Allied World Assurance 10-Q 23 Oct 13 2013 Q3 Quarterly report Financial data

Document_And_Entity_Informatio

Document And Entity Information	9 Months Ended
	Sep. 30, 2013	Oct. 14, 2013
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	30-Sep-13
Document Fiscal Period Focus	Q3
Document Fiscal Year Focus	2013
Entity Registrant Name	Allied World Assurance Co Holdings, AG
Entity Central Index Key	1163348
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		33,761,646

Unaudited_Condensed_Consolidat

Unaudited Condensed Consolidated Balance Sheets (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
ASSETS:
Fixed maturity investments trading, at fair value (amortized cost: 2013: $5,996,065; 2012: $6,473,429)	$6,047,883	$6,626,454
Equity securities trading, at fair value (cost: 2013: $679,228; 2012: $480,312)	708,929	523,949
Other invested assets	905,025	783,534
Total investments	7,661,837	7,933,937
Cash and cash equivalents	953,047	681,879
Restricted cash	257,517	183,485
Insurance balances receivable	740,112	510,532
Funds held	375,131	336,368
Prepaid reinsurance	327,052	277,406
Reinsurance recoverable	1,226,034	1,141,110
Accrued investment income	25,471	29,135
Net deferred acquisition costs	145,951	108,010
Goodwill	268,376	268,376
Intangible assets	49,464	51,365
Balances receivable on sale of investments	237,031	418,879
Net deferred tax assets	41,832	25,580
Other assets	68,665	63,884
Total assets	12,377,520	12,029,946
LIABILITIES:
Reserve for losses and loss expenses	5,780,781	5,645,549
Unearned premiums	1,515,746	1,218,021
Reinsurance balances payable	193,643	136,264
Balances due on purchases of investments	497,974	759,934
Senior notes	798,426	798,215
Dividends payable	16,952	0
Accounts payable and accrued liabilities	130,070	145,628
Total liabilities	8,933,592	8,703,611
Commitments and contingencies
SHAREHOLDERS' EQUITY:
Common shares, 2013: par value CHF 12.30 per share and 2012: par value CHF 12.64 per share, (2013: 34,972,795; 2012: 36,369,868 shares issued and 2013: 33,814,920; 2012: 34,797,781 shares outstanding)	424,837	454,980
Treasury shares, at cost (2013: 1,157,875; 2012: 1,572,087)	-85,845	-113,818
Retained earnings	3,104,936	2,985,173
Total shareholders' equity	3,443,928	3,326,335
Total liabilities and shareholders' equity	$12,377,520	$12,029,946

Unaudited_Condensed_Consolidat1

Unaudited Condensed Consolidated Balance Sheets (Parenthetical)(USD ($))	Sep. 30, 2013	Dec. 31, 2012
In Thousands, except Share data, unless otherwise specified
Fixed maturity investments trading, amortized cost	$5,996,065	$6,473,429
Equity securities trading, cost	$679,228	$480,312
Common shares, issued	34,972,795	36,369,868
Common shares, outstanding	33,814,920	34,797,781
Treasury stock, shares	1,157,875	1,572,087

Unaudited_Condensed_Consolidat2

Unaudited Condensed Consolidated Statements Of Operations And Comprehensive Income (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
REVENUES:
Gross premiums written	$580,893	$504,420	$2,183,174	$1,832,219
Premiums ceded	-127,816	-112,883	-453,823	-357,019
Net premiums written	453,077	391,537	1,729,351	1,475,200
Change in unearned premiums	57,696	49,480	-248,079	-202,546
Net premiums earned	510,773	441,017	1,481,272	1,272,654
Net investment income	39,271	39,121	110,294	128,781
Net realized investment gains (losses)	27,487	149,813	-8,074	292,057
Total revenues	577,531	629,951	1,583,492	1,693,492
EXPENSES:
Net losses and loss expenses	276,970	258,948	807,276	724,530
Acquisition costs	65,114	51,086	186,416	149,812
General and administrative expenses	88,553	78,572	251,818	222,917
Amortization of intangible assets	633	633	1,900	1,900
Interest expense	14,094	13,822	42,416	41,579
Foreign exchange loss (gain)	4,353	1,023	7,361	-77
Total expenses	449,717	404,084	1,297,187	1,140,661
Income before income taxes	127,814	225,867	286,305	552,831
Income tax expense	4,971	6,220	6,332	18,677
NET INCOME	122,843	219,647	279,973	534,154
Other comprehensive loss:
Unrealized gains on investments arising during the period net of applicable deferred income tax benefit for three and nine months ended September 30, 2012: $15 and ($81), respectively	0	-29	0	150
Reclassification adjustment for net realized investment gains included in net income, net of applicable income tax	0	0	0	-13,249
Other comprehensive loss	0	-29	0	-13,099
COMPREHENSIVE INCOME	$122,843	$219,618	$279,973	$521,055
PER SHARE DATA
Basic earnings per share	$3.61	$6.16	$8.15	$14.68
Diluted earnings per share	$3.54	$6	$7.97	$14.28
Weighted average common shares outstanding	33,991,359	35,652,768	34,340,227	36,379,514
Weighted average common shares and common share equivalents outstanding	34,728,193	36,616,734	35,131,092	37,393,093
Dividends paid per share	$0.50	$0.75	$0.88	$1.50

Unaudited_Condensed_Consolidat3

Unaudited Condensed Consolidated Statements Of Operations And Comprehensive Income (Parenthetical) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Deferred income tax benefit (expense) for unrealized (losses) gains on investments arising during the period	$0	($15)	$0	$81

Unaudited_Condensed_Consolidat4

Unaudited Condensed Consolidated Statements Of Shareholders' Equity (USD $)	Total	Share Capital [Member]	Additional Paid-In Capital [Member]		Treasury Shares [Member]		Accumulated Other Comprehensive Income [Member]	Retained Earnings [Member]
In Thousands
Balance, at beginning of period at Dec. 31, 2011	$3,149,022	$557,153	$78,225		($136,590)		$14,484	$2,635,750
Net income	534,154							534,154
Dividends - par value reduction	-40,419	-40,419
Other comprehensive loss	-13,099						-13,099
Stock compensation	13,176		-23,050	[1]	36,226	[1]
Share repurchases	-207,048				-207,048
Shares cancelled	0	-39,488	-55,175		186,468			-91,805
Balance, at end of period at Sep. 30, 2012	3,435,786	477,246	0		-120,944		1,385	3,078,099
Balance, at beginning of period at Dec. 31, 2012	3,326,335	454,980	0		-113,818		0	2,985,173
Net income	279,973							279,973
Dividends - par value reduction	-12,981	-12,981
Dividends	-34,069							-34,069
Other comprehensive loss	0
Stock compensation	7,815		0	[1]	26,093	[1]		-18,278	[1]
Share repurchases	-123,145				-123,145
Shares cancelled	0	-17,162	0		125,025			-107,863
Balance, at end of period at Sep. 30, 2013	$3,443,928	$424,837	$0		($85,845)		$0	$3,104,936
[1]	Includes stock compensation expense for the period and shares issued out of treasury for awards exercised or vested.

Unaudited_Condensed_Consolidat5

Unaudited Condensed Consolidated Statements Of Cash Flows (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
CASH FLOWS PROVIDED BY OPERATING ACTIVITIES:
Net income	$279,973	$534,154
Adjustments to reconcile net income to cash provided by operating activities:
Net realized gains on sales of investments	-76,104	-63,625
Mark to market adjustments	80,136	-225,425
Stock compensation expense	9,282	13,118
Undistributed income of equity method investments	-4,313	0
Changes in:
Reserve for losses and loss expenses, net of reinsurance recoverables	50,308	151,041
Unearned premiums, net of prepaid reinsurance	248,079	202,545
Insurance balances receivable	-229,580	-121,422
Funds held	-38,763	18,602
Reinsurance balances payable	57,379	-4,107
Net deferred acquisition costs	-37,941	-27,193
Net deferred tax assets	-16,252	1,850
Accounts payable and accrued liabilities	-18,540	-2,996
Other items, net	34,508	23,677
Net cash provided by operating activities	338,172	500,219
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of trading securities	-4,955,817	-6,328,719
Purchases of other invested assets	-211,501	-52,578
Sales of available for sale securities	0	215,318
Sales of trading securities	5,137,280	5,778,138
Sales of other invested assets	189,155	110,429
Purchases of fixed assets	-4,171	-2,041
Change in restricted cash	-74,032	35,685
Net cash provided by (used in) investing activities	80,914	-243,768
CASH FLOWS USED IN FINANCING ACTIVITIES:
Dividends paid - partial par value reduction	-12,981	-54,721
Dividends paid	-17,117	0
Proceeds from the exercise of stock options	8,465	9,104
Share repurchases	-120,163	-204,746
Net cash used in financing activities	-141,796	-250,363
Effect of exchange rate changes on foreign currency cash	-6,122	-567
NET INCREASE IN CASH AND CASH EQUIVALENTS	271,168	5,521
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	681,879	633,996
CASH AND CASH EQUIVALENTS, END OF PERIOD	953,047	639,517
Supplemental disclosure of cash flow information:
- Cash paid for income taxes	17,249	18,912
- Cash paid for interest expense	$45,750	$45,750

General

General	9 Months Ended
	Sep. 30, 2013
General [Abstract]
General	1. GENERAL
	Allied World Assurance Company Holdings, AG, a Swiss holding company (“Allied World Switzerland”), through its wholly-owned subsidiaries (collectively, the “Company”), provides property and casualty insurance and reinsurance on a worldwide basis through operations in Bermuda, the United States, Europe, Hong Kong and Singapore. References to $ are to the lawful currency of the United States and to CHF are to the lawful currency of Switzerland.

Basis_of_Preparation_and_Conso

Basis of Preparation and Consolidation	9 Months Ended
	Sep. 30, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Preparation and Consolidation	2. BASIS OF PREPARATION AND CONSOLIDATION
	These unaudited condensed consolidated financial statements include the accounts of the Company and have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) for interim financial information and with Article 10 of Regulation S-X as promulgated by the U.S. Securities and Exchange Commission (“SEC”). Accordingly, they do not include all of the information and footnotes required by U.S. GAAP for complete financial statements. In the opinion of management, these unaudited condensed consolidated financial statements reflect all adjustments that are normal and recurring in nature and necessary for a fair presentation of financial position and results of operations as of the end of and for the periods presented. The results of operations for any interim period are not necessarily indicative of the results for a full year.
	The preparation of financial statements in conformity with U.S. 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. The significant estimates reflected in the Company's financial statements include, but are not limited to:
	•       The premium estimates for certain reinsurance agreements,
	•       Recoverability of deferred acquisition costs,
	•       The reserve for outstanding losses and loss expenses,
	•       Valuation of ceded reinsurance recoverables,
	•       Determination of impairment of goodwill and other intangible assets, and
	•       Valuation of financial instruments.
	Intercompany accounts and transactions have been eliminated on consolidation and all entities meeting consolidation requirements have been included in the unaudited condensed consolidated financial statements. To facilitate comparison of information across periods, certain reclassifications have been made to prior year amounts to conform to the current year's presentation.
	These unaudited condensed consolidated financial statements, including these notes, should be read in conjunction with the Company's audited consolidated financial statements, and related notes thereto, included in the Company's Annual Report on Form 10-K for the year ended December 31, 2012.

New_Accounting_Pronouncements

New Accounting Pronouncements	9 Months Ended
	Sep. 30, 2013
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
New Accounting Pronouncements	3. NEW ACCOUNTING PRONOUNCEMENTS
	In December 2011 (with a clarification amendment issued in January 2013), the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update 2011-11, “Disclosures about Offsetting Assets and Liabilities” (“ASU 2011-11”). The objective of ASU 2011-11 was to enhance disclosures about derivatives, repurchase agreements and reverse repurchase agreements, securities borrowing and securities lending transactions to the extent they are subject to master netting arrangements or similar agreements. The Company adopted ASU 2011-11 on January 1, 2013. The adoption of ASU 2011-11 did not have an impact on the presentation of the financial statements.
	In July 2013, the FASB issued Accounting Standards Update 2013-10, “Inclusion of the Fed Funds Effective Swap Rate (or Overnight Index Swap Rate) as a Benchmark Interest Rate for Hedge Accounting Purposes” (“ASU 2013-10”). Prior to the issuance of ASU 2013-10, only interest rates on U.S. Treasury securities and the London Interbank Offered Rate (“LIBOR”) swap rate were considered benchmark interest rates in the application of hedge accounting under U.S. GAAP. ASU 2013-10 permits the Fed Funds Effective Swap Rate to be used as a U.S. benchmark interest rate for hedge accounting purposes in addition to U.S. Treasury securities and LIBOR. ASU 2013-10 was effective prospectively for qualifying new or redesignated hedging relationships entered into on or after July 17, 2013. ASU 2013-10 did not have any impact on the financial statements upon adoption, as the Company does not apply hedge accounting.

Investments

Investments	9 Months Ended
	Sep. 30, 2013
Investments [Abstract]
Investments	4. INVESTMENTS
	a) Trading Securities
	Securities accounted for at fair value with changes in fair value recognized in the unaudited condensed consolidated statements of operations and comprehensive income (“consolidated income statements”) by category are as follows:
			30-Sep-13						31-Dec-12
			Fair Value			Amortized Cost			Fair Value			Amortized Cost
	U.S. Government and Government agencies		$	1,850,335		$	1,851,002		$	1,865,913		$	1,854,198
	Non-U.S. Government and Government agencies			209,625			214,114			261,627			253,657
	States, municipalities and political subdivisions			95,381			95,661			40,444			39,342
	Corporate debt:
		Financial institutions		841,930			826,763			866,140			835,587
		Industrials		1,146,793			1,140,636			1,153,909			1,139,706
		Utilities		74,764			74,865			69,153			67,463
	Mortgage-backed			1,373,161			1,341,303			1,958,373			1,877,854
	Asset-backed			455,894			451,721			410,895			405,622
	Total fixed maturity investments		$	6,047,883		$	5,996,065		$	6,626,454		$	6,473,429
			30-Sep-13						31-Dec-12
			Fair Value			Original Cost			Fair Value			Original Cost
	Equity securities		$	708,929		$	679,228		$	523,949		$	480,312
	Other invested assets			767,071			680,986			655,888			606,521
			$	1,476,000		$	1,360,214		$	1,179,837		$	1,086,833
	Other invested assets, included in the table above, include investments in private equity funds, hedge funds and a high yield loan fund that are accounted for at fair value, but excludes other private securities described below in Note 4(b) that are accounted for using the equity method of accounting.
	b) Other Invested Assets
	Details regarding the carrying value, redemption characteristics and unfunded investment commitments of the other invested assets portfolio as of September 30, 2013 were as follows:
				Carrying			Investments			Estimated		Investments
				Value as of			with			Remaining		without			Redemption
				September 30,			Redemption			Restriction		Redemption		Redemption	Notice	Unfunded
	Fund Type			2013			Restrictions			Period		Restrictions(1)		Frequency(1)	Period(1)	Commitments
	Private equity			$	131,449		$	131,449		3 - 10 Years		$	0			$	252,571
	Mezzanine debt				58,116			58,116		8 - 10 Years			0				203,969
	Distressed				8,991			8,991		4 -5 Years			0				5,102
	Total private equity structures				198,556			198,556					0				461,642
	Distressed				150,775			134,127		1 - 2 Years			16,648	Quarterly	45 - 65 Days		0
	Equity long/short				116,019			0					116,019	Quarterly	30 - 60 Days		0
	Multi-strategy				131,224			0					131,224	Quarterly	45 - 90 Days		0
	Global macro				19,641			0					19,641	Monthly	3 Days		0
	Event driven				18,445			0					18,445	Annual	60 Days		0
	Relative value credit				101,311			0					101,311	Quarterly	60 Days		0
	Total hedge funds				537,415			134,127					403,288				0
	Other private securities				137,954			0					137,954				5,000
	High yield loan fund				31,100			0					31,100	Monthly	30 Days		0
	Total other invested assets			$	905,025		$	332,683				$	572,342			$	466,642
	____________
	(1)       The redemption frequency and notice periods only apply to the investments without redemption restrictions. Some or all of these investments may be subject to a gate.
	In general, the Company has invested in hedge funds that require at least 30 days' notice of redemption and may be redeemed on a monthly, quarterly, semi-annual, annual or longer basis, depending on the fund. Certain hedge funds have lock-up periods ranging from one to three years from initial investment. A lock-up period refers to the initial amount of time an investor is contractually required to invest before having the ability to redeem. Funds that provide for periodic redemptions may, depending on the funds' governing documents, have the ability to deny or delay a redemption request, called a “gate.” The fund may implement this restriction because the aggregate amount of redemption requests as of a particular date exceeds a specified level, generally ranging from 15% to 25% of the fund's net assets. The gate is a method for executing an orderly redemption process to reduce the possibility of adversely affecting investors in the fund. Typically, the imposition of a gate delays a portion of the requested redemption, with the remaining portion settled in cash sometime after the redemption date. Certain funds may impose a redemption fee on early redemptions. Interests in private equity funds cannot be redeemed because the investments include restrictions that do not allow for redemption until termination of the fund.
	The following describes each investment type:
	Private equity funds: Primary funds may invest in companies and general partnership interests. Secondary funds buy limited partnership interests from existing limited partners of primary private equity funds. As owners of private equity funds seek liquidity, they can sell their existing investments, plus any remaining commitment, to secondary market participants. These funds cannot be redeemed because the investments include restrictions that do not allow for redemption until termination of the fund.
	Mezzanine debt funds: Mezzanine debt funds primarily focus on providing capital to upper middle market and middle market companies and private equity sponsors, in connection with leveraged buyouts, mergers and acquisitions, recapitalizations, growth financings and other corporate transactions. The most common position in the capital structure will be between the senior secured debt holder and the equity; however, the funds will utilize a flexible approach when structuring investments, which may include secured debt, subordinated debt, preferred stock and/or private equity. These funds cannot be redeemed because the investments include restrictions that do not allow for redemption until termination of the fund.
	Distressed funds: In distressed debt investing, managers take positions in the debt of companies experiencing significant financial difficulties, including bankruptcy, or in certain positions of the capital structure of structured securities. The manager relies on the fundamental analysis of these securities, including the claims on the assets and the likely return to bondholders. Certain funds cannot be redeemed because the investments include restrictions that do not allow for redemption until termination of the fund.
	Equity long/short funds: In equity long/short funds, managers take long positions in companies they deem to be undervalued and short positions in companies they deem to be overvalued. Long/short managers may invest in countries, regions or sectors and vary by their use of leverage and by their targeted net long position.
	Multi-strategy funds: These funds may utilize many strategies employed by specialized funds including distressed investing, equity long/short, merger arbitrage, convertible arbitrage, fixed income arbitrage and macro trading.
	Global macro funds: These funds focus on a top-down analysis of global markets as influenced by major political and economic trends or events. Global macro managers develop investment strategies that aim to forecast movements in interest rates, fund flows, political changes and other wide-ranging systematic factors. The portfolios of these funds can include long or short positions in equities, fixed-income securities, currencies and commodities in the form of cash or derivatives instruments.
	Event driven funds: Event driven strategies seek to deploy capital into specific securities whose returns are affected by a specific event that affects the value of one or more securities of a company. Returns for such securities are linked primarily to the specific outcome of the events and not by the overall direction of the bond or stock markets. Examples could include mergers and acquisitions (arbitrage), corporate restructurings and spin-offs, and capital structure arbitrage.
	Relative value credit funds: These funds seek to take exposure to credit-sensitive securities, long and/or short, based upon credit analysis of issuers and securities and credit market views.
	Other private securities: These securities include strategic non-controlling minority investments in private asset management companies and other insurance related investments that are accounted for using the equity method of accounting.
	High yield loan fund: A long-only private mutual fund that invests in high yield fixed income securities.
	c) Net Investment Income
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
		2013			2012			2013			2012
	Fixed maturity investments	$	31,179		$	36,778		$	96,366		$	120,883
	Equity securities		6,110			5,211			13,718			13,706
	Other invested assets		5,809			164			11,116			4,387
	Cash and cash equivalents		302			640			1,319			1,797
	Expenses		-4,129			-3,672			-12,225			-11,992
Net investment income	$	39,271		$	39,121		$	110,294		$	128,781
Net investment income from other invested assets included the distributed and undistributed net income from investments accounted for using the equity method of accounting for the three and nine months ended September 30, 2013.
d) Components of Realized Gains and Losses
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
		2013			2012			2013			2012
Gross realized gains on sale of invested assets		$	51,915		$	27,210		$	154,387		$	119,154
Gross realized losses on sale of invested assets			-40,770			-6,686			-82,812			-43,355
Net realized and unrealized (losses) gains on derivatives			-4,169			-962			3,392			-192
Mark-to-market gains (losses):
	Fixed maturity investments, trading		30,383			99,821			-101,205			144,024
	Equity securities, trading		-17,198			18,913			-18,555			38,516
	Other invested assets, trading		7,326			11,517			36,719			33,910
Net realized investment gains (losses)		$	27,487		$	149,813		$	-8,074		$	292,057
Proceeds from sale of available for sale securities		$	0		$	1,000		$	0		$	214,716
e) Pledged Assets
As of September 30, 2013 and December 31, 2012, $ 2,556,660 and $2,141,249, respectively, of cash and cash equivalents and investments were deposited, pledged or held in trust accounts in favor of ceding companies and other counterparties or government authorities to comply with reinsurance contract provisions, insurance laws and other contract provisions.
In addition, as of September 30, 2013 and December 31, 2012, a further $ 1,063,086 and $1,225,155, respectively, of cash and cash equivalents and investments were pledged as collateral for the Company's letter of credit facilities. See Note 8(d) to the Consolidated Financial Statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2012 for details on the Company's credit facilities.

