	0	0
Other income (loss)	0	0
Total revenues	-7,021	-10,007
Expenses
Losses and loss adjustment expenses	0	0
Acquisition expenses	0	0
Other operating expenses	0	0
Interest expense	-7,021	-10,007
Net foreign exchange (gains) losses	-26,871	822
Total expenses	-33,892	-9,185
Income (loss) before income taxes	26,871	-822
Income tax expense (benefit)	0	0
Income (loss) before equity in net income of subsidiaries	26,871	-822
Equity in net income of subsidiaries	-312,319	-220,812
Net income	-285,448	-221,634
Amounts attributable to noncontrolling interests	0	0
Net income available to Arch	-285,448	-221,634
Preferred dividends	0	0
Net income available to Arch common shareholders	-285,448	-221,634
Comprehensive income available to Arch	($344,943)	($269,339)

Guarantor_Financial_Informatio4

Guarantor Financial Information - Condensed consolidating statement of cash flows (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Operating Activities
Net Cash Provided by (Used in) Operating Activities	$85,461	$198,169
Investing Activities
Purchases of fixed maturity investments	-7,030,731	-7,131,071
Purchases of equity securities	-125,863	-89,227
Purchases of other investments	-685,497	-304,454
Proceeds from the sale of fixed maturity investments	6,857,459	7,014,281
Proceeds from the sale of equity securities	125,906	49,614
Proceeds from sales, redemptions and maturities of other investments	605,263	331,176
Proceeds from redemptions and maturities of fixed maturity investments	272,657	168,484
Net sales (purchases) of short-term investments	66,283	156,262
Change in cash collateral related to securities lending	-5,529	4,669
Contributions to subsidiaries	0	0
Intercompany loans issued	0	0
Purchase of business, net of cash acquired	-2,432	-235,578
Purchases of furniture, equipment and other assets	-32,897	-5,382
Net Cash Provided By (Used For) Investing Activities	44,619	-41,226
Financing Activities
Purchases of common shares under share repurchase program	-162,898	0
Proceeds from common shares issued, net	-412	3,021
Proceeds from intercompany borrowings	0
Repayments of intercompany borrowings		0
Change in cash collateral related to securities lending	5,529	-4,669
Third party investment in non-redeemable noncontrolling interests	0	796,903
Third party investment in redeemable noncontrolling interests	0	186,893
Dividends paid to redeemable noncontrolling interests	-4,816	0
Dividends paid to parent	0	0
Other	29,779	1,700
Preferred dividends paid	-5,484	-5,484
Net Cash Provided By (Used For) Financing Activities	-138,302	978,364
Effects of exchange rate changes on foreign currency cash	-6,468	241
Increase (decrease) in cash	-14,690	1,135,548
Cash beginning of year	485,702	434,057
Cash end of period	471,012	1,569,605
Reportable Legal Entities | ACGL (Parent Guarantor)
Operating Activities
Net Cash Provided by (Used in) Operating Activities	173,508	11,685
Investing Activities
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 sales, redemptions and maturities of other investments	0	0
Proceeds from redemptions and maturities of fixed maturity investments	0	0
Net sales (purchases) of short-term investments	83	-16
Change in cash collateral related to securities lending	0	0
Contributions to subsidiaries	0	0
Intercompany loans issued	0	0
Purchase of business, net of cash acquired	0	0
Purchases of furniture, equipment and other assets	-23	-76
Net Cash Provided By (Used For) Investing Activities	60	-92
Financing Activities
Purchases of common shares under share repurchase program	-162,898
Proceeds from common shares issued, net	-412	3,021
Proceeds from intercompany borrowings	0
Repayments of intercompany borrowings		0
Change in cash collateral related to securities lending	0	0
Third party investment in non-redeemable noncontrolling interests		0
Third party investment in redeemable noncontrolling interests		0
Dividends paid to redeemable noncontrolling interests	0
Dividends paid to parent	0	0
Other	0	0
Preferred dividends paid	-5,484	-5,484
Net Cash Provided By (Used For) Financing Activities	-168,794	-2,463
Effects of exchange rate changes on foreign currency cash	0	0
Increase (decrease) in cash	4,774	9,130
Cash beginning of year	3,218	3,223
Cash end of period	7,992	12,353
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)
Operating Activities
Net Cash Provided by (Used in) Operating Activities	-8,950	7,151
Investing Activities
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	9,001	0
Proceeds from the sale of equity securities	0	0
Proceeds from sales, redemptions and maturities of other investments	0	0
Proceeds from redemptions and maturities of fixed maturity investments	0	0
Net sales (purchases) of short-term investments	-16	360,269
Change in cash collateral related to securities lending	0	0
Contributions to subsidiaries	0	-312,207
Intercompany loans issued	-7,500	0
Purchase of business, net of cash acquired	0	0
Purchases of furniture, equipment and other assets	0	0