Derivative_Instruments

Derivative Instruments	9 Months Ended
	Sep. 30, 2013
Derivative Instruments [Abstract]
Derivative Instruments	5. DERIVATIVE INSTRUMENTS
	As of September 30, 2013 and December 31, 2012, none of the Company's derivatives were designated as hedges for accounting purposes. The following table summarizes information on the location and amounts of derivative fair values on the unaudited condensed consolidated balance sheets (“consolidated balance sheets”):
			30-Sep-13											31-Dec-12
			Asset						Liability					Asset				Liability
			Derivative			Asset			Derivative			Liability		Derivative		Asset		Derivative		Liability
			Notional			Derivative			Notional			Derivative		Notional		Derivative		Notional		Derivative
			Amount			Fair Value			Amount			Fair Value		Amount		Fair Value		Amount		Fair Value
	Put options		$	0		$	0		$	0		$	0	$	5,152	$	532	$	0	$	0
	Foreign exchange contracts			197,811			2,493			279,050			7,062		127,712		1,713		194,566		2,656
	Interest rate swaps			33,000			5			327,000			172		0		0		0		0
	Total derivatives		$	230,811		$	2,498		$	606,050		$	7,234	$	132,864	$	2,245	$	194,566	$	2,656
	Derivative assets and derivative liabilities relating to the put options are classified within “equity securities trading, at fair value” on the consolidated balance sheets. All other asset and liability derivatives are classified within “other assets” or “accounts payable and accrued liabilities” on the consolidated balance sheets.
	The following table provides the net realized and unrealized gains (losses) on derivatives not designated as hedges recorded on the consolidated income statements:
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
		2013			2012			2013			2012
	Foreign exchange contracts	$	-2,336		$	-676		$	-1,091		$	-96
	Total included in foreign exchange (loss) gain		-2,336			-676			-1,091			-96
	Put options		0			0			-3,822			-336
	Foreign exchange contracts		-4,164			-2,751			1,925			-641
	Interest rate futures and swaps		-5			1,789			5,289			785
	Total included in net realized investment gains		-4,169			-962			3,392			-192
	Total realized and unrealized (losses) gains on derivatives	$	-6,505		$	-1,638		$	2,301		$	-288
	Derivative Instruments Not Designated as Hedging Instruments
	The Company is exposed to foreign currency risk in its investment portfolio. Accordingly, the fair values of the Company's investment portfolio are partially influenced by the change in foreign exchange rates. These foreign currency hedging activities have not been designated as specific hedges for financial reporting purposes.
	The Company's insurance and reinsurance subsidiaries and branches operate in various foreign countries and consequently the Company's underwriting portfolio is exposed to foreign currency risk. The Company manages foreign currency risk by seeking to match liabilities under the insurance policies and reinsurance contracts that it writes and that are payable in foreign currencies with cash and investments that are denominated in such currencies. When necessary, the Company may also use derivatives to economically hedge un-matched foreign currency exposures, specifically forward contracts and currency options.
	The Company also purchases and sells interest rate future and interest rate swap contracts to actively manage the duration and yield curve positioning of its fixed income portfolio. Interest rate futures and interest rate swaps can efficiently increase or decrease the overall duration of the portfolio. Additionally, interest rate future and interest rate swap contracts can be utilized to obtain the desired position along the yield curve in order to protect against certain future yield curve shapes.
	The Company also purchases options to actively manage the Company's equity portfolio.

Fair_Value_Of_Financial_Instru

Fair Value Of Financial Instruments	9 Months Ended
	Sep. 30, 2013
Fair Value Of Financial Instruments [Abstract]
Fair Value Of Financial Instruments	6. FAIR VALUE OF FINANCIAL INSTRUMENTS
	In accordance with U.S. GAAP, fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. There is a three-level valuation hierarchy for disclosure of fair value measurements. The valuation hierarchy is based upon whether the inputs to the valuation of an asset or liability are observable or unobservable in the market at the measurement date, with quoted market prices being the highest level (Level 1) and unobservable inputs being the lowest level (Level 3). A fair value measurement will fall within the level of the hierarchy based on the input that is significant to determining such measurement. The three levels are defined as follows:
	•       Level 1: Observable inputs to the valuation methodology that are quoted prices (unadjusted) for identical assets or liabilities in active markets.
	•       Level 2: Observable inputs to the valuation methodology other than quoted market prices (unadjusted) for identical assets or liabilities in active markets. Level 2 inputs include quoted prices for similar assets and liabilities in active markets, quoted prices for identical assets in markets that are not active and inputs other than quoted prices that are observable for the asset or liability, either directly or indirectly, for substantially the full term of the asset or liability.
	•       Level 3: Inputs to the valuation methodology that are unobservable for the asset or liability.
	The following table shows the fair value of the Company's financial instruments and where in the fair value hierarchy the fair value measurements are included as of the dates indicated below:
				Carrying		Total
	30-Sep-13			Amount		Fair Value		Level 1		Level 2		Level 3
	Fixed maturity investments:
		U.S. Government and Government agencies		$	1,850,335	$	1,850,335	$	1,522,185	$	328,150	$	0
		Non-U.S. Government and Government agencies			209,625		209,625		0		209,625		0
		States, municipalities and political subdivisions			95,381		95,381		0		95,381		0
		Corporate debt			2,063,487		2,063,487		0		2,063,487		0
		Mortgage-backed			1,373,161		1,373,161		0		1,183,907		189,254
		Asset-backed			455,894		455,894		0		375,861		80,033
	Total fixed maturity investments				6,047,883		6,047,883		1,522,185		4,256,411		269,287
	Equity securities				708,929		708,929		641,458		0		67,471
	Other invested assets				767,071		767,071		0		0		767,071
	Total investments			$	7,523,883	$	7,523,883	$	2,163,643	$	4,256,411	$	1,103,829
	Derivative assets:
		Foreign exchange contracts		$	2,493	$	2,493	$	0	$	2,493	$	0
		Interest rate futures			5		5		0		5		0
	Derivative liabilities:
		Foreign exchange contracts		$	7,062	$	7,062	$	0	$	7,062	$	0
		Interest rate swaps			172		172		0		172		0
	Senior notes			$	798,426	$	904,010	$	0	$	904,010	$	0
				Carrying		Total
	31-Dec-12			Amount		Fair Value		Level 1		Level 2		Level 3
	Fixed maturity investments:
		U.S. Government and Government agencies		$	1,865,913	$	1,865,913	$	1,529,158	$	336,755	$	0
		Non-U.S. Government and Government agencies			261,627		261,627		0		261,627		0
		States, municipalities and political subdivisions			40,444		40,444		0		40,444		0
		Corporate debt			2,089,202		2,089,202		0		2,089,202		0
		Mortgage-backed			1,958,373		1,958,373		0		1,790,548		167,825
		Asset-backed			410,895		410,895		0		348,649		62,246
	Total fixed maturity investments				6,626,454		6,626,454		1,529,158		4,867,225		230,071
	Equity securities				523,949		523,949		469,269		0		54,680
	Other invested assets				655,888		655,888		0		0		655,888
	Total investments			$	7,806,291	$	7,806,291	$	1,998,427	$	4,867,225	$	940,639
	Derivative assets:
		Foreign exchange contracts		$	1,713	$	1,713	$	0	$	1,713	$	0
	Derivative liabilities:
		Foreign exchange contracts		$	2,656	$	2,656	$	0	$	2,656	$	0
	Senior notes			$	798,215	$	918,627	$	0	$	918,627	$	0
	“Other invested assets” excluded other private securities that the Company did not measure at fair value, but are accounted for using the equity method of accounting. Derivative assets and derivative liabilities relating to foreign exchange contracts and interest rate swaps are classified within “other assets” or “accounts payable and accrued liabilities” on the consolidated balance sheets.
	The following describes the valuation techniques used by the Company to determine the fair value of financial instruments held as of the balance sheet date.
	U.S. Government and Government agencies: Comprised primarily of bonds issued by the U.S. Treasury, the Federal Home Loan Bank, the Federal Home Loan Mortgage Corporation and the Federal National Mortgage Association. The fair values of the Company's U.S. government securities are based on quoted market prices in active markets and are included in the Level 1 fair value hierarchy. The Company believes the market for U.S. Treasury securities is an actively traded market given the high level of daily trading volume. The fair values of U.S. government agency securities are priced using the spread above the risk-free yield curve. As the yields for the risk-free yield curve and the spreads for these securities are observable market inputs, the fair values of U.S. government agency securities are included in the Level 2 fair value hierarchy.
	Non-U.S. Government and Government agencies: Comprised of fixed income obligations of non-U.S. governmental entities. The fair values of these securities are based on prices obtained from international indices and are included in the Level 2 fair value hierarchy.
	States, municipalities and political subdivisions: Comprised of fixed income obligations of U.S. domiciled state and municipality entities. The fair values of these securities are based on prices obtained from the new issue market, and are included in the Level 2 fair value hierarchy.
	Corporate debt: Comprised of bonds issued by or loan obligations of corporations that are diversified across a wide range of issuers and industries. The fair values of corporate debt that are short-term are priced using spread above the LIBOR yield curve, and the fair values of corporate debt that are long-term are priced using the spread above the risk-free yield curve. The spreads are sourced from broker-dealers, trade prices and the new issue market. As the significant inputs used to price corporate bonds are observable market inputs, the fair values of corporate debt are included in the Level 2 fair value hierarchy.
	Mortgage-backed: Primarily comprised of residential and commercial mortgages originated by both U.S. government agencies (such as the Federal National Mortgage Association) and non-U.S. government agencies. The fair values of mortgage-backed securities originated by U.S. government agencies and non-U.S. government agencies are based on a pricing model that incorporates prepayment speeds and spreads to determine appropriate average life of mortgage-backed securities. The spreads are sourced from broker-dealers, trade prices and the new issue market. As the significant inputs used to price the mortgage-backed securities are observable market inputs, the fair values of these securities are included in the Level 2 fair value hierarchy, unless the significant inputs used to price the mortgage-backed securities are broker-dealer quotes and the Company is not able to determine if those quotes are based on observable market inputs, in which case the fair value is included in the Level 3 hierarchy.
	Asset-backed: Principally comprised of bonds backed by pools of automobile loan receivables, home equity loans, credit card receivables and collateralized loan obligations originated by a variety of financial institutions. The fair values of asset-backed securities are priced using prepayment speed and spread inputs that are sourced from the new issue market or broker-dealer quotes. As the significant inputs used to price the asset-backed securities are observable market inputs, the fair values of these securities are included in the Level 2 fair value hierarchy, unless the significant inputs used to price the asset-backed securities are broker-dealer quotes and the Company is not able to determine if those quotes are based on observable market inputs, in which case the fair value is included in the Level 3 hierarchy.
	Equity securities: Comprised of common and preferred stocks and mutual funds. Equities are generally included in the Level 1 fair value hierarchy as prices are obtained from market exchanges in active markets. Non-U.S. mutual funds where the net asset value is not provided on a daily basis are included in the Level 3 fair value hierarchy.
	Other invested assets: Comprised of funds invested in a range of diversified strategies. In accordance with U.S. GAAP, the fair values of the funds are based on the net asset value of the funds as reported by the fund manager that the Company believes is an unobservable input, and as such, the fair values of those funds are included in the Level 3 fair value hierarchy.
	Derivative instruments: The fair value of foreign exchange contracts, interest rate futures and interest rate swaps are priced from quoted market prices for similar exchange-traded derivatives and pricing valuation models that utilize independent market data inputs.  The fair value of derivatives are included in the Level 2 fair value hierarchy.
	Senior notes: The fair value of the senior notes is based on reported trades. The fair value of the senior notes is included in the Level 2 fair value hierarchy.
	The Company measures the fair value of certain assets on a non-recurring basis, generally quarterly, annually or when events or changes in circumstances indicate that the carrying amount of the assets may not be recoverable. These assets include investments accounted for using the equity method, goodwill and intangible assets. The Company uses a variety of techniques to measure the fair value of these assets when appropriate, as described below:
	Investments accounted for using the equity method: When the Company determines that the carrying value of these assets may not be recoverable, the Company records the assets at fair value with the loss recognized in income. In such cases, the Company measures the fair value of these assets using discounted cash flow models.
	Goodwill and intangible assets: The Company tests goodwill and intangible assets for impairment whenever events or changes in circumstances indicate that the carrying amount may not be recoverable, but at least annually for goodwill and indefinite-lived intangibles. If the Company determines that goodwill and intangible assets may be impaired, the Company uses techniques, including discounted expected future cash flows and market multiple models, to measure fair value.
	The following is a reconciliation of the beginning and ending balance of financial instruments using significant unobservable inputs (Level 3):
		Other invested assets		Mortgage-backed		Asset-backed		Equities
	Three Months Ended September 30, 2013
	Opening balance	$	714,391	$	198,003	$	61,285	$	53,499
	Realized and unrealized gains (losses) included in net income		9,403		464		-313		3,972
Purchases		67,554		69,775		16,969		10,000
Sales		-24,277		-79,001		-1,302		0
Transfers into Level 3 from Level 2		0		13		3,394		0
Transfers out of Level 3 into Level 2 (1)		0		0		0		0
Ending balance	$	767,071	$	189,254	$	80,033	$	67,471
Three Months Ended September 30, 2012
Opening balance	$	520,890	$	157,959	$	117,586	$	0
Realized and unrealized gains included in net income		11,871		4,855		988		0
Purchases		34,800		40,481		7,466		0
Sales		-2,859		-48,728		-7,326		0
Transfers into Level 3 from Level 2		0		14,730		12,495		0
Transfers out of Level 3 into Level 2 (1)		0		-27		-55,442		0
Ending balance	$	564,702	$	169,270	$	75,767	$	0
	Other invested assets		Mortgage-backed		Asset-backed		Equities
Nine Months Ended September 30, 2013
Opening balance	$	655,888	$	167,825	$	62,246	$	54,680
Realized and unrealized gains (losses) included in net income		53,365		-5,910		-791		2,791
Purchases		237,506		102,369		42,956		10,000
Sales		-179,688		-69,968		-26,728		0
Transfers into Level 3 from Level 2		0		5,073		2,350		0
Transfers out of Level 3 into Level 2 (1)		0		-10,135		0		0
Ending balance	$	767,071	$	189,254	$	80,033	$	67,471
Nine Months Ended September 30, 2012
Opening balance	$	540,409	$	249,204	$	94,745	$	0
Realized and unrealized gains included in net income		26,753		10,951		1,643		0
Purchases		52,578		50,302		32,573		0
Sales		-55,038		-124,940		-57,325		0
Transfers into Level 3 from Level 2		0		18,461		15,835		0
Transfers out of Level 3 into Level 2 (1)		0		-34,708		-11,704		0
Ending balance	$	564,702	$	169,270	$	75,767	$	0
____________
(1) Transfers out of Level 3 are primarily attributable to the availability of market observable information.
The Company attempts to verify the significant inputs used by broker-dealers in determining the fair value of the securities priced by them. If the Company could not obtain sufficient information to determine if the broker-dealers were using significant observable inputs, then such securities have been transferred to the Level 3 fair value hierarchy. The Company believes the prices obtained from the broker-dealers are the best estimate of fair value of the securities being priced as the broker-dealers are typically involved in the initial pricing of the security, and the Company has compared the price per the broker-dealer to other pricing sources and noted no material differences. The Company recognizes transfers between levels at the end of the reporting period. There were no transfers between Level 1 and Level 2 during the period.
The Company's external investment accounting service provider receives prices from internationally recognized independent pricing services to measure the fair values of its fixed maturity investments. Pricing sources are evaluated and selected in a manner to ensure that the most reliable sources are used. The Company uses a pricing service ranking to consistently select the most appropriate pricing service in instances where it receives multiple quotes on the same security. The Company obtains multiple quotes for the majority of its securities. The independent pricing sources obtain market quotations and actual transaction prices for securities that have quoted prices in active markets. Each pricing service 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 service uses observable market inputs, including, but not limited to, reported trades, benchmark yields, broker-dealer quotes, interest rates, prepayment speeds, default rates and such other inputs as are available from market sources to determine a reasonable fair value.
All of the Company's securities classified as Level 3, other than investments in other invested assets, are valued based on unadjusted broker-dealer quotes. This includes less liquid securities such as lower quality asset-backed securities, commercial mortgage-backed securities and residential mortgage-backed securities. The primary valuation inputs include monthly payment information, the probability of default, loss severity rates and estimated prepayment rates. Significant changes in these inputs in isolation would result in a significantly lower or higher fair value measurement. In general, a change in the assumption of the probability of default is accompanied by a directionally similar change in the assumption used for the loss severity in an event of default and prepayment rates.
The Company records the unadjusted price provided and validates this price through a process that includes, but is not limited to, monthly and/or quarterly: (i) comparison of prices between two independent sources, with significant differences requiring additional price sources; (ii) quantitative analysis (e.g., comparing the quarterly return for each managed portfolio to their target benchmark, with significant differences identified and investigated); (iii) evaluation of methodologies used by external parties to calculate fair value, including a review of the inputs used for pricing; (iv) comparing the price to the Company's knowledge of the current investment market; and (v) back-testing, which includes randomly selecting purchased or sold securities and comparing the executed prices to the fair value estimates from the pricing service. In addition to internal controls, management relies on the effectiveness of the valuation controls in place at the Company's external investment accounting service provider (supported by a Statement on Standards for Attestation Engagements No. 16 report) in conjunction with regular discussion and analysis of the investment portfolio's structure and performance.

Reserve_For_Losses_And_Loss_Ex

Reserve For Losses And Loss Expenses	9 Months Ended
	Sep. 30, 2013
Reserve For Losses And Loss Expenses [Abstract]
Reserve For Losses And Loss Expenses	7. RESERVE FOR LOSSES AND LOSS EXPENSES
	The reserve for losses and loss expenses consists of the following:
		September 30,			December 31,
		2013			2012
	Outstanding loss reserves	$	1,517,049		$	1,539,114
	Reserves for losses incurred but not reported		4,263,732			4,106,435
	Reserve for losses and loss expenses	$	5,780,781		$	5,645,549
	The table below is a reconciliation of the beginning and ending liability for unpaid losses and loss expenses. Losses incurred and paid are reflected net of reinsurance recoverables.
			Three Months Ended					Nine Months Ended
			September 30,					September 30,
			2013			2012		2013		2012
	Gross liability at beginning of period		$	5,696,865		$	5,377,518	$	5,645,549	$	5,225,143
	Reinsurance recoverable at beginning of period			-1,179,525			-1,073,612		-1,141,110		-1,002,919
	Net liability at beginning of period			4,517,340			4,303,906		4,504,439		4,222,224
	Net losses incurred related to:
		Current year		338,420			315,102		961,224		862,088
		Prior years		-61,450			-56,154		-153,948		-137,558
		Total incurred		276,970			258,948		807,276		724,530
	Net paid losses related to:
		Current year		30,399			16,323		54,983		36,163
		Prior years		213,252			179,666		696,137		542,077
		Total paid		243,651			195,989		751,120		578,240
	Foreign exchange revaluation			4,088			6,400		-5,848		4,751
	Net liability at end of period			4,554,747			4,373,265		4,554,747		4,373,265
	Reinsurance recoverable at end of period			1,226,034			1,077,522		1,226,034		1,077,522
	Gross liability at end of period		$	5,780,781		$	5,450,787	$	5,780,781	$	5,450,787
	For the three months ended September 30, 2013, the Company had net favorable reserve development in its international insurance and reinsurance segments due to actual loss emergence being lower than initially expected. The majority of the net favorable reserve development was recognized in the 2007 through 2011 loss years across most lines of business. In addition, the reinsurance segment recognized net favorable reserve development for the 2012 loss year due to the low level of reported property losses. This was partially offset by adverse development in the U.S. insurance segment in the 2011 and 2012 loss years primarily caused by adverse development on reported claims for certain healthcare, errors and omissions and private/not for profit directors' and officers'.
	For the nine months ended September 30, 2013, the Company had net favorable reserve development in its international insurance and reinsurance segments due to actual loss emergence being lower than initially expected, for most loss years. The reinsurance segment recognized net favorable reserve development for the 2012 loss year due to the low level of reported property losses. This was partially offset by adverse development in the U.S. insurance segment in the 2011 and 2012 loss years for certain healthcare, errors and omissions and not-for-profit director's and officers' classes of business.
	For the three months ended September 30, 2012, the Company had net favorable reserve development in each of its segments due to actual loss emergence being lower than initially expected. The majority of the net favorable reserve development was recognized by each segment in the 2004 through 2008 loss years. The Company had net unfavorable reserve development in its U.S. insurance segment in the 2010 and 2011 loss years, primarily due to higher than expected losses on a terminated program, and in its international insurance segment in the 2011 loss year, primarily due to an individual full-limit general casualty claim.
	For the nine months ended September 30, 2012, the Company had net favorable reserve development in each of its segments due to actual loss emergence being lower than initially expected. The majority of the net favorable reserve development was recognized in the international insurance and reinsurance segments in the 2004 through 2008 loss years.
	While the Company has experienced favorable reserve development in its insurance and reinsurance lines, there is no assurance that conditions and trends that have affected the development of liabilities in the past will continue. It is not appropriate to extrapolate future redundancies based on prior years' development. The methodology of estimating loss reserves is periodically reviewed to ensure that the key assumptions used in the actuarial models continue to be appropriate.

Income_Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2013
Income Tax Disclosure [Abstract]
Income Taxes	8. INCOME TAXES
	Under Swiss law, a resident company is subject to income tax at the federal, cantonal and communal levels that is levied on net income. Income attributable to permanent establishments or real estate located abroad is excluded from the Swiss tax base. Allied World Switzerland is a holding company and, therefore, is exempt from cantonal and communal income tax. As a result, Allied World Switzerland is subject to Swiss income tax only at the federal level. Allied World Switzerland is a resident of the Canton of Zug and, as such, is subject to an annual cantonal and communal capital tax on the taxable equity of Allied World Switzerland. Allied World Switzerland has a Swiss operating company resident in the Canton of Zug. The operating company is subject to federal, cantonal and communal income tax and to annual cantonal and communal capital tax.
	Under current Bermuda law, Allied World Assurance Company Holdings, Ltd (“Allied World Bermuda”) and its Bermuda subsidiaries are not required to pay taxes in Bermuda on either income or capital gains. Allied World Bermuda and Allied World Assurance Company, Ltd have received an assurance from the Bermuda Minister of Finance under the Exempted Undertakings Tax Protection Act 1966 of Bermuda, that in the event of any such taxes being imposed, Allied World Bermuda and Allied World Assurance Company, Ltd will be exempted until March 2035.
	Certain subsidiaries of Allied World Switzerland file U.S. federal income tax returns and various U.S. state income tax returns, as well as income tax returns in the United Kingdom, Ireland, Switzerland, Hong Kong and Singapore. To the best of the Company's knowledge, there are no income tax examinations pending by any tax authority.
	Management has deemed all material tax positions to have a greater than 50% likelihood of being sustained based on technical merits if challenged. The Company does not expect any material unrecognized tax benefits within 12 months of September 30, 2013.

Shareholders_Equity

Shareholders' Equity	9 Months Ended
	Sep. 30, 2013
Shareholders' Equity [Abstract]
Shareholders' Equity	9. SHAREHOLDERS' EQUITY
	a) Authorized shares
	The issued share capital consists of the following:
		September 30,		December 31,
		2013		2012
	Common shares issued and fully paid, 2013: CHF 12.30 per share; 2012: CHF 12.64 per share		34,972,795		36,369,868
	Share capital at end of period	$	424,837	$	454,980
		Nine Months
		Ended
		September 30,
		2013
	Shares issued at beginning of period	36,369,868
	Shares cancelled	-1,397,073
	Total shares issued at end of period	34,972,795
	Treasury shares issued at beginning of period	1,572,087
	Shares repurchased	1,367,833
	Shares issued out of treasury	-384,972
	Shares cancelled	-1,397,073
	Total treasury shares at end of period	1,157,875
	Total shares outstanding at end of period	33,814,920
	During the nine months ended September 30, 2013, 1,367,833 voting shares repurchased and designated for cancellation were constructively retired and cancelled.
	Effective July 9, 2013, the Company cancelled 29,240 non-voting shares held in treasury and 1,538,686 shares previously repurchased and constructively retired, following a required filing with the Swiss Commercial Register in Zug.
	Allied World Switzerland's articles of association authorized its Board of Directors to increase the share capital by a maximum of up to CHF 92,259 or 7,500,728 voting shares.
	b) Dividends
	The Company paid the following dividends during the nine months ended September 30, 2013:
		Partial
		Par Value		Dividend		Total
		Reduction		Per		Amount
	Dividend Paid	Per Share		Share		Paid
	12-Mar-13	CHF	0.34	$	0.375	$	12,981
	3-Jul-13	-	-	$	0.5	$	17,117
	On May 3, 2012, the shareholders approved the Company's proposal to pay cash dividends in the form of a distribution by way of par value reductions. The aggregate reduction amount was paid to shareholders in four installments of $0.375 per share, with the last of such quarterly dividend payments being made on March 12, 2013.
	On May 2, 2013, the shareholders approved the Company's proposal to pay cash dividends in the form of a distribution out of general legal reserve from capital contributions. The distribution amount will be paid to shareholders in quarterly dividends of $0.50 per share. The first dividend was paid on July 3, 2013. The second dividend was paid on October 3, 2013. The Company expects to pay the remaining dividends in January 2014 and April 2014.
	c) Share Repurchases
	In May 2012, the Company established a new share repurchase program in order to repurchase up to $500,000 of its common shares. Repurchases may be effected from time to time through open market purchases, privately negotiated transactions, tender offers or otherwise. The timing, form and amount of the share repurchases under the program will depend on a variety of factors, including market conditions, the Company's capital position, legal requirements and other factors. Under the terms of this new share repurchase program, common shares repurchased shall be designated for cancellation at acquisition and shall be cancelled upon shareholder approval.
	Shares repurchased by the Company and not designated for cancellation are classified as “Treasury shares, at cost” on the consolidated balance sheets. The Company will issue shares out of treasury principally related to the Company's employee benefit plans. Shares repurchased and designated for cancellation are constructively retired and recorded as a share cancellation.
	The Company's share repurchases were as follows:
		Three Months Ended				Nine Months Ended
		September 30,				September 30,
		2013		2012		2013		2012
	Common shares repurchased		427,388		605,898		1,367,833		2,942,085
	Total cost of shares repurchased	$	40,574	$	47,590	$	123,145	$	207,048
	Average price per share	$	94.93	$	78.54	$	90.03	$	70.37

Earnings_Per_Share

Earnings Per Share	9 Months Ended
	Sep. 30, 2013
Earnings Per Share [Abstract]
Earnings Per Share	10. EARNINGS PER SHARE
	The following table sets forth the comparison of basic and diluted earnings per share:
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
		2013			2012			2013			2012
	Basic earnings per share:
	Net income	$	122,843		$	219,647		$	279,973		$	534,154
	Weighted average common shares outstanding		33,991,359			35,652,768			34,340,227			36,379,514
	Basic earnings per share	$	3.61		$	6.16		$	8.15		$	14.68
			Three Months Ended						Nine Months Ended
			September 30,						September 30,
			2013			2012			2013			2012
	Diluted earnings per share:
	Net income		$	122,843		$	219,647		$	279,973		$	534,154
	Weighted average common shares outstanding			33,991,359			35,652,768			34,340,227			36,379,514
	Share equivalents:
	Options			500,114			461,373			497,458			429,393
	Restricted stock units and performance-based equity awards			236,133			501,633			292,369			583,226
	Employee share purchase plan			587			960			1,038			960
	Weighted average common shares and common share equivalents
		outstanding - diluted		34,728,193			36,616,734			35,131,092			37,393,093
	Diluted earnings per share		$	3.54		$	6		$	7.97		$	14.28
	For the three and nine months ended September 30, 2013, no employee stock options and restricted stock units (“RSUs”) were considered anti-dilutive.
	For the three and nine months ended September 30, 2012, a weighted average of 221,008 and 338,395 employee stock options and RSUs were considered anti-dilutive and were therefore excluded from the calculation of the diluted earnings per share, respectively.

Segment_Information

Segment Information	9 Months Ended
	Sep. 30, 2013
Segment Information [Abstract]
Segment Information	11. SEGMENT INFORMATION
	The determination of reportable segments is based on how senior management monitors the Company's underwriting operations. Management monitors the performance of its direct underwriting operations based on the geographic location of the Company's offices, the markets and customers served and the type of accounts written. The Company is currently organized into three operating segments: U.S. insurance, international insurance and reinsurance. All product lines fall within these classifications.
	The U.S. insurance segment includes the Company's direct specialty insurance operations in the United States. This segment provides both direct property and specialty casualty insurance primarily to non-Fortune 1000 North American domiciled accounts. The international insurance segment includes the Company's direct insurance operations in Bermuda, Europe, Singapore and Hong Kong. This segment provides both direct property and casualty insurance primarily to Fortune 1000 North American domiciled accounts from the Bermuda office and direct property and specialty casualty insurance to our non-North American domiciled accounts from the European, Singapore and Hong Kong offices. The reinsurance segment includes the Company's reinsurance operations in the United States, Bermuda, Europe, Singapore and Hong Kong. This segment provides reinsurance of property, general casualty, professional liability, specialty lines and property catastrophe coverages written by insurance companies. The Company presently writes reinsurance on both a treaty and a facultative basis, targeting several niche reinsurance markets.
	Responsibility and accountability for the results of underwriting operations are assigned by major line of business within each segment. Because the Company does not manage its assets by segment, investment income, interest expense and total assets are not allocated to individual reportable segments. General and administrative expenses are allocated to segments based on various factors, including staff count and each segment's proportional share of gross premiums written.
	Management measures results for each segment on the basis of the “loss and loss expense ratio,” “acquisition cost ratio,” “general and administrative expense ratio” and the “combined ratio.” The “loss and loss expense ratio” is derived by dividing net losses and loss expenses by net premiums earned. The “acquisition cost ratio” is derived by dividing acquisition costs by net premiums earned. The “general and administrative expense ratio” is derived by dividing general and administrative expenses by net premiums earned. The “combined ratio” is the sum of the “loss and loss expense ratio,” the “acquisition cost ratio” and the “general and administrative expense ratio.”
	The following tables provide a summary of the segment results:
						International
	Three Months Ended September 30, 2013		U.S. Insurance			Insurance			Reinsurance			Total
	Gross premiums written		$	308,709		$	132,881		$	139,303		$	580,893
	Net premiums written			238,792			75,632			138,653			453,077
	Net premiums earned			207,602			87,554			215,617			510,773
	Net losses and loss expenses			-141,222			-31,094			-104,654			-276,970
	Acquisition costs			-28,426			282			-36,970			-65,114
	General and administrative expenses			-41,616			-26,450			-20,487			-88,553
	Underwriting (loss) income			-3,662			30,292			53,506			80,136
	Net investment income												39,271
	Net realized investment gains												27,487
	Amortization of intangible assets												-633
	Interest expense												-14,094
	Foreign exchange loss												-4,353
	Income before income taxes											$	127,814
	Loss and loss expense ratio			68.00%			35.50%			48.50%			54.20%
	Acquisition cost ratio			13.70%			-0.30%			17.10%			12.70%
	General and administrative expense ratio			20.00%			30.20%			9.50%			17.30%
	Combined ratio			101.70%			65.40%			75.10%			84.20%
						International
	Three Months Ended September 30, 2012		U.S. Insurance			Insurance			Reinsurance			Total
	Gross premiums written		$	263,129		$	121,315		$	119,976		$	504,420
	Net premiums written			200,779			71,199			119,559			391,537
	Net premiums earned			173,948			85,329			181,740			441,017
	Net losses and loss expenses			-109,111			-15,099			-134,738			-258,948
	Acquisition costs			-22,696			266			-28,656			-51,086
	General and administrative expenses			-37,388			-22,920			-18,264			-78,572
	Underwriting income			4,753			47,576			82			52,411
	Net investment income												39,121
	Net realized investment gains												149,813
	Amortization of intangible assets												-633
	Interest expense												-13,822
	Foreign exchange loss												-1,023
	Income before income taxes											$	225,867
	Loss and loss expense ratio			62.70%			17.70%			74.10%			58.70%
	Acquisition cost ratio			13.00%			-0.30%			15.80%			11.60%
	General and administrative expense ratio			21.50%			26.90%			10.00%			17.80%
	Combined ratio			97.20%			44.30%			99.90%			88.10%
						International
	Nine Months Ended September 30, 2013		U.S. Insurance			Insurance			Reinsurance			Total
	Gross premiums written		$	872,024		$	453,990		$	857,160		$	2,183,174
	Net premiums written			652,464			259,771			817,116			1,729,351
	Net premiums earned			593,477			258,809			628,986			1,481,272
	Net losses and loss expenses			-398,910			-90,997			-317,369			-807,276
	Acquisition costs			-78,824			1,489			-109,081			-186,416
	General and administrative expenses			-119,514			-75,374			-56,930			-251,818
	Underwriting (loss) income			-3,771			93,927			145,606			235,762
	Net investment income												110,294
	Net realized investment losses												-8,074
	Amortization of intangible assets												-1,900
	Interest expense												-42,416
	Foreign exchange loss												-7,361
	Income before income taxes											$	286,305
	Loss and loss expense ratio			67.20%			35.20%			50.50%			54.50%
	Acquisition cost ratio			13.30%			-0.60%			17.30%			12.60%
	General and administrative expense ratio			20.10%			29.10%			9.10%			17.00%
	Combined ratio			100.60%			63.70%			76.90%			84.10%
						International
Nine Months Ended September 30, 2012		U.S. Insurance			Insurance			Reinsurance			Total
Gross premiums written		$	733,314		$	418,498		$	680,407		$	1,832,219
Net premiums written			551,286			255,150			668,764			1,475,200
Net premiums earned			490,091			247,805			534,758			1,272,654
Net losses and loss expenses			-309,889			-75,432			-339,209			-724,530
Acquisition costs			-63,918			1,376			-87,270			-149,812
General and administrative expenses			-103,162			-66,969			-52,786			-222,917
Underwriting income			13,122			106,780			55,493			175,395
Net investment income												128,781
Net realized investment gains												292,057
Amortization of intangible assets												-1,900
Interest expense												-41,579
Foreign exchange gain												77
Income before income taxes											$	552,831
Loss and loss expense ratio			63.20%			30.40%			63.40%			56.90%
Acquisition cost ratio			13.00%			-0.60%			16.30%			11.80%
General and administrative expense ratio			21.00%			27.00%			9.90%			17.50%
Combined ratio			97.20%			56.80%			89.60%			86.20%
The following table shows an analysis of the Company's gross premiums written by geographic location of the Company's subsidiaries and branches. All intercompany premiums have been eliminated.
	Three Months Ended						Nine Months Ended
	September 30,						September 30,
	2013			2012			2013			2012
United States	$	377,618		$	312,807		$	1,296,212		$	1,051,355
Bermuda		111,103			108,955			550,815			484,313
Europe		52,004			46,836			198,747			182,220
Singapore		35,515			31,902			122,546			101,213
Hong Kong		4,653			3,920			14,854			13,118
Total gross premiums written	$	580,893		$	504,420		$	2,183,174		$	1,832,219
Europe includes gross premiums written attributable to Switzerland of $10,509 and $7,039 for the three months ended September 30, 2013 and 2012, respectively, and $57,183 and $40,474 for the nine months ended September 30, 2013 and 2012, respectively.