Net Cash Provided By (Used For) Investing Activities	1,485	48,062
Financing Activities
Purchases of common shares under share repurchase program	0
Proceeds from common shares issued, net	0	0
Proceeds from intercompany borrowings	7,500
Repayments of intercompany borrowings		-10,250
Change in cash collateral related to securities lending	0	0
Third party investment in non-redeemable noncontrolling interests		0
Third party investment in redeemable noncontrolling interests		0
Dividends paid to redeemable noncontrolling interests	0
Dividends paid to parent	0	0
Other	0	0
Preferred dividends paid	0	0
Net Cash Provided By (Used For) Financing Activities	7,500	-10,250
Effects of exchange rate changes on foreign currency cash	0	0
Increase (decrease) in cash	35	44,963
Cash beginning of year	2,787	509
Cash end of period	2,822	45,472
Reportable Legal Entities | Other ACGL Subsidiaries
Operating Activities
Net Cash Provided by (Used in) Operating Activities	106,903	194,833
Investing Activities
Purchases of fixed maturity investments	-7,030,731	-7,131,071
Purchases of equity securities	-125,863	-89,227
Purchases of other investments	-685,497	-304,454
Proceeds from the sale of fixed maturity investments	6,848,458	7,014,281
Proceeds from the sale of equity securities	125,906	49,614
Proceeds from sales, redemptions and maturities of other investments	605,263	331,176
Proceeds from redemptions and maturities of fixed maturity investments	272,657	168,484
Net sales (purchases) of short-term investments	66,216	-203,991
Change in cash collateral related to securities lending	-5,529	4,669
Contributions to subsidiaries	-5,500	-100,000
Intercompany loans issued	-7,500	10,250
Purchase of business, net of cash acquired	-2,432	-235,578
Purchases of furniture, equipment and other assets	-32,874	-5,306
Net Cash Provided By (Used For) Investing Activities	22,574	-491,153
Financing Activities
Purchases of common shares under share repurchase program	0
Proceeds from common shares issued, net	5,500	412,207
Proceeds from intercompany borrowings	7,500
Repayments of intercompany borrowings		0
Change in cash collateral related to securities lending	5,529	-4,669
Third party investment in non-redeemable noncontrolling interests		796,903
Third party investment in redeemable noncontrolling interests		186,893
Dividends paid to redeemable noncontrolling interests	-4,816
Dividends paid to parent	-186,000	-15,500
Other	29,779	1,700
Preferred dividends paid	0	0
Net Cash Provided By (Used For) Financing Activities	-142,508	1,377,534
Effects of exchange rate changes on foreign currency cash	-6,468	241
Increase (decrease) in cash	-19,499	1,081,455
Cash beginning of year	479,697	430,325
Cash end of period	460,198	1,511,780
Consolidating Adjustments and Eliminations
Operating Activities
Net Cash Provided by (Used in) Operating Activities	-186,000	-15,500
Investing Activities
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 sales, redemptions and maturities 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 cash collateral related to securities lending	0	0
Contributions to subsidiaries	5,500	412,207
Intercompany loans issued	15,000	-10,250
Purchase of business, net of cash acquired	0	0
Purchases of furniture, equipment and other assets	0	0
Net Cash Provided By (Used For) Investing Activities	20,500	401,957
Financing Activities
Purchases of common shares under share repurchase program	0
Proceeds from common shares issued, net	-5,500	-412,207
Proceeds from intercompany borrowings	-15,000
Repayments of intercompany borrowings		10,250
Change in cash collateral related to securities lending	0	0
Third party investment in non-redeemable noncontrolling interests		0
Third party investment in redeemable noncontrolling interests		0
Dividends paid to redeemable noncontrolling interests	0
Dividends paid to parent	186,000	15,500
Other	0	0
Preferred dividends paid	0	0
Net Cash Provided By (Used For) Financing Activities	165,500	-386,457
Effects of exchange rate changes on foreign currency cash	0	0
Increase (decrease) in cash	0	0
Cash beginning of year	0	0
Cash end of period	$0	$0

Transactions_with_Related_Part1

Transactions with Related Parties (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Related Party Transaction [Line Items]
Investment commitments	$1,220,000,000
Purchases of other investments	685,497,000	304,454,000
Common director | The Carlyle Group
Related Party Transaction [Line Items]
Investment commitments	545,900,000
Unfunded commitments	393,800,000
Purchases of other investments	23,400,000	19,100,000
Aggregate distributions received	$7,100,000	$35,100,000

Subsequent_Events_Details

Subsequent Events (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	31-May-08
Subsequent Event [Line Items]
Investment in joint venture, at cost	$100,000,000	$100,000,000
Gulf Re
Subsequent Event [Line Items]
Investment in joint venture, at cost			100,000,000
Joint venture, ownership percentage			50.00%
Joint venture, initial capital amount			$200,000,000