Commitments_And_Contingencies

Commitments And Contingencies	9 Months Ended
	Sep. 30, 2013
Commitments And Contingencies [Abstract]
Commitments And Contingencies	12. COMMITMENTS AND CONTINGENCIES
	The Company, in common with the insurance industry in general, is subject to litigation and arbitration in the normal course of its business. These legal proceedings generally relate to claims asserted by or against the Company in the ordinary course of insurance or reinsurance operations. Estimated amounts payable under these proceedings are included in the reserve for losses and loss expenses in the Company's consolidated balance sheets. As of September 30, 2013, the Company was not a party to any material legal proceedings arising outside the ordinary course of business that management believes will have a material adverse effect on the Company's results of operations, financial position or cash flow.

Condensed_Consolidated_Guarant

Condensed Consolidated Guarantor Financial Statements	9 Months Ended
	Sep. 30, 2013
Condensed Consolidated Guarantor Financial Statements [Abstract]
Condensed consolidated guarantor financial statements	13. CONDENSED CONSOLIDATED GUARANTOR FINANCIAL STATEMENTS
	The following tables present unaudited condensed consolidating financial information as of September 30, 2013 and December 31, 2012 and for the three and nine months ended September 30, 2013 and 2012 for Allied World Switzerland (the “Parent Guarantor”) and Allied World Bermuda (the “Subsidiary Issuer”). The Subsidiary Issuer is a direct, 100%-owned subsidiary of the Parent Guarantor. Investments in subsidiaries are accounted for by the Parent Guarantor under the equity method for purposes of the supplemental consolidating presentation. Earnings of subsidiaries are reflected in the Parent Guarantor's investment accounts and earnings. The Parent Guarantor fully and unconditionally guarantees the senior notes issued by the Subsidiary Issuer.
	Unaudited Condensed Consolidating Balance Sheet:
		Allied World			Allied World
		Switzerland			Bermuda			Other Allied						Allied World
		(Parent			(Subsidiary			World			Consolidating			Switzerland
	As of September 30, 2013	Guarantor)			Issuer)			Subsidiaries			Adjustments			Consolidated
	ASSETS:
	Investments	$	0		$	0		$	7,661,837		$	0		$	7,661,837
	Cash and cash equivalents		56,060			8,388			888,599			0			953,047
	Insurance balances receivable		0			0			740,112			0			740,112
	Funds held		0			0			375,131			0			375,131
	Reinsurance recoverable		0			0			1,226,034			0			1,226,034
	Net deferred acquisition costs		0			0			145,951			0			145,951
	Goodwill and intangible assets		0			0			317,840			0			317,840
	Balances receivable on sale of investments		0			0			237,031			0			237,031
	Investments in subsidiaries		3,407,988			4,509,074			0			-7,917,062			0
	Due (to) from subsidiaries		-967			-14,679			15,646			0			0
	Other assets		1,897			5,038			713,602			0			720,537
	Total assets	$	3,464,978		$	4,507,821		$	12,321,783		$	-7,917,062		$	12,377,520
	LIABILITIES:
	Reserve for losses and loss expenses	$	0		$	0		$	5,780,781		$	0		$	5,780,781
	Unearned premiums		0			0			1,515,746			0			1,515,746
	Reinsurance balances payable		0			0			193,643			0			193,643
	Balances due on purchases of investments		0			0			497,974			0			497,974
	Senior notes		0			798,426			0			0			798,426
	Other liabilities		21,050			17,457			108,515			0			147,022
	Total liabilities		21,050			815,883			8,096,659			0			8,933,592
	Total shareholders’ equity		3,443,928			3,691,938			4,225,124			-7,917,062			3,443,928
	Total liabilities and shareholders’ equity	$	3,464,978		$	4,507,821		$	12,321,783		$	-7,917,062		$	12,377,520
		Allied World			Allied World
		Switzerland			Bermuda			Other Allied						Allied World
		(Parent			(Subsidiary			World			Consolidating			Switzerland
	As of December 31, 2012	Guarantor)			Issuer)			Subsidiaries			Adjustments			Consolidated
	ASSETS:
	Investments	$	0		$	0		$	7,933,937		$	0		$	7,933,937
	Cash and cash equivalents		19,997			11,324			650,558			0			681,879
	Insurance balances receivable		0			0			510,532			0			510,532
	Funds held		0			0			336,368			0			336,368
	Reinsurance recoverable		0			0			1,141,110			0			1,141,110
	Net deferred acquisition costs		0			0			108,010			0			108,010
	Goodwill and intangible assets		0			0			319,741			0			319,741
	Balances receivable on sale of investments		0			0			418,879			0			418,879
	Investments in subsidiaries		3,337,446			4,768,769			0			-8,106,215			0
	Due (to) from subsidiaries		-23,864			-7,173			31,037			0			0
	Other assets		1,499			6,081			571,910			0			579,490
	Total assets	$	3,335,078		$	4,779,001		$	12,022,082		$	-8,106,215		$	12,029,946
	LIABILITIES:
	Reserve for losses and loss expenses	$	0		$	0		$	5,645,549		$	0		$	5,645,549
	Unearned premiums		0			0			1,218,021			0			1,218,021
	Reinsurance balances payable		0			0			136,264			0			136,264
	Balances due on purchases of investments		0			0			759,934			0			759,934
	Senior notes		0			798,215			0			0			798,215
	Other liabilities		8,743			17,727			119,158			0			145,628
	Total liabilities		8,743			815,942			7,878,926			0			8,703,611
	Total shareholders’ equity		3,326,335			3,963,059			4,143,156			-8,106,215			3,326,335
	Total liabilities and shareholders’ equity	$	3,335,078		$	4,779,001		$	12,022,082		$	-8,106,215		$	12,029,946
	Unaudited Condensed Consolidating Income Statement:
			Allied World			Allied World
			Switzerland			Bermuda			Other Allied						Allied World
			(Parent			(Subsidiary			World			Consolidating			Switzerland
	Three Months Ended September 30, 2013		Guarantor)			Issuer)			Subsidiaries			Adjustments			Consolidated
	Net premiums earned		$	0		$	0		$	510,773		$	0		$	510,773
	Net investment income			3			0			39,268			0			39,271
Net realized investment losses			0			0			27,487			0			27,487
Net losses and loss expenses			0			0			-276,970			0			-276,970
Acquisition costs			0			0			-65,114			0			-65,114
General and administrative expenses			-7,323			-4,961			-76,269			0			-88,553
Amortization of intangible assets			0			0			-633			0			-633
Interest expense			0			-13,838			-256			0			-14,094
Foreign exchange gain (loss)			-13			-212			-4,128			0			-4,353
Income tax (expense) benefit			0			0			-4,971			0			-4,971
Equity in earnings of consolidated subsidiaries			130,176			150,653			0			-280,829			0
NET INCOME (LOSS)		$	122,843		$	131,642		$	149,187		$	-280,829		$	122,843
Other comprehensive income			0			0			0			0			0
COMPREHENSIVE INCOME (LOSS)		$	122,843		$	131,642		$	149,187		$	-280,829		$	122,843
		Allied World			Allied World
		Switzerland			Bermuda			Other Allied						Allied World
		(Parent			(Subsidiary			World			Consolidating			Switzerland
Three Months Ended September 30, 2012		Guarantor)			Issuer)			Subsidiaries			Adjustments			Consolidated
Net premiums earned		$	0		$	0		$	441,017		$	0		$	441,017
Net investment income			7			6			39,108			0			39,121
Net realized investment gains			0			0			149,813			0			149,813
Net losses and loss expenses			0			0			-258,948			0			-258,948
Acquisition costs			0			0			-51,086			0			-51,086
General and administrative expenses			-6,013			-235			-72,324			0			-78,572
Amortization of intangible assets			0			0			-633			0			-633
Interest expense			0			-13,822			0			0			-13,822
Foreign exchange gain (loss)			-206			-83			-734			0			-1,023
Income tax (expense) benefit			0			0			-6,220			0			-6,220
Equity in earnings of consolidated subsidiaries			225,859			231,471			0			-457,330			0
NET INCOME (LOSS)		$	219,647		$	217,337		$	239,993		$	-457,330		$	219,647
Unrealized losses on investments arising during the period
	net of applicable deferred income tax benefit of $15		-29			0			-29			29			-29
Reclassification adjustment for net realized investment
	gains included in net income, net of applicable income tax		0			0			0			0			0
Other comprehensive loss			-29			0			-29			29			-29
COMPREHENSIVE INCOME (LOSS)		$	219,618		$	217,337		$	239,964		$	-457,301		$	219,618
		Allied World			Allied World
		Switzerland			Bermuda			Other Allied						Allied World
		(Parent			(Subsidiary			World			Consolidating			Switzerland
Nine Months Ended September 30, 2013		Guarantor)			Issuer)			Subsidiaries			Adjustments			Consolidated
Net premiums earned		$	0		$	0		$	1,481,272		$	0		$	1,481,272
Net investment income			11			4			110,279			0			110,294
Net realized investment losses			0			0			-8,074			0			-8,074
Net losses and loss expenses			0			0			-807,276			0			-807,276
Acquisition costs			0			0			-186,416			0			-186,416
General and administrative expenses			-26,875			-5,873			-219,070			0			-251,818
Amortization of intangible assets			0			0			-1,900			0			-1,900
Interest expense			0			-41,503			-913			0			-42,416
Foreign exchange loss (gain)			261			-935			-6,687			0			-7,361
Income tax (expense) benefit			0			0			-6,332			0			-6,332
Equity in earnings of consolidated subsidiaries			306,576			353,280			0			-659,856			0
NET INCOME (LOSS)		$	279,973		$	304,973		$	354,883		$	-659,856		$	279,973
Other comprehensive income			0			0			0			0			0
COMPREHENSIVE INCOME (LOSS)		$	279,973		$	304,973		$	354,883		$	-659,856		$	279,973
		Allied World			Allied World
		Switzerland			Bermuda			Other Allied						Allied World
		(Parent			(Subsidiary			World			Consolidating			Switzerland
Nine Months Ended September 30, 2012		Guarantor)			Issuer)			Subsidiaries			Adjustments			Consolidated
Net premiums earned		$	0		$	0		$	1,272,654		$	0		$	1,272,654
Net investment income			21			17			128,743			0			128,781
Net realized investment gains			0			0			292,057			0			292,057
Net losses and loss expenses			0			0			-724,530			0			-724,530
Acquisition costs			0			0			-149,812			0			-149,812
General and administrative expenses			-14,247			-2,689			-205,981			0			-222,917
Amortization of intangible assets			0			0			-1,900			0			-1,900
Interest expense			0			-41,579			0			0			-41,579
Foreign exchange gain (loss)			343			-150			-116			0			77
Income tax (expense) benefit			71			0			-18,748			0			-18,677
Equity in earnings of consolidated subsidiaries			547,966			580,880			0			-1,128,846			0
NET INCOME (LOSS)		$	534,154		$	536,479		$	592,367		$	-1,128,846		$	534,154
Unrealized gains on investments arising during the period
	net of applicable deferred income tax expense of $81		150			0			150			-150			150
Reclassification adjustment for net realized investment
	gains included in net income, net of applicable income tax		-13,249			0			-13,249			13,249			-13,249
Other comprehensive loss			-13,099			0			-13,099			13,099			-13,099
COMPREHENSIVE INCOME (LOSS)		$	521,055		$	536,479		$	579,268		$	-1,115,747		$	521,055
Unaudited Condensed Consolidating Cash Flows:
		Allied World			Allied World
		Switzerland			Bermuda			Other Allied						Allied World
		(Parent			(Subsidiary			World			Consolidating			Switzerland
Nine Months Ended September 30, 2013		Guarantor)			Issuer)			Subsidiaries			Adjustments			Consolidated
CASH FLOWS PROVIDED BY (USED IN)
	OPERATING ACTIVITIES	$	177,859		$	-2,936		$	157,127		$	0		$	332,050
CASH FLOWS PROVIDED BY (USED IN)
	INVESTING ACTIVITIES:
Purchases trading securities			0			0			-4,955,817			0			-4,955,817
Purchases of other invested assets			0			0			-211,501			0			-211,501
Sales of available for sale securities			0			0			0			0			0
Sales of trading securities			0			0			5,137,280			0			5,137,280
Sales of other invested assets			0			0			189,155			0			189,155
Other			0			0			-78,203			0			-78,203
	Net cash provided by (used in) investing activities		0			0			80,914			0			80,914
CASH FLOWS PROVIDED BY (USED IN)
	FINANCING ACTIVITIES:
Dividends paid - partial par value reduction			-12,981			0			0			0			-12,981
Dividends paid			-17,117			0			0			0			-17,117
Proceeds from the exercise of stock options			8,465			0			0			0			8,465
Share repurchases			-120,163			0			0			0			-120,163
	Net cash provided by (used in) financing activities		-141,796			0			0			0			-141,796
NET INCREASE (DECREASE) IN CASH AND
	CASH EQUIVALENTS		36,063			-2,936			238,041			0			271,168
CASH AND CASH EQUIVALENTS, BEGINNING
	OF PERIOD		19,997			11,324			650,558			0			681,879
CASH AND CASH EQUIVALENTS, END OF
	PERIOD	$	56,060		$	8,388		$	888,599		$	0		$	953,047
		Allied World			Allied World
		Switzerland			Bermuda			Other Allied						Allied World
		(Parent			(Subsidiary			World			Consolidating			Switzerland
Nine Months Ended September 30, 2012		Guarantor)			Issuer)			Subsidiaries			Adjustments			Consolidated
CASH FLOWS PROVIDED BY (USED IN)
	OPERATING ACTIVITIES:	$	162,141		$	687		$	336,824		$	0		$	499,652
CASH FLOWS PROVIDED BY (USED IN)
	INVESTING ACTIVITIES:
Purchases of available for sale securities			0			0			0			0			0
Purchases of trading securities			0			0			-6,328,719			0			-6,328,719
Purchases of other invested assets			0			0			-52,578			0			-52,578
Sales of available for sale securities			0			0			215,318			0			215,318
Sales of trading securities			0			0			5,778,138			0			5,778,138
Sales of other invested assets			0			0			110,429			0			110,429
Other			0			0			33,644			0			33,644
	Net cash provided by (used in) investing activities		0			0			-243,768			0			-243,768
CASH FLOWS PROVIDED BY (USED IN)
	FINANCING ACTIVITIES:
Partial par value reduction			-54,721			0			0			0			-54,721
Proceeds from the exercise of stock options			9,104			0			0			0			9,104
Share repurchases			-204,746			0			0			0			-204,746
	Net cash provided by (used in) financing activities		-250,363			0			0			0			-250,363
NET INCREASE (DECREASE) IN CASH AND
	CASH EQUIVALENTS		-88,222			687			93,056			0			5,521
CASH AND CASH EQUIVALENTS, BEGINNING
	OF PERIOD		112,672			8,886			512,438			0			633,996
CASH AND CASH EQUIVALENTS, END OF
	PERIOD	$	24,450		$	9,573		$	605,494		$	0		$	639,517
Notes to Parent Company Condensed Financial Information
a) Dividends
Allied World Switzerland received cash dividends from its subsidiaries of $237,000 and $285,000 for the nine months ended September 30, 2013 and 2012, respectively. Such dividends are included in “cash flows provided by (used in) operating activities” in the unaudited condensed consolidating cash flows.

Subsequent_events

Subsequent events	9 Months Ended
	Sep. 30, 2013
Subsequent Events [Abstract]
Subsequent events	14. SUBSEQUENT EVENTS
	On October 3, 2013, the Company paid a quarterly dividend of $0.50 per share to shareholders of record on September 24, 2013.

Significant_Accounting_Policie

Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2013
Significant Accounting Policies [Abstract]
Basis of Accounting	These unaudited condensed consolidated financial statements include the accounts of the Company and have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) for interim financial information and with Article 10 of Regulation S-X as promulgated by the U.S. Securities and Exchange Commission (“SEC”). Accordingly, they do not include all of the information and footnotes required by U.S. GAAP for complete financial statements. In the opinion of management, these unaudited condensed consolidated financial statements reflect all adjustments that are normal and recurring in nature and necessary for a fair presentation of financial position and results of operations as of the end of and for the periods presented. The results of operations for any interim period are not necessarily indicative of the results for a full year.
	Intercompany accounts and transactions have been eliminated on consolidation and all entities meeting consolidation requirements have been included in the unaudited condensed consolidated financial statements. To facilitate comparison of information across periods, certain reclassifications have been made to prior year amounts to conform to the current year's presentation.
Use Of Estimates	The preparation of financial statements in conformity with U.S. 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. The significant estimates reflected in the Company's financial statements include, but are not limited to:
	•       The premium estimates for certain reinsurance agreements,
	•       Recoverability of deferred acquisition costs,
	•       The reserve for outstanding losses and loss expenses,
	•       Valuation of ceded reinsurance recoverables,
	•       Determination of impairment of goodwill and other intangible assets, and
	•       Valuation of financial instruments.

Investments_Tables

Investments (Tables)	9 Months Ended
	Sep. 30, 2013
Investments [Abstract]
Schedule of trading securities			30-Sep-13						31-Dec-12
			Fair Value			Amortized Cost			Fair Value			Amortized Cost
	U.S. Government and Government agencies		$	1,850,335		$	1,851,002		$	1,865,913		$	1,854,198
	Non-U.S. Government and Government agencies			209,625			214,114			261,627			253,657
	States, municipalities and political subdivisions			95,381			95,661			40,444			39,342
	Corporate debt:
		Financial institutions		841,930			826,763			866,140			835,587
		Industrials		1,146,793			1,140,636			1,153,909			1,139,706
		Utilities		74,764			74,865			69,153			67,463
	Mortgage-backed			1,373,161			1,341,303			1,958,373			1,877,854
	Asset-backed			455,894			451,721			410,895			405,622
	Total fixed maturity investments		$	6,047,883		$	5,996,065		$	6,626,454		$	6,473,429
			30-Sep-13						31-Dec-12
			Fair Value			Original Cost			Fair Value			Original Cost
	Equity securities		$	708,929		$	679,228		$	523,949		$	480,312
	Other invested assets			767,071			680,986			655,888			606,521
			$	1,476,000		$	1,360,214		$	1,179,837		$	1,086,833
Schedule of other invested assets				Carrying			Investments			Estimated		Investments
				Value as of			with			Remaining		without			Redemption
				September 30,			Redemption			Restriction		Redemption		Redemption	Notice	Unfunded
	Fund Type			2013			Restrictions			Period		Restrictions(1)		Frequency(1)	Period(1)	Commitments
	Private equity			$	131,449		$	131,449		3 - 10 Years		$	0			$	252,571
	Mezzanine debt				58,116			58,116		8 - 10 Years			0				203,969
	Distressed				8,991			8,991		4 -5 Years			0				5,102
	Total private equity structures				198,556			198,556					0				461,642
	Distressed				150,775			134,127		1 - 2 Years			16,648	Quarterly	45 - 65 Days		0
	Equity long/short				116,019			0					116,019	Quarterly	30 - 60 Days		0
	Multi-strategy				131,224			0					131,224	Quarterly	45 - 90 Days		0
	Global macro				19,641			0					19,641	Monthly	3 Days		0
	Event driven				18,445			0					18,445	Annual	60 Days		0
	Relative value credit				101,311			0					101,311	Quarterly	60 Days		0
	Total hedge funds				537,415			134,127					403,288				0
	Other private securities				137,954			0					137,954				5,000
	High yield loan fund				31,100			0					31,100	Monthly	30 Days		0
	Total other invested assets			$	905,025		$	332,683				$	572,342			$	466,642
	(1)       The redemption frequency and notice periods only apply to the investments without redemption restrictions. Some or all of these investments may be subject to a gate.
Schedule of net investment income		Three Months Ended						Nine Months Ended
		September 30,						September 30,
		2013			2012			2013			2012
	Fixed maturity investments	$	31,179		$	36,778		$	96,366		$	120,883
	Equity securities		6,110			5,211			13,718			13,706
	Other invested assets		5,809			164			11,116			4,387
	Cash and cash equivalents		302			640			1,319			1,797
	Expenses		-4,129			-3,672			-12,225			-11,992
	Net investment income	$	39,271		$	39,121		$	110,294		$	128,781
Components of realized gains and losses			Three Months Ended						Nine Months Ended
			September 30,						September 30,
			2013			2012			2013			2012
	Gross realized gains on sale of invested assets		$	51,915		$	27,210		$	154,387		$	119,154
	Gross realized losses on sale of invested assets			-40,770			-6,686			-82,812			-43,355
	Net realized and unrealized (losses) gains on derivatives			-4,169			-962			3,392			-192
	Mark-to-market gains (losses):
		Fixed maturity investments, trading		30,383			99,821			-101,205			144,024
		Equity securities, trading		-17,198			18,913			-18,555			38,516
		Other invested assets, trading		7,326			11,517			36,719			33,910
	Net realized investment gains (losses)		$	27,487		$	149,813		$	-8,074		$	292,057
	Proceeds from sale of available for sale securities		$	0		$	1,000		$	0		$	214,716

Derivative_Instruments_Tables

Derivative Instruments (Tables)	9 Months Ended
	Sep. 30, 2013
Derivative Instruments [Abstract]
Location and amounts of derivative fair values on the unaudited condensed consolidated balance sheets			30-Sep-13											31-Dec-12
			Asset						Liability					Asset				Liability
			Derivative			Asset			Derivative			Liability		Derivative		Asset		Derivative		Liability
			Notional			Derivative			Notional			Derivative		Notional		Derivative		Notional		Derivative
			Amount			Fair Value			Amount			Fair Value		Amount		Fair Value		Amount		Fair Value
	Put options		$	0		$	0		$	0		$	0	$	5,152	$	532	$	0	$	0
	Foreign exchange contracts			197,811			2,493			279,050			7,062		127,712		1,713		194,566		2,656
	Interest rate swaps			33,000			5			327,000			172		0		0		0		0
	Total derivatives		$	230,811		$	2,498		$	606,050		$	7,234	$	132,864	$	2,245	$	194,566	$	2,656
Unrealized and realized gains (losses) on derivatives recorded in the unaudited condensed consolidated income statements		Three Months Ended						Nine Months Ended
		September 30,						September 30,
		2013			2012			2013			2012
	Foreign exchange contracts	$	-2,336		$	-676		$	-1,091		$	-96
	Total included in foreign exchange (loss) gain		-2,336			-676			-1,091			-96
	Put options		0			0			-3,822			-336
	Foreign exchange contracts		-4,164			-2,751			1,925			-641
	Interest rate futures and swaps		-5			1,789			5,289			785
	Total included in net realized investment gains		-4,169			-962			3,392			-192
	Total realized and unrealized (losses) gains on derivatives	$	-6,505		$	-1,638		$	2,301		$	-288

Fair_Value_Of_Financial_Instru1

Fair Value Of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2013
Fair Value Of Financial Instruments [Abstract]
Fair value hierarchy and related disclosures				Carrying		Total
	30-Sep-13			Amount		Fair Value		Level 1		Level 2		Level 3
	Fixed maturity investments:
		U.S. Government and Government agencies		$	1,850,335	$	1,850,335	$	1,522,185	$	328,150	$	0
		Non-U.S. Government and Government agencies			209,625		209,625		0		209,625		0
		States, municipalities and political subdivisions			95,381		95,381		0		95,381		0
		Corporate debt			2,063,487		2,063,487		0		2,063,487		0
		Mortgage-backed			1,373,161		1,373,161		0		1,183,907		189,254
		Asset-backed			455,894		455,894		0		375,861		80,033
	Total fixed maturity investments				6,047,883		6,047,883		1,522,185		4,256,411		269,287
	Equity securities				708,929		708,929		641,458		0		67,471
	Other invested assets				767,071		767,071		0		0		767,071
	Total investments			$	7,523,883	$	7,523,883	$	2,163,643	$	4,256,411	$	1,103,829
	Derivative assets:
		Foreign exchange contracts		$	2,493	$	2,493	$	0	$	2,493	$	0
		Interest rate futures			5		5		0		5		0
	Derivative liabilities:
		Foreign exchange contracts		$	7,062	$	7,062	$	0	$	7,062	$	0
		Interest rate swaps			172		172		0		172		0
	Senior notes			$	798,426	$	904,010	$	0	$	904,010	$	0
				Carrying		Total
	31-Dec-12			Amount		Fair Value		Level 1		Level 2		Level 3
	Fixed maturity investments:
		U.S. Government and Government agencies		$	1,865,913	$	1,865,913	$	1,529,158	$	336,755	$	0
		Non-U.S. Government and Government agencies			261,627		261,627		0		261,627		0
		States, municipalities and political subdivisions			40,444		40,444		0		40,444		0
		Corporate debt			2,089,202		2,089,202		0		2,089,202		0
		Mortgage-backed			1,958,373		1,958,373		0		1,790,548		167,825
		Asset-backed			410,895		410,895		0		348,649		62,246
	Total fixed maturity investments				6,626,454		6,626,454		1,529,158		4,867,225		230,071
	Equity securities				523,949		523,949		469,269		0		54,680
	Other invested assets				655,888		655,888		0		0		655,888
	Total investments			$	7,806,291	$	7,806,291	$	1,998,427	$	4,867,225	$	940,639
	Derivative assets:
		Foreign exchange contracts		$	1,713	$	1,713	$	0	$	1,713	$	0
	Derivative liabilities:
		Foreign exchange contracts		$	2,656	$	2,656	$	0	$	2,656	$	0
	Senior notes			$	798,215	$	918,627	$	0	$	918,627	$	0
Fair value measurements using significant unobservable inputs (Level 3)		Other invested assets		Mortgage-backed		Asset-backed		Equities
	Three Months Ended September 30, 2013
	Opening balance	$	714,391	$	198,003	$	61,285	$	53,499
	Realized and unrealized gains (losses) included in net income		9,403		464		-313		3,972
	Purchases		67,554		69,775		16,969		10,000
	Sales		-24,277		-79,001		-1,302		0
	Transfers into Level 3 from Level 2		0		13		3,394		0
	Transfers out of Level 3 into Level 2 (1)		0		0		0		0
	Ending balance	$	767,071	$	189,254	$	80,033	$	67,471
	Three Months Ended September 30, 2012
	Opening balance	$	520,890	$	157,959	$	117,586	$	0
	Realized and unrealized gains included in net income		11,871		4,855		988		0
	Purchases		34,800		40,481		7,466		0
	Sales		-2,859		-48,728		-7,326		0
	Transfers into Level 3 from Level 2		0		14,730		12,495		0
	Transfers out of Level 3 into Level 2 (1)		0		-27		-55,442		0
	Ending balance	$	564,702	$	169,270	$	75,767	$	0
		Other invested assets		Mortgage-backed		Asset-backed		Equities
	Nine Months Ended September 30, 2013
	Opening balance	$	655,888	$	167,825	$	62,246	$	54,680
	Realized and unrealized gains (losses) included in net income		53,365		-5,910		-791		2,791
	Purchases		237,506		102,369		42,956		10,000
	Sales		-179,688		-69,968		-26,728		0
	Transfers into Level 3 from Level 2		0		5,073		2,350		0
	Transfers out of Level 3 into Level 2 (1)		0		-10,135		0		0
	Ending balance	$	767,071	$	189,254	$	80,033	$	67,471
	Nine Months Ended September 30, 2012
	Opening balance	$	540,409	$	249,204	$	94,745	$	0
	Realized and unrealized gains included in net income		26,753		10,951		1,643		0
	Purchases		52,578		50,302		32,573		0
	Sales		-55,038		-124,940		-57,325		0
	Transfers into Level 3 from Level 2		0		18,461		15,835		0
	Transfers out of Level 3 into Level 2 (1)		0		-34,708		-11,704		0
	Ending balance	$	564,702	$	169,270	$	75,767	$	0
	(1) Transfers out of Level 3 are primarily attributable to the availability of market observable information.

Reserve_For_Losses_And_Loss_Ex1

Reserve For Losses And Loss Expenses (Tables)	9 Months Ended
	Sep. 30, 2013
Reserve For Losses And Loss Expenses [Abstract]
Components of liabilty for unpaid losses and loss expenses		September 30,			December 31,
		2013			2012
	Outstanding loss reserves	$	1,517,049		$	1,539,114
	Reserves for losses incurred but not reported		4,263,732			4,106,435
	Reserve for losses and loss expenses	$	5,780,781		$	5,645,549
Reconciliation of beginning and ending liability for unpaid losses and loss expenses			Three Months Ended					Nine Months Ended
			September 30,					September 30,
			2013			2012		2013		2012
	Gross liability at beginning of period		$	5,696,865		$	5,377,518	$	5,645,549	$	5,225,143
	Reinsurance recoverable at beginning of period			-1,179,525			-1,073,612		-1,141,110		-1,002,919
	Net liability at beginning of period			4,517,340			4,303,906		4,504,439		4,222,224
	Net losses incurred related to:
		Current year		338,420			315,102		961,224		862,088
		Prior years		-61,450			-56,154		-153,948		-137,558
		Total incurred		276,970			258,948		807,276		724,530
	Net paid losses related to:
		Current year		30,399			16,323		54,983		36,163
		Prior years		213,252			179,666		696,137		542,077
		Total paid		243,651			195,989		751,120		578,240
	Foreign exchange revaluation			4,088			6,400		-5,848		4,751
	Net liability at end of period			4,554,747			4,373,265		4,554,747		4,373,265
	Reinsurance recoverable at end of period			1,226,034			1,077,522		1,226,034		1,077,522
	Gross liability at end of period		$	5,780,781		$	5,450,787	$	5,780,781	$	5,450,787

Shareholders_Equity_Tables

Shareholders' Equity (Tables)	9 Months Ended
	Sep. 30, 2013
Shareholders' Equity [Abstract]
Issued share capital		September 30,		December 31,
		2013		2012
	Common shares issued and fully paid, 2013: CHF 12.30 per share; 2012: CHF 12.64 per share		34,972,795		36,369,868
	Share capital at end of period	$	424,837	$	454,980
Schedule of common stock outstanding roll forward		Nine Months
		Ended
		September 30,
		2013
	Shares issued at beginning of period	36,369,868
	Shares cancelled	-1,397,073
	Total shares issued at end of period	34,972,795
	Treasury shares issued at beginning of period	1,572,087
	Shares repurchased	1,367,833
	Shares issued out of treasury	-384,972
	Shares cancelled	-1,397,073
	Total treasury shares at end of period	1,157,875
	Total shares outstanding at end of period	33,814,920
Dividends paid		Partial
		Par Value		Dividend		Total
		Reduction		Per		Amount
	Dividend Paid	Per Share		Share		Paid
	12-Mar-13	CHF	0.34	$	0.375	$	12,981
	3-Jul-13	-	-	$	0.5	$	17,117
Share repurchases		Three Months Ended				Nine Months Ended
		September 30,				September 30,
		2013		2012		2013		2012
	Common shares repurchased		427,388		605,898		1,367,833		2,942,085
	Total cost of shares repurchased	$	40,574	$	47,590	$	123,145	$	207,048
	Average price per share	$	94.93	$	78.54	$	90.03	$	70.37

Earnings_Per_Share_Tables

Earnings Per Share (Tables)	9 Months Ended
	Sep. 30, 2013
Earnings Per Share [Abstract]
Comparison Of Basic And Diluted Earnings Per Share		Three Months Ended						Nine Months Ended
		September 30,						September 30,
		2013			2012			2013			2012
	Basic earnings per share:
	Net income	$	122,843		$	219,647		$	279,973		$	534,154
	Weighted average common shares outstanding		33,991,359			35,652,768			34,340,227			36,379,514
	Basic earnings per share	$	3.61		$	6.16		$	8.15		$	14.68
			Three Months Ended						Nine Months Ended
			September 30,						September 30,
			2013			2012			2013			2012
	Diluted earnings per share:
	Net income		$	122,843		$	219,647		$	279,973		$	534,154
	Weighted average common shares outstanding			33,991,359			35,652,768			34,340,227			36,379,514
	Share equivalents:
	Options			500,114			461,373			497,458			429,393
	Restricted stock units and performance-based equity awards			236,133			501,633			292,369			583,226
	Employee share purchase plan			587			960			1,038			960
	Weighted average common shares and common share equivalents
		outstanding - diluted		34,728,193			36,616,734			35,131,092			37,393,093
	Diluted earnings per share		$	3.54		$	6		$	7.97		$	14.28

Segment_Information_Tables

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2013
Segment Information [Abstract]
Summary of segment results						International
	Three Months Ended September 30, 2013		U.S. Insurance			Insurance			Reinsurance			Total
	Gross premiums written		$	308,709		$	132,881		$	139,303		$	580,893
	Net premiums written			238,792			75,632			138,653			453,077
	Net premiums earned			207,602			87,554			215,617			510,773
	Net losses and loss expenses			-141,222			-31,094			-104,654			-276,970
	Acquisition costs			-28,426			282			-36,970			-65,114
	General and administrative expenses			-41,616			-26,450			-20,487			-88,553
	Underwriting (loss) income			-3,662			30,292			53,506			80,136
	Net investment income												39,271
	Net realized investment gains												27,487
	Amortization of intangible assets												-633
	Interest expense												-14,094
	Foreign exchange loss												-4,353
	Income before income taxes											$	127,814
	Loss and loss expense ratio			68.00%			35.50%			48.50%			54.20%
	Acquisition cost ratio			13.70%			-0.30%			17.10%			12.70%
	General and administrative expense ratio			20.00%			30.20%			9.50%			17.30%
	Combined ratio			101.70%			65.40%			75.10%			84.20%
						International
	Three Months Ended September 30, 2012		U.S. Insurance			Insurance			Reinsurance			Total
	Gross premiums written		$	263,129		$	121,315		$	119,976		$	504,420
	Net premiums written			200,779			71,199			119,559			391,537
	Net premiums earned			173,948			85,329			181,740			441,017
	Net losses and loss expenses			-109,111			-15,099			-134,738			-258,948
	Acquisition costs			-22,696			266			-28,656			-51,086
	General and administrative expenses			-37,388			-22,920			-18,264			-78,572
	Underwriting income			4,753			47,576			82			52,411
	Net investment income												39,121
	Net realized investment gains												149,813
	Amortization of intangible assets												-633
	Interest expense												-13,822
	Foreign exchange loss												-1,023
	Income before income taxes											$	225,867
	Loss and loss expense ratio			62.70%			17.70%			74.10%			58.70%
	Acquisition cost ratio			13.00%			-0.30%			15.80%			11.60%
	General and administrative expense ratio			21.50%			26.90%			10.00%			17.80%
	Combined ratio			97.20%			44.30%			99.90%			88.10%
						International
	Nine Months Ended September 30, 2013		U.S. Insurance			Insurance			Reinsurance			Total
	Gross premiums written		$	872,024		$	453,990		$	857,160		$	2,183,174
	Net premiums written			652,464			259,771			817,116			1,729,351
	Net premiums earned			593,477			258,809			628,986			1,481,272
	Net losses and loss expenses			-398,910			-90,997			-317,369			-807,276
	Acquisition costs			-78,824			1,489			-109,081			-186,416
	General and administrative expenses			-119,514			-75,374			-56,930			-251,818
	Underwriting (loss) income			-3,771			93,927			145,606			235,762
	Net investment income												110,294
	Net realized investment losses												-8,074
	Amortization of intangible assets												-1,900
	Interest expense												-42,416
	Foreign exchange loss												-7,361
	Income before income taxes											$	286,305
	Loss and loss expense ratio			67.20%			35.20%			50.50%			54.50%
	Acquisition cost ratio			13.30%			-0.60%			17.30%			12.60%
	General and administrative expense ratio			20.10%			29.10%			9.10%			17.00%
	Combined ratio			100.60%			63.70%			76.90%			84.10%
						International
	Nine Months Ended September 30, 2012		U.S. Insurance			Insurance			Reinsurance			Total
	Gross premiums written		$	733,314		$	418,498		$	680,407		$	1,832,219
	Net premiums written			551,286			255,150			668,764			1,475,200
	Net premiums earned			490,091			247,805			534,758			1,272,654
	Net losses and loss expenses			-309,889			-75,432			-339,209			-724,530
	Acquisition costs			-63,918			1,376			-87,270			-149,812
General and administrative expenses			-103,162			-66,969			-52,786			-222,917
Underwriting income			13,122			106,780			55,493			175,395
Net investment income												128,781
Net realized investment gains												292,057
Amortization of intangible assets												-1,900
Interest expense												-41,579
Foreign exchange gain												77
Income before income taxes											$	552,831
Loss and loss expense ratio			63.20%			30.40%			63.40%			56.90%
Acquisition cost ratio			13.00%			-0.60%			16.30%			11.80%
General and administrative expense ratio			21.00%			27.00%			9.90%			17.50%
Combined ratio			97.20%			56.80%			89.60%			86.20%
Gross premiums written by geographic location		Three Months Ended						Nine Months Ended
		September 30,						September 30,
		2013			2012			2013			2012
	United States	$	377,618		$	312,807		$	1,296,212		$	1,051,355
	Bermuda		111,103			108,955			550,815			484,313
	Europe		52,004			46,836			198,747			182,220
	Singapore		35,515			31,902			122,546			101,213
	Hong Kong		4,653			3,920			14,854			13,118
	Total gross premiums written	$	580,893		$	504,420		$	2,183,174		$	1,832,219

Condensed_Consolidated_Guarant1

Condensed Consolidated Guarantor Financial Statements (Tables)	9 Months Ended
	Sep. 30, 2013
Condensed Consolidated Guarantor Financial Statements [Abstract]
Schedule of condensed consolidating balance sheet	Unaudited Condensed Consolidating Balance Sheet:
		Allied World			Allied World
		Switzerland			Bermuda			Other Allied						Allied World
		(Parent			(Subsidiary			World			Consolidating			Switzerland
	As of September 30, 2013	Guarantor)			Issuer)			Subsidiaries			Adjustments			Consolidated
	ASSETS:
	Investments	$	0		$	0		$	7,661,837		$	0		$	7,661,837
	Cash and cash equivalents		56,060			8,388			888,599			0			953,047
	Insurance balances receivable		0			0			740,112			0			740,112
	Funds held		0			0			375,131			0			375,131
	Reinsurance recoverable		0			0			1,226,034			0			1,226,034
	Net deferred acquisition costs		0			0			145,951			0			145,951
	Goodwill and intangible assets		0			0			317,840			0			317,840
	Balances receivable on sale of investments		0			0			237,031			0			237,031
	Investments in subsidiaries		3,407,988			4,509,074			0			-7,917,062			0
	Due (to) from subsidiaries		-967			-14,679			15,646			0			0
	Other assets		1,897			5,038			713,602			0			720,537
	Total assets	$	3,464,978		$	4,507,821		$	12,321,783		$	-7,917,062		$	12,377,520
	LIABILITIES:
	Reserve for losses and loss expenses	$	0		$	0		$	5,780,781		$	0		$	5,780,781
	Unearned premiums		0			0			1,515,746			0			1,515,746
	Reinsurance balances payable		0			0			193,643			0			193,643
	Balances due on purchases of investments		0			0			497,974			0			497,974
	Senior notes		0			798,426			0			0			798,426
	Other liabilities		21,050			17,457			108,515			0			147,022
	Total liabilities		21,050			815,883			8,096,659			0			8,933,592
	Total shareholders’ equity		3,443,928			3,691,938			4,225,124			-7,917,062			3,443,928
	Total liabilities and shareholders’ equity	$	3,464,978		$	4,507,821		$	12,321,783		$	-7,917,062		$	12,377,520
		Allied World			Allied World
		Switzerland			Bermuda			Other Allied						Allied World
		(Parent			(Subsidiary			World			Consolidating			Switzerland
	As of December 31, 2012	Guarantor)			Issuer)			Subsidiaries			Adjustments			Consolidated
	ASSETS:
	Investments	$	0		$	0		$	7,933,937		$	0		$	7,933,937
	Cash and cash equivalents		19,997			11,324			650,558			0			681,879
	Insurance balances receivable		0			0			510,532			0			510,532
	Funds held		0			0			336,368			0			336,368
	Reinsurance recoverable		0			0			1,141,110			0			1,141,110
	Net deferred acquisition costs		0			0			108,010			0			108,010
	Goodwill and intangible assets		0			0			319,741			0			319,741
	Balances receivable on sale of investments		0			0			418,879			0			418,879
	Investments in subsidiaries		3,337,446			4,768,769			0			-8,106,215			0
	Due (to) from subsidiaries		-23,864			-7,173			31,037			0			0
	Other assets		1,499			6,081			571,910			0			579,490
	Total assets	$	3,335,078		$	4,779,001		$	12,022,082		$	-8,106,215		$	12,029,946
	LIABILITIES:
	Reserve for losses and loss expenses	$	0		$	0		$	5,645,549		$	0		$	5,645,549
	Unearned premiums		0			0			1,218,021			0			1,218,021
	Reinsurance balances payable		0			0			136,264			0			136,264
	Balances due on purchases of investments		0			0			759,934			0			759,934
	Senior notes		0			798,215			0			0			798,215
	Other liabilities		8,743			17,727			119,158			0			145,628
	Total liabilities		8,743			815,942			7,878,926			0			8,703,611
	Total shareholders’ equity		3,326,335			3,963,059			4,143,156			-8,106,215			3,326,335
	Total liabilities and shareholders’ equity	$	3,335,078		$	4,779,001		$	12,022,082		$	-8,106,215		$	12,029,946
Schedule of condensed consolidating income statement	Unaudited Condensed Consolidating Income Statement:
			Allied World			Allied World
			Switzerland			Bermuda			Other Allied						Allied World
			(Parent			(Subsidiary			World			Consolidating			Switzerland
	Three Months Ended September 30, 2013		Guarantor)			Issuer)			Subsidiaries			Adjustments			Consolidated
	Net premiums earned		$	0		$	0		$	510,773		$	0		$	510,773
	Net investment income			3			0			39,268			0			39,271
	Net realized investment losses			0			0			27,487			0			27,487
	Net losses and loss expenses			0			0			-276,970			0			-276,970
	Acquisition costs			0			0			-65,114			0			-65,114
	General and administrative expenses			-7,323			-4,961			-76,269			0			-88,553
	Amortization of intangible assets			0			0			-633			0			-633
	Interest expense			0			-13,838			-256			0			-14,094
	Foreign exchange gain (loss)			-13			-212			-4,128			0			-4,353
	Income tax (expense) benefit			0			0			-4,971			0			-4,971
	Equity in earnings of consolidated subsidiaries			130,176			150,653			0			-280,829			0
	NET INCOME (LOSS)		$	122,843		$	131,642		$	149,187		$	-280,829		$	122,843
	Other comprehensive income			0			0			0			0			0
	COMPREHENSIVE INCOME (LOSS)		$	122,843		$	131,642		$	149,187		$	-280,829		$	122,843
			Allied World			Allied World
			Switzerland			Bermuda			Other Allied						Allied World
			(Parent			(Subsidiary			World			Consolidating			Switzerland
	Three Months Ended September 30, 2012		Guarantor)			Issuer)			Subsidiaries			Adjustments			Consolidated
	Net premiums earned		$	0		$	0		$	441,017		$	0		$	441,017
	Net investment income			7			6			39,108			0			39,121
	Net realized investment gains			0			0			149,813			0			149,813
	Net losses and loss expenses			0			0			-258,948			0			-258,948
	Acquisition costs			0			0			-51,086			0			-51,086
	General and administrative expenses			-6,013			-235			-72,324			0			-78,572
	Amortization of intangible assets			0			0			-633			0			-633
	Interest expense			0			-13,822			0			0			-13,822
	Foreign exchange gain (loss)			-206			-83			-734			0			-1,023
	Income tax (expense) benefit			0			0			-6,220			0			-6,220
	Equity in earnings of consolidated subsidiaries			225,859			231,471			0			-457,330			0
	NET INCOME (LOSS)		$	219,647		$	217,337		$	239,993		$	-457,330		$	219,647
	Unrealized losses on investments arising during the period
		net of applicable deferred income tax benefit of $15		-29			0			-29			29			-29
	Reclassification adjustment for net realized investment
		gains included in net income, net of applicable income tax		0			0			0			0			0
	Other comprehensive loss			-29			0			-29			29			-29
	COMPREHENSIVE INCOME (LOSS)		$	219,618		$	217,337		$	239,964		$	-457,301		$	219,618
			Allied World			Allied World
			Switzerland			Bermuda			Other Allied						Allied World
			(Parent			(Subsidiary			World			Consolidating			Switzerland
	Nine Months Ended September 30, 2013		Guarantor)			Issuer)			Subsidiaries			Adjustments			Consolidated
	Net premiums earned		$	0		$	0		$	1,481,272		$	0		$	1,481,272
	Net investment income			11			4			110,279			0			110,294
	Net realized investment losses			0			0			-8,074			0			-8,074
	Net losses and loss expenses			0			0			-807,276			0			-807,276
	Acquisition costs			0			0			-186,416			0			-186,416
	General and administrative expenses			-26,875			-5,873			-219,070			0			-251,818
	Amortization of intangible assets			0			0			-1,900			0			-1,900
	Interest expense			0			-41,503			-913			0			-42,416
	Foreign exchange loss (gain)			261			-935			-6,687			0			-7,361
	Income tax (expense) benefit			0			0			-6,332			0			-6,332
	Equity in earnings of consolidated subsidiaries			306,576			353,280			0			-659,856			0
	NET INCOME (LOSS)		$	279,973		$	304,973		$	354,883		$	-659,856		$	279,973
	Other comprehensive income			0			0			0			0			0
	COMPREHENSIVE INCOME (LOSS)		$	279,973		$	304,973		$	354,883		$	-659,856		$	279,973
			Allied World			Allied World
			Switzerland			Bermuda			Other Allied						Allied World
			(Parent			(Subsidiary			World			Consolidating			Switzerland
	Nine Months Ended September 30, 2012		Guarantor)			Issuer)			Subsidiaries			Adjustments			Consolidated
	Net premiums earned		$	0		$	0		$	1,272,654		$	0		$	1,272,654
Net investment income			21			17			128,743			0			128,781
Net realized investment gains			0			0			292,057			0			292,057
Net losses and loss expenses			0			0			-724,530			0			-724,530
Acquisition costs			0			0			-149,812			0			-149,812
General and administrative expenses			-14,247			-2,689			-205,981			0			-222,917
Amortization of intangible assets			0			0			-1,900			0			-1,900
Interest expense			0			-41,579			0			0			-41,579
Foreign exchange gain (loss)			343			-150			-116			0			77
Income tax (expense) benefit			71			0			-18,748			0			-18,677
Equity in earnings of consolidated subsidiaries			547,966			580,880			0			-1,128,846			0
NET INCOME (LOSS)		$	534,154		$	536,479		$	592,367		$	-1,128,846		$	534,154
Unrealized gains on investments arising during the period
	net of applicable deferred income tax expense of $81		150			0			150			-150			150
Reclassification adjustment for net realized investment
	gains included in net income, net of applicable income tax		-13,249			0			-13,249			13,249			-13,249
Other comprehensive loss			-13,099			0			-13,099			13,099			-13,099
COMPREHENSIVE INCOME (LOSS)		$	521,055		$	536,479		$	579,268		$	-1,115,747		$	521,055
Schedule of condensed consolidating cash flows	Unaudited Condensed Consolidating Cash Flows:
			Allied World			Allied World
			Switzerland			Bermuda			Other Allied						Allied World
			(Parent			(Subsidiary			World			Consolidating			Switzerland
	Nine Months Ended September 30, 2013		Guarantor)			Issuer)			Subsidiaries			Adjustments			Consolidated
	CASH FLOWS PROVIDED BY (USED IN)
		OPERATING ACTIVITIES	$	177,859		$	-2,936		$	157,127		$	0		$	332,050
	CASH FLOWS PROVIDED BY (USED IN)
		INVESTING ACTIVITIES:
	Purchases trading securities			0			0			-4,955,817			0			-4,955,817
	Purchases of other invested assets			0			0			-211,501			0			-211,501
	Sales of available for sale securities			0			0			0			0			0
	Sales of trading securities			0			0			5,137,280			0			5,137,280
	Sales of other invested assets			0			0			189,155			0			189,155
	Other			0			0			-78,203			0			-78,203
		Net cash provided by (used in) investing activities		0			0			80,914			0			80,914
	CASH FLOWS PROVIDED BY (USED IN)
		FINANCING ACTIVITIES:
	Dividends paid - partial par value reduction			-12,981			0			0			0			-12,981
	Dividends paid			-17,117			0			0			0			-17,117
	Proceeds from the exercise of stock options			8,465			0			0			0			8,465
	Share repurchases			-120,163			0			0			0			-120,163
		Net cash provided by (used in) financing activities		-141,796			0			0			0			-141,796
	NET INCREASE (DECREASE) IN CASH AND
		CASH EQUIVALENTS		36,063			-2,936			238,041			0			271,168
	CASH AND CASH EQUIVALENTS, BEGINNING
		OF PERIOD		19,997			11,324			650,558			0			681,879
	CASH AND CASH EQUIVALENTS, END OF
		PERIOD	$	56,060		$	8,388		$	888,599		$	0		$	953,047
			Allied World			Allied World
			Switzerland			Bermuda			Other Allied						Allied World
			(Parent			(Subsidiary			World			Consolidating			Switzerland
	Nine Months Ended September 30, 2012		Guarantor)			Issuer)			Subsidiaries			Adjustments			Consolidated
	CASH FLOWS PROVIDED BY (USED IN)
		OPERATING ACTIVITIES:	$	162,141		$	687		$	336,824		$	0		$	499,652
	CASH FLOWS PROVIDED BY (USED IN)
		INVESTING ACTIVITIES:
	Purchases of available for sale securities			0			0			0			0			0
	Purchases of trading securities			0			0			-6,328,719			0			-6,328,719
	Purchases of other invested assets			0			0			-52,578			0			-52,578
	Sales of available for sale securities			0			0			215,318			0			215,318
	Sales of trading securities			0			0			5,778,138			0			5,778,138
	Sales of other invested assets			0			0			110,429			0			110,429
	Other			0			0			33,644			0			33,644
		Net cash provided by (used in) investing activities		0			0			-243,768			0			-243,768
	CASH FLOWS PROVIDED BY (USED IN)
		FINANCING ACTIVITIES:
	Partial par value reduction			-54,721			0			0			0			-54,721
	Proceeds from the exercise of stock options			9,104			0			0			0			9,104
	Share repurchases			-204,746			0			0			0			-204,746
		Net cash provided by (used in) financing activities		-250,363			0			0			0			-250,363
	NET INCREASE (DECREASE) IN CASH AND
		CASH EQUIVALENTS		-88,222			687			93,056			0			5,521
	CASH AND CASH EQUIVALENTS, BEGINNING
		OF PERIOD		112,672			8,886			512,438			0			633,996
	CASH AND CASH EQUIVALENTS, END OF
		PERIOD	$	24,450		$	9,573		$	605,494		$	0		$	639,517

Investments_Narrative_Details

Investments (Narrative) (Details) (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013	Dec. 31, 2012
	Minimum [Member]	Minimum [Member]	Maximum [Member]	Maximum [Member]	Compliance With Reinsurance Contract Provisions Or Insurance Laws [Member]	Compliance With Reinsurance Contract Provisions Or Insurance Laws [Member]	Pledged As Collateral For Letter Of Credit Facilities [Member]	Pledged As Collateral For Letter Of Credit Facilities [Member]
	Subject To Redemption Restriction [Member]	Not Subject To Redemption Restriction [Member]	Subject To Redemption Restriction [Member]	Not Subject To Redemption Restriction [Member]
		Hedge Funds [Member]		Hedge Funds [Member]
Pledged assets:
Pledged assets					$2,556,660	$2,141,249	$1,063,086	$1,225,155
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Period of redemption restriction	1 year		3 years
Required notification period for redemption		P30D
Gate trigger, percentage		15.00%		25.00%

Investments_Schedule_Of_Tradin

Investments (Schedule Of Trading Securities) (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	$6,047,883	$6,626,454
Fixed Maturities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Amortized Cost	5,996,065	6,473,429
U.S. Government And Government Agencies [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	1,850,335	1,865,913
Trading securities, Amortized Cost	1,851,002	1,854,198
Non-U.S. Government And Government Agencies [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	209,625	261,627
Trading securities, Amortized Cost	214,114	253,657
States, Municipalities And Political Subdivisions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	95,381	40,444
Trading securities, Amortized Cost	95,661	39,342
Corporate Debt, Financial Institutions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	841,930	866,140
Trading securities, Amortized Cost	826,763	835,587
Corporate Debt, Industrials [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	1,146,793	1,153,909
Trading securities, Amortized Cost	1,140,636	1,139,706
Corporate Debt, Utilities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	74,764	69,153
Trading securities, Amortized Cost	74,865	67,463
Mortgage-Backed [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	1,373,161	1,958,373
Trading securities, Amortized Cost	1,341,303	1,877,854
Asset-Backed [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading fixed maturity investments	455,894	410,895
Trading securities, Amortized Cost	451,721	405,622
Equity securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	708,929	523,949
Trading securities, Amortized Cost	679,228	480,312
Other Invested Assets [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	767,071	655,888
Trading securities, Amortized Cost	680,986	606,521
Equity Securities And Other Invested Assets [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	1,476,000	1,179,837
Trading securities, Amortized Cost	$1,360,214	$1,086,833

Investments_Schedule_Of_Other_

Investments (Schedule Of Other Investments) (Details) (USD $)

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

In Thousands, unless otherwise specified

Investments with redemption restrictions [Member]

Investments without redemption restrictions [Member]

Total Private Equity Structures [Member]

Total Private Equity Structures [Member]

Total Private Equity Structures [Member]

Private Equity [Member]

Private Equity [Member]

Private Equity [Member]

Private Equity [Member]

Private Equity [Member]

Mezzanine Debt [Member]

Mezzanine Debt [Member]

Mezzanine Debt [Member]

Mezzanine Debt [Member]

Mezzanine Debt [Member]

Distressed [Member]

Distressed [Member]

Distressed [Member]

Distressed [Member]

Distressed [Member]

Hedge Funds [Member]

Hedge Funds [Member]

Hedge Funds [Member]

Hedge Funds [Member]

Distressed Funds [Member]

Distressed Funds [Member]

Distressed Funds [Member]

Distressed Funds [Member]

Distressed Funds [Member]

Distressed Funds [Member]

Distressed Funds [Member]

Equity Long/Short [Member]

Equity Long/Short [Member]

Equity Long/Short [Member]

Equity Long/Short [Member]

Equity Long/Short [Member]

Multi-Strategy Funds [Member]

Multi-Strategy Funds [Member]

Multi-Strategy Funds [Member]

Multi-Strategy Funds [Member]

Multi-Strategy Funds [Member]

Global Macro [Member]

Global Macro [Member]

Global Macro [Member]

Event Driven Funds [Member]

Event Driven Funds [Member]

Event Driven Funds [Member]

Event Driven Funds [Member]

Relative Value Credit [Member]

Relative Value Credit [Member]

Relative Value Credit [Member]

Other Private Securities [Member]

Other Private Securities [Member]

Other Private Securities [Member]

High Yield Loan Fund [Member]

High Yield Loan Fund [Member]

High Yield Loan Fund [Member]

Investments with redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments without redemption restrictions [Member]

Investments with redemption restrictions [Member]

Investments without redemption restrictions [Member]

Minimum [Member]

Maximum [Member]

Minimum [Member]

Maximum [Member]

Minimum [Member]

Maximum [Member]

Minimum [Member]

Minimum [Member]

Maximum [Member]

Minimum [Member]

Maximum [Member]

Minimum [Member]

Maximum [Member]

Minimum [Member]

Maximum [Member]

Maximum [Member]

Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]

Other invested assets

$905,025

$783,534

$332,683

$572,342

[1]

$198,556

$198,556

$0

[1]

$131,449

$131,449

$0

[1]

$58,116

$58,116

$0

[1]

$8,991

$8,991

$0

[1]

$537,415

$134,127

$403,288

[1]

$150,775

$134,127

$16,648

[1]

$116,019

$0

$116,019

[1]

$131,224

$0

$131,224

[1]

$19,641

$0

$19,641

[1]

$18,445

$0

$18,445

[1]

$101,311

$0

$101,311

[1]

$137,954

$0

$137,954

[1]

$31,100

$0

$31,100

[1]

Estimated remaining restriction period

3 years

10 years

8 years

10 years

4 years

5 years

1 year

2 years

Redemption frequency

Quarterly

[1]

Quarterly

[1]

Quarterly

[1]

Monthly

[1]

Annual

[1]

Quarterly

[1]

Monthly

[1]

Required notification period for redemption

P30D

P45D

[1]

P65D

[1]

P30D

[1]

P60D

[1]

P45D

[1]

P90D

[1]

P3D

[1]

P60D

[1]

[1]

P60D

[1]

P30D

[1]

Unfunded Commitments

$466,642

$461,642

$252,571

$203,969

$5,102

$0

$0

$0

$0

$0

$0

$0

$5,000

$0

[1]

The redemption frequency and notice periods only apply to the investments without redemption restrictions. Some or all of these investments may be subject to a gate.

Investments_Schedule_Of_Net_In

Investments (Schedule Of Net Investment Income) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net investment income	$39,271	$39,121	$110,294	$128,781
Fixed maturity investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net investment income	31,179	36,778	96,366	120,883
Equity securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net investment income	6,110	5,211	13,718	13,706
Other invested assets [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net investment income	5,809	164	11,116	4,387
Cash and cash equivalents [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net investment income	302	640	1,319	1,797
Expenses [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net investment income	($4,129)	($3,672)	($12,225)	($11,992)

Investments_Components_Of_Real

Investments (Components Of Realized Gains And Losses) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Gain (Loss) on Investments [Line Items]
Gross realized gains on sale of invested assets	$51,915	$27,210	$154,387	$119,154
Gross realized losses on sale of invested assets	-40,770	-6,686	-82,812	-43,355
Net realized and unrealized (losses) gains on derivatives	-4,169	-962	3,392	-192
Net realized investment gains (losses)	27,487	149,813	-8,074	292,057
Proceeds from sale of available for sale securities	0	1,000	0	214,716
Fixed maturity investments, trading
Gain (Loss) on Investments [Line Items]
Mark to market changes gains (losses)	30,383	99,821	-101,205	144,024
Equity securities [Member]
Gain (Loss) on Investments [Line Items]
Mark to market changes gains (losses)	-17,198	18,913	-18,555	38,516
Other invested assets [Member]
Gain (Loss) on Investments [Line Items]
Mark to market changes gains (losses)	$7,326	$11,517	$36,719	$33,910

Derivative_Instruments_Locatio

Derivative Instruments (Location and Amounts of Derivative Fair Values On The Condensed Consolidated Balance Sheets) (Details) (Derivatives not designated as hedging instruments [Member], USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Derivatives, Fair Value [Line Items]
Asset Derivative Notional Amount	$230,811	$132,864
Asset Derivative Fair Value	2,498	2,245
Liability Derivative Notional Amount	606,050	194,566
Liability Derivative Fair Value	7,234	2,656
Put options [Member] | Equity securities trading, at fair value [Member]
Derivatives, Fair Value [Line Items]
Asset Derivative Notional Amount	0	5,152
Asset Derivative Fair Value	0	532
Liability Derivative Notional Amount	0	0
Liability Derivative Fair Value	0	0
Foreign exchange contracts | Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivative Notional Amount	197,811	127,712
Asset Derivative Fair Value	2,493	1,713
Foreign exchange contracts | Accounts payable and accrued liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivative Notional Amount	279,050	194,566
Liability Derivative Fair Value	7,062	2,656
Interest rate swaps | Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivative Notional Amount	33,000	0
Asset Derivative Fair Value	5	0
Interest rate swaps | Accounts payable and accrued liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivative Notional Amount	327,000	0
Liability Derivative Fair Value	$172	$0

Derivative_Instruments_Locatio1

Derivative Instruments (Location and Amounts of Unrealized And Realized Gains (Losses) On Derivatives Recorded In The Condensed Consolidated Income Statements) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized (losses) gains on derivatives	($6,505)	($1,638)	$2,301	($288)
Foreign exchange (loss) gain [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized (losses) gains on derivatives	-2,336	-676	-1,091	-96
Foreign exchange (loss) gain [Member] | Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized (losses) gains on derivatives	-2,336	-676	-1,091	-96
Net realized investment gains [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized (losses) gains on derivatives	-4,169	-962	3,392	-192
Net realized investment gains [Member] | Put options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized (losses) gains on derivatives	0	0	-3,822	-336
Net realized investment gains [Member] | Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized (losses) gains on derivatives	-4,164	-2,751	1,925	-641
Net realized investment gains [Member] | Interest rate futures
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized (losses) gains on derivatives	($5)	$1,789	$5,289	$785

Fair_Value_Of_Financial_Instru2

Fair Value Of Financial Instruments (Schedule Of Fair Value Of Financial Instruments) (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	$6,047,883	$6,626,454
Equity securities	708,929	523,949
Other invested assets	905,025	783,534
Total investments	7,661,837	7,933,937
Senior notes	798,426	798,215
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	6,047,883	6,626,454
Equity securities	708,929	523,949
Other invested assets	767,071	655,888
Total investments	7,523,883	7,806,291
Senior notes	798,426	798,215
Fair Value, Measurements, Recurring [Member] | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	2,493	1,713
Derivative liabilities	7,062	2,656
Fair Value, Measurements, Recurring [Member] | Interest rate futures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	5
Fair Value, Measurements, Recurring [Member] | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	172
Fair Value, Measurements, Recurring [Member] | U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	1,850,335	1,865,913
Fair Value, Measurements, Recurring [Member] | Non-U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	209,625	261,627
Fair Value, Measurements, Recurring [Member] | States, Municipalities And Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	95,381	40,444
Fair Value, Measurements, Recurring [Member] | Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	2,063,487	2,089,202
Fair Value, Measurements, Recurring [Member] | Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	1,373,161	1,958,373
Fair Value, Measurements, Recurring [Member] | Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	455,894	410,895
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	6,047,883	6,626,454
Equity securities	708,929	523,949
Other invested assets	767,071	655,888
Total investments	7,523,883	7,806,291
Senior notes	904,010	918,627
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	2,493	1,713
Derivative liabilities	7,062	2,656
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Interest rate futures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	5
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	172
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	1,850,335	1,865,913
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Non-U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	209,625	261,627
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | States, Municipalities And Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	95,381	40,444
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	2,063,487	2,089,202
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	1,373,161	1,958,373
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	455,894	410,895
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	1,522,185	1,529,158
Equity securities	641,458	469,269
Other invested assets	0	0
Total investments	2,163,643	1,998,427
Senior notes	0	0
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Interest rate futures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	0
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	1,522,185	1,529,158
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Non-U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | States, Municipalities And Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	4,256,411	4,867,225
Equity securities	0	0
Other invested assets	0	0
Total investments	4,256,411	4,867,225
Senior notes	904,010	918,627
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	2,493	1,713
Derivative liabilities	7,062	2,656
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Interest rate futures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	5
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	172
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	328,150	336,755
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Non-U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	209,625	261,627
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | States, Municipalities And Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	95,381	40,444
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	2,063,487	2,089,202
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	1,183,907	1,790,548
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	375,861	348,649
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	269,287	230,071
Equity securities	67,471	54,680
Other invested assets	767,071	655,888
Total investments	1,103,829	940,639
Senior notes	0	0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Interest rate futures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Non-U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | States, Municipalities And Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	189,254	167,825
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	$80,033	$62,246

Fair_Value_Of_Financial_Instru3

Fair Value Of Financial Instruments (Schedule Of Fair Value Measurement Using Significant Unobservable Inputs Level Three) (Details) (USD $)	3 Months Ended				9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Other Invested Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Opening balance	$714,391		$520,890		$655,888		$540,409
Realized and unrealized gains (losses) included in net income	9,403		11,871		53,365		26,753
Purchases	67,554		34,800		237,506		52,578
Sales	-24,277		-2,859		-179,688		-55,038
Transfers into Level 3 from Level 2	0		0		0		0
Transfers out of Level 3 to Level 2	0	[1]	0	[1]	0	[1]	0	[1]
Ending balance	767,071		564,702		767,071		564,702
Mortgage Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Opening balance	198,003		157,959		167,825		249,204
Realized and unrealized gains (losses) included in net income	464		4,855		-5,910		10,951
Purchases	69,775		40,481		102,369		50,302
Sales	-79,001		-48,728		-69,968		-124,940
Transfers into Level 3 from Level 2	13		14,730		5,073		18,461
Transfers out of Level 3 to Level 2	0	[1]	-27	[1]	-10,135	[1]	-34,708	[1]
Ending balance	189,254		169,270		189,254		169,270
Asset Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Opening balance	61,285		117,586		62,246		94,745
Realized and unrealized gains (losses) included in net income	-313		988		-791		1,643
Purchases	16,969		7,466		42,956		32,573
Sales	-1,302		-7,326		-26,728		-57,325
Transfers into Level 3 from Level 2	3,394		12,495		2,350		15,835
Transfers out of Level 3 to Level 2	0	[1]	-55,442	[1]	0	[1]	-11,704	[1]
Ending balance	80,033		75,767		80,033		75,767
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Opening balance	53,499				54,680
Realized and unrealized gains (losses) included in net income	3,972				2,791
Purchases	10,000				10,000
Ending balance	$67,471				$67,471
[1]	Transfers out of Level 3 are primarily attributable to the availability of market observable information.

Reserve_For_Losses_And_Loss_Ex2

Reserve For Losses And Loss Expenses (Schedule Of Reserve For Losses And Loss Expenses) (Details) (USD $)	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
In Thousands, unless otherwise specified
Reserve For Losses And Loss Expenses [Abstract]
Outstanding loss reserves	$1,517,049		$1,539,114
Reserves for losses incurred but not reported	4,263,732		4,106,435
Reserve for losses and loss expenses	$5,780,781	$5,696,865	$5,645,549	$5,450,787	$5,377,518	$5,225,143

Reserve_For_Losses_And_Loss_Ex3

Reserve For Losses And Loss Expenses (Schedule Of Reconciliation Of Unpaid Losses And Loss Expenses) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Reserve For Losses And Loss Expenses [Abstract]
Gross liability at beginning of year	$5,696,865	$5,377,518	$5,645,549	$5,225,143
Reinsurance recoverable at beginning of year	-1,179,525	-1,073,612	-1,141,110	-1,002,919
Net liability at beginning of year	4,517,340	4,303,906	4,504,439	4,222,224
Net losses incurred related to:
Current year	338,420	315,102	961,224	862,088
Prior years	-61,450	-56,154	-153,948	-137,558
Total incurred	276,970	258,948	807,276	724,530
Net paid losses related to:
Current year	30,399	16,323	54,983	36,163
Prior years	213,252	179,666	696,137	542,077
Total paid	243,651	195,989	751,120	578,240
Foreign exchange revaluation	4,088	6,400	-5,848	4,751
Net liability at end of year	4,554,747	4,373,265	4,554,747	4,373,265
Reinsurance recoverable at end of year	1,226,034	1,077,522	1,226,034	1,077,522
Gross liability at end of year	$5,780,781	$5,450,787	$5,780,781	$5,450,787

Shareholders_Equity_Narrative_

Shareholders' Equity (Narrative) (Details) (CHF)	0 Months Ended	9 Months Ended
In Thousands, except Share data, unless otherwise specified	Jul. 09, 2013	Sep. 30, 2013
Class of Stock [Line Items]
Shares cancelled, shares		1,397,073
Maximum authorized value increase of registered share capital		92,259
Voting Common Stock [Member]
Class of Stock [Line Items]
Shares cancelled, shares		940,445
Maximum authorized increased voting shares		7,500,728
Nonvoting Common Stock [Member]
Class of Stock [Line Items]
Shares cancelled, shares	29,240
Common Shares Repurchased And Constructively Retired [Member]
Class of Stock [Line Items]
Shares cancelled, shares	1,538,686	1,367,833

Shareholders_Equity_Issued_Sha

Shareholders' Equity (Issued Share Capital) (Details)	Sep. 30, 2013	Sep. 30, 2013	Dec. 31, 2012	Dec. 31, 2012
In Thousands, except Share data, unless otherwise specified	USD ($)	CHF	USD ($)	CHF
Shareholders' Equity [Abstract]
Common shares, issued	34,972,795	34,972,795	36,369,868	36,369,868
Common shares, par value		12.3		12.64
Common stock value	$424,837		$454,980

Shareholders_Equity_Schedule_O

Shareholders' Equity (Schedule Of Common Stock Outstanding Roll Forward) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Shares issued:
Total shares issued at beginning of period			36,369,868
Shares cancelled, shares			-1,397,073
Total shares issued at end of period	34,972,795		34,972,795
Treasury shares:
Treasury shares issued, balance at beginning of period			1,572,087
Shares repurchased, shares	427,388	605,898	1,367,833	2,942,085
Shares issued out of treasury			-384,972
Shares cancelled, shares			-1,397,073
Total treasury shares at end of period	1,157,875		1,157,875
Total shares outstanding at end of period	33,814,920		33,814,920		34,797,781

Shareholders_Equity_Schedule_O1

Shareholders' Equity (Schedule Of Dividends Paid) (Details)	0 Months Ended	3 Months Ended		9 Months Ended		0 Months Ended
In Thousands, except Per Share data, unless otherwise specified	3-May-12	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Jul. 03, 2013	2-May-13	Mar. 12, 2013	Mar. 12, 2013	3-May-12
	installments	USD ($)	USD ($)	USD ($)	USD ($)	Voting Common Stock [Member]	Voting Common Stock [Member]	Voting Common Stock [Member]	Voting Common Stock [Member]	Voting Common Stock [Member]
						USD ($)	USD ($)	Par Value Reduction [Member]	Par Value Reduction [Member]	Par Value Reduction [Member]
								USD ($)	CHF	USD ($)
Dividends:
Dividends declared per share							$0.50			$0.38
Dividends paid per common share		$0.50	$0.75	$0.88	$1.50	$0.50		$0.38	0.34
Payment of quarterly dividends						$17,117		$12,981
Par value reductions, number of installments	4

Shareholders_Equity_Schedule_O2

Shareholders' Equity (Schedule Of Share Repurchases) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Share repurchases:
Shares repurchased, shares	427,388	605,898	1,367,833	2,942,085
Cost of shares repurchased	$40,574,000	$47,590,000	$123,145,000	$207,048,000
Shares repurchased, average price per share	$94.93	$78.54	$90.03	$70.37
Repurchase authorization approved			$500,000,000

Earnings_Per_Share_Narrative_D

Earnings Per Share (Narrative) (Details) (Stock Options And Rsus [Member])	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Stock Options And Rsus [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share	0	221,008	0	338,395

Earnings_Per_Share_Comparison_

Earnings Per Share (Comparison Of Basic And Diluted Earnings Per Share) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Basic earnings per share:
Net income	$122,843	$219,647	$279,973	$534,154
Weighted average common shares outstanding	33,991,359	35,652,768	34,340,227	36,379,514
Basic earnings per share	$3.61	$6.16	$8.15	$14.68
Diluted earnings per share:
Net income	$122,843	$219,647	$279,973	$534,154
Weighted average common shares outstanding	33,991,359	35,652,768	34,340,227	36,379,514
Share equivalents:
Options	500,114	461,373	497,458	429,393
Restricted stock units and performance-based equity awards	236,133	501,633	292,369	583,226
Employee share purchase plan	587	960	1,038	960
Weighted average common shares and common share equivalents outstanding - diluted	34,728,193	36,616,734	35,131,092	37,393,093
Diluted earnings per share	$3.54	$6	$7.97	$14.28

Segment_Information_Summary_Of

Segment Information (Summary Of Segment Results) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Gross premiums written	$580,893	$504,420	$2,183,174	$1,832,219
Net premiums written	453,077	391,537	1,729,351	1,475,200
Net premiums earned	510,773	441,017	1,481,272	1,272,654
Net losses and loss expenses	-276,970	-258,948	-807,276	-724,530
Acquisition costs	-65,114	-51,086	-186,416	-149,812
General and administrative expenses	-88,553	-78,572	-251,818	-222,917
Underwriting (loss) income	80,136	52,411	235,762	175,395
Net investment income	39,271	39,121	110,294	128,781
Net realized investment gains (losses)	27,487	149,813	-8,074	292,057
Amortization of intangible assets	-633	-633	-1,900	-1,900
Interest expense	-14,094	-13,822	-42,416	-41,579
Foreign exchange (loss) gain	-4,353	-1,023	-7,361	77
Income before income taxes	127,814	225,867	286,305	552,831
Loss and loss expense ratio	54.20%	58.70%	54.50%	56.90%
Acquisition cost ratio	12.70%	11.60%	12.60%	11.80%
General and administrative expense ratio	17.30%	17.80%	17.00%	17.50%
Combined ratio	84.20%	88.10%	84.10%	86.20%
U.S. Insurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	308,709	263,129	872,024	733,314
Net premiums written	238,792	200,779	652,464	551,286
Net premiums earned	207,602	173,948	593,477	490,091
Net losses and loss expenses	-141,222	-109,111	-398,910	-309,889
Acquisition costs	-28,426	-22,696	-78,824	-63,918
General and administrative expenses	-41,616	-37,388	-119,514	-103,162
Underwriting (loss) income	-3,662	4,753	-3,771	13,122
Loss and loss expense ratio	68.00%	62.70%	67.20%	63.20%
Acquisition cost ratio	13.70%	13.00%	13.30%	13.00%
General and administrative expense ratio	20.00%	21.50%	20.10%	21.00%
Combined ratio	101.70%	97.20%	100.60%	97.20%
International Insurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	132,881	121,315	453,990	418,498
Net premiums written	75,632	71,199	259,771	255,150
Net premiums earned	87,554	85,329	258,809	247,805
Net losses and loss expenses	-31,094	-15,099	-90,997	-75,432
Acquisition costs	282	266	1,489	1,376
General and administrative expenses	-26,450	-22,920	-75,374	-66,969
Underwriting (loss) income	30,292	47,576	93,927	106,780
Loss and loss expense ratio	35.50%	17.70%	35.20%	30.40%
Acquisition cost ratio	-0.30%	-0.30%	-0.60%	-0.60%
General and administrative expense ratio	30.20%	26.90%	29.10%	27.00%
Combined ratio	65.40%	44.30%	63.70%	56.80%
Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	139,303	119,976	857,160	680,407
Net premiums written	138,653	119,559	817,116	668,764
Net premiums earned	215,617	181,740	628,986	534,758
Net losses and loss expenses	-104,654	-134,738	-317,369	-339,209
Acquisition costs	-36,970	-28,656	-109,081	-87,270
General and administrative expenses	-20,487	-18,264	-56,930	-52,786
Underwriting (loss) income	$53,506	$82	$145,606	$55,493
Loss and loss expense ratio	48.50%	74.10%	50.50%	63.40%
Acquisition cost ratio	17.10%	15.80%	17.30%	16.30%
General and administrative expense ratio	9.50%	10.00%	9.10%	9.90%
Combined ratio	75.10%	99.90%	76.90%	89.60%

Segment_Information_Gross_Prem

Segment Information (Gross Premiums Written By Geographic Location) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$580,893	$504,420	$2,183,174	$1,832,219
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	377,618	312,807	1,296,212	1,051,355
Bermuda [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	111,103	108,955	550,815	484,313
Europe [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	52,004	46,836	198,747	182,220
Singapore [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	35,515	31,902	122,546	101,213
Hong Kong [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	4,653	3,920	14,854	13,118
Switzerland [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$10,509	$7,039	$57,183	$40,474

Condensed_Consolidated_Guarant2

Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Balance Sheet) (Details) (USD $)	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
In Thousands, unless otherwise specified
ASSETS:
Investments	$7,661,837		$7,933,937
Cash and cash equivalents	953,047		681,879	639,517		633,996
Insurance balances receivable	740,112		510,532
Funds held	375,131		336,368
Reinsurance recoverable	1,226,034	1,179,525	1,141,110	1,077,522	1,073,612	1,002,919
Net deferred acquisition costs	145,951		108,010
Goodwill and intangible assets	317,840		319,741
Balances receivable on sale of investments	237,031		418,879
Investments in subsidiaries	0		0
Due (to) from subsidiaries	0		0
Other assets	720,537		579,490
Total assets	12,377,520		12,029,946
LIABILITIES
Reserve for losses and loss expenses	5,780,781	5,696,865	5,645,549	5,450,787	5,377,518	5,225,143
Unearned premiums	1,515,746		1,218,021
Reinsurance balances payable	193,643		136,264
Balances due on purchases of investments	497,974		759,934
Senior notes	798,426		798,215
Other liabilities	147,022		145,628
Total liabilities	8,933,592		8,703,611
Total shareholders' equity	3,443,928		3,326,335	3,435,786		3,149,022
Total liabilities and shareholders' equity	12,377,520		12,029,946
Allied World Switzerland (Parent Guarantor) [Member]
ASSETS:
Investments	0		0
Cash and cash equivalents	56,060		19,997	24,450		112,672
Insurance balances receivable	0		0
Funds held	0		0
Reinsurance recoverable	0		0
Net deferred acquisition costs	0		0
Goodwill and intangible assets	0		0
Balances receivable on sale of investments	0		0
Investments in subsidiaries	3,407,988		3,337,446
Due (to) from subsidiaries	-967		-23,864
Other assets	1,897		1,499
Total assets	3,464,978		3,335,078
LIABILITIES
Reserve for losses and loss expenses	0		0
Unearned premiums	0		0
Reinsurance balances payable	0		0
Balances due on purchases of investments	0		0
Senior notes	0		0
Other liabilities	21,050		8,743
Total liabilities	21,050		8,743
Total shareholders' equity	3,443,928		3,326,335
Total liabilities and shareholders' equity	3,464,978		3,335,078
Allied World Bermuda (Subsidiary Issuer) [Member]
ASSETS:
Investments	0		0
Cash and cash equivalents	8,388		11,324	9,573		8,886
Insurance balances receivable	0		0
Funds held	0		0
Reinsurance recoverable	0		0
Net deferred acquisition costs	0		0
Goodwill and intangible assets	0		0
Balances receivable on sale of investments	0		0
Investments in subsidiaries	4,509,074		4,768,769
Due (to) from subsidiaries	-14,679		-7,173
Other assets	5,038		6,081
Total assets	4,507,821		4,779,001
LIABILITIES
Reserve for losses and loss expenses	0		0
Unearned premiums	0		0
Reinsurance balances payable	0		0
Balances due on purchases of investments	0		0
Senior notes	798,426		798,215
Other liabilities	17,457		17,727
Total liabilities	815,883		815,942
Total shareholders' equity	3,691,938		3,963,059
Total liabilities and shareholders' equity	4,507,821		4,779,001
Other Allied World Subsidiaries [Member]
ASSETS:
Investments	7,661,837		7,933,937
Cash and cash equivalents	888,599		650,558	605,494		512,438
Insurance balances receivable	740,112		510,532
Funds held	375,131		336,368
Reinsurance recoverable	1,226,034		1,141,110
Net deferred acquisition costs	145,951		108,010
Goodwill and intangible assets	317,840		319,741
Balances receivable on sale of investments	237,031		418,879
Investments in subsidiaries	0		0
Due (to) from subsidiaries	15,646		31,037
Other assets	713,602		571,910
Total assets	12,321,783		12,022,082
LIABILITIES
Reserve for losses and loss expenses	5,780,781		5,645,549
Unearned premiums	1,515,746		1,218,021
Reinsurance balances payable	193,643		136,264
Balances due on purchases of investments	497,974		759,934
Senior notes	0		0
Other liabilities	108,515		119,158
Total liabilities	8,096,659		7,878,926
Total shareholders' equity	4,225,124		4,143,156
Total liabilities and shareholders' equity	12,321,783		12,022,082
Consolidating Adjustments [Member]
ASSETS:
Investments	0		0
Cash and cash equivalents	0		0
Insurance balances receivable	0		0
Funds held	0		0
Reinsurance recoverable	0		0
Net deferred acquisition costs	0		0
Goodwill and intangible assets	0		0
Balances receivable on sale of investments	0		0
Investments in subsidiaries	-7,917,062		-8,106,215
Due (to) from subsidiaries	0		0
Other assets	0		0
Total assets	-7,917,062		-8,106,215
LIABILITIES
Reserve for losses and loss expenses	0		0
Unearned premiums	0		0
Reinsurance balances payable	0		0
Balances due on purchases of investments	0		0
Senior notes	0		0
Other liabilities	0		0
Total liabilities	0		0
Total shareholders' equity	-7,917,062		-8,106,215
Total liabilities and shareholders' equity	($7,917,062)		($8,106,215)

Condensed_Consolidated_Guarant3

Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Income Statement) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Condensed Financial Statements, Captions [Line Items]
Net premiums earned	$510,773	$441,017	$1,481,272	$1,272,654
Net investment income	39,271	39,121	110,294	128,781
Net realized investment losses	27,487	149,813	-8,074	292,057
Net losses and loss expenses	-276,970	-258,948	-807,276	-724,530
Acquisition costs	-65,114	-51,086	-186,416	-149,812
General and administrative expenses	-88,553	-78,572	-251,818	-222,917
Amortization of intangible assets	-633	-633	-1,900	-1,900
Interest expense	-14,094	-13,822	-42,416	-41,579
Foreign exchange (loss) gain	-4,353	-1,023	-7,361	77
Income tax (expense) benefit	-4,971	-6,220	-6,332	-18,677
Equity in earnings of consolidated subsidiaries	0	0	0	0
NET INCOME	122,843	219,647	279,973	534,154
Unrealized gains on investments arising during the period net of applicable deferred income tax benefit for three and nine months ended September 30, 2012: $15 and ($81), respectively	0	-29	0	150
Reclassification adjustment for net realized investment gains included in net income, net of applicable income tax	0	0	0	-13,249
Other comprehensive loss	0	-29	0	-13,099
COMPREHENSIVE INCOME (LOSS)	122,843	219,618	279,973	521,055
Parenthetical disclosures:
Deferred income tax (benefit) for unrealized gains on investments arising during the period	0	15	0	-81
Allied World Switzerland (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
Net premiums earned	0	0	0	0
Net investment income	3	7	11	21
Net realized investment losses	0	0	0	0
Net losses and loss expenses	0	0	0	0
Acquisition costs	0	0	0	0
General and administrative expenses	-7,323	-6,013	-26,875	-14,247
Amortization of intangible assets	0	0	0	0
Interest expense	0	0	0	0
Foreign exchange (loss) gain	-13	-206	261	343
Income tax (expense) benefit	0	0	0	71
Equity in earnings of consolidated subsidiaries	130,176	225,859	306,576	547,966
NET INCOME	122,843	219,647	279,973	534,154
Unrealized gains on investments arising during the period net of applicable deferred income tax benefit for three and nine months ended September 30, 2012: $15 and ($81), respectively		-29		150
Reclassification adjustment for net realized investment gains included in net income, net of applicable income tax		0		-13,249
Other comprehensive loss	0	-29	0	-13,099
COMPREHENSIVE INCOME (LOSS)	122,843	219,618	279,973	521,055
Allied World Bermuda (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
Net premiums earned	0	0	0	0
Net investment income	0	6	4	17
Net realized investment losses	0	0	0	0
Net losses and loss expenses	0	0	0	0
Acquisition costs	0	0	0	0
General and administrative expenses	-4,961	-235	-5,873	-2,689
Amortization of intangible assets	0	0	0	0
Interest expense	-13,838	-13,822	-41,503	-41,579
Foreign exchange (loss) gain	-212	-83	-935	-150
Income tax (expense) benefit	0	0	0	0
Equity in earnings of consolidated subsidiaries	150,653	231,471	353,280	580,880
NET INCOME	131,642	217,337	304,973	536,479
Unrealized gains on investments arising during the period net of applicable deferred income tax benefit for three and nine months ended September 30, 2012: $15 and ($81), respectively		0		0
Reclassification adjustment for net realized investment gains included in net income, net of applicable income tax		0		0
Other comprehensive loss	0	0	0	0
COMPREHENSIVE INCOME (LOSS)	131,642	217,337	304,973	536,479
Other Allied World Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net premiums earned	510,773	441,017	1,481,272	1,272,654
Net investment income	39,268	39,108	110,279	128,743
Net realized investment losses	27,487	149,813	-8,074	292,057
Net losses and loss expenses	-276,970	-258,948	-807,276	-724,530
Acquisition costs	-65,114	-51,086	-186,416	-149,812
General and administrative expenses	-76,269	-72,324	-219,070	-205,981
Amortization of intangible assets	-633	-633	-1,900	-1,900
Interest expense	-256	0	-913	0
Foreign exchange (loss) gain	-4,128	-734	-6,687	-116
Income tax (expense) benefit	-4,971	-6,220	-6,332	-18,748
Equity in earnings of consolidated subsidiaries	0	0	0	0
NET INCOME	149,187	239,993	354,883	592,367
Unrealized gains on investments arising during the period net of applicable deferred income tax benefit for three and nine months ended September 30, 2012: $15 and ($81), respectively		-29		150
Reclassification adjustment for net realized investment gains included in net income, net of applicable income tax		0		-13,249
Other comprehensive loss	0	-29	0	-13,099
COMPREHENSIVE INCOME (LOSS)	149,187	239,964	354,883	579,268
Parenthetical disclosures:
Deferred income tax (benefit) for unrealized gains on investments arising during the period		15	0	-81
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net premiums earned	0	0	0	0
Net investment income	0	0	0	0
Net realized investment losses	0	0	0	0
Net losses and loss expenses	0	0	0	0
Acquisition costs	0	0	0	0
General and administrative expenses	0	0	0	0
Amortization of intangible assets	0	0	0	0
Interest expense	0	0	0	0
Foreign exchange (loss) gain	0	0	0	0
Income tax (expense) benefit	0	0	0	0
Equity in earnings of consolidated subsidiaries	-280,829	-457,330	-659,856	-1,128,846
NET INCOME	-280,829	-457,330	-659,856	-1,128,846
Unrealized gains on investments arising during the period net of applicable deferred income tax benefit for three and nine months ended September 30, 2012: $15 and ($81), respectively		29		-150
Reclassification adjustment for net realized investment gains included in net income, net of applicable income tax		0		13,249
Other comprehensive loss	0	29	0	13,099
COMPREHENSIVE INCOME (LOSS)	($280,829)	($457,301)	($659,856)	($1,115,747)

Condensed_Consolidated_Guarant4

Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Cash Flows) (Details) (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Condensed Financial Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	$332,050	$499,652
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of trading securities	-4,955,817	-6,328,719
Purchases of other invested assets	-211,501	-52,578
Sales of available for sale securities	0	215,318
Sales of trading securities	5,137,280	5,778,138
Sales of other invested assets	189,155	110,429
Other	-78,203	33,644
Net cash provided by (used in) investing activities	80,914	-243,768
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Dividends paid - partial par value reduction	-12,981	-54,721
Dividends paid	-17,117	0
Proceeds from the exercise of stock options	8,465	9,104
Share repurchases	-120,163	-204,746
Net cash used in financing activities	-141,796	-250,363
NET INCREASE IN CASH AND CASH EQUIVALENTS	271,168	5,521
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	681,879	633,996
CASH AND CASH EQUIVALENTS, END OF PERIOD	953,047	639,517
Allied World Switzerland (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	177,859	162,141
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Dividends paid - partial par value reduction	-12,981	-54,721
Dividends paid	-17,117
Proceeds from the exercise of stock options	8,465	9,104
Share repurchases	-120,163	-204,746
Other		0
Net cash used in financing activities	-141,796	-250,363
NET INCREASE IN CASH AND CASH EQUIVALENTS	36,063	-88,222
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	19,997	112,672
CASH AND CASH EQUIVALENTS, END OF PERIOD	56,060	24,450
Allied World Bermuda (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	-2,936	687
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
NET INCREASE IN CASH AND CASH EQUIVALENTS	-2,936	687
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	11,324	8,886
CASH AND CASH EQUIVALENTS, END OF PERIOD	8,388	9,573
Other Allied World Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	157,127	336,824
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of trading securities	-4,955,817	-6,328,719
Purchases of other invested assets	-211,501	-52,578
Sales of available for sale securities	0	215,318
Sales of trading securities	5,137,280	5,778,138
Sales of other invested assets	189,155	110,429
Other	-78,203	33,644
Net cash provided by (used in) investing activities	80,914	-243,768
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
NET INCREASE IN CASH AND CASH EQUIVALENTS	238,041	93,056
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	650,558	512,438
CASH AND CASH EQUIVALENTS, END OF PERIOD	888,599	605,494
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	0
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
NET INCREASE IN CASH AND CASH EQUIVALENTS	0
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	0
CASH AND CASH EQUIVALENTS, END OF PERIOD	$0

Condensed_Consolidated_Guarant5

Condensed Consolidated Guarantor Financial Statements (Notes to Parent Company Condensed Financial Information) (Details) (Allied World Switzerland (Parent Guarantor) [Member], USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Allied World Switzerland (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
Cash dividends received from subsidiaries during the period	$237,000	$285,000

Subsequent_Events_Details

Subsequent Events (Details) (USD $)	3 Months Ended		9 Months Ended		0 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Oct. 03, 2013
					Subsequent Event [Member]
Subsequent Event [Line Items]
Dividends paid per common share	$0.50	$0.75	$0.88	$1.50	$0.50