Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Cash Flows) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	$ 628,856	$ 548,208	$ 450,854
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of available for sale securities	0	(727)	(133,079)
Purchases of trading securities	(8,006,237)	(7,854,053)	(12,816,343)
Purchases of other invested assets	(333,031)	(281,957)	(165,505)
Sales of available for sale securities	236,978	609,833	2,687,129
Sales of trading securities	7,892,907	6,942,291	10,936,501
Sales of other invested assets	53,741	71,543	9,746
Other	(104,091)	3,946	(18,386)
Net cash (used in) provided by investing activities	(259,733)	(509,124)	500,063
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Dividends paid - par value reduction	(67,825)	(28,600)	0
Dividends paid	0	0	(47,712)
Proceeds from the exercise of stock options	10,527	6,837	9,598
Proceeds from senior notes	0	0	296,682
Share repurchases	(263,942)	(86,700)	(674,662)
Repurchase of founder warrants	0	(53,620)	(70,016)
Other		0
Net cash used in financing activities	(321,240)	(162,083)	(486,110)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	47,883	(122,999)	464,807
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	633,996	756,995	292,188
CASH AND CASH EQUIVALENTS, END OF PERIOD	681,879	633,996	756,995
Allied World Switzerland (Parent Guarantor)
Condensed Financial Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	338,565	87,592	5,535
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Capital contribution to subsidiary	(110,000)
Other		(4,496)
Net cash (used in) provided by investing activities	(110,000)	(4,496)
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Dividends paid - par value reduction	(67,825)	(28,600)
Proceeds from the exercise of stock options	10,527	6,837
Share repurchases	(263,942)	(86,700)	(27,047)
Capital contributions on redomestication			160,000
Other		(449)
Net cash used in financing activities	(321,240)	(108,912)	132,953
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(92,675)	(25,816)	138,488
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	112,672	138,488
CASH AND CASH EQUIVALENTS, END OF PERIOD	19,997	112,672	138,488
Allied World Bermuda (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	2,438	(62,404)	690,100
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Other			(2,004)
Net cash (used in) provided by investing activities			(2,004)
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Dividends paid			(47,712)
Proceeds from the exercise of stock options			9,598
Proceeds from senior notes			296,682
Share repurchases			(647,615)
Repurchase of founder warrants		(53,620)	(70,016)
Capital contributions on redomestication			(160,000)
Other		(753)	2,781
Net cash used in financing activities		(54,373)	(616,282)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	2,438	(116,777)	71,814
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	8,886	125,663	53,849
CASH AND CASH EQUIVALENTS, END OF PERIOD	11,324	8,886	125,663
Other Allied World Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	287,853	523,020	505,219
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of available for sale securities		(727)	(133,079)
Purchases of trading securities	(8,006,237)	(7,854,053)	(12,816,343)
Purchases of other invested assets	(333,031)	(281,957)	(165,505)
Sales of available for sale securities	236,978	609,833	2,687,129
Sales of trading securities	7,892,907	6,942,291	10,936,501
Sales of other invested assets	53,741	71,543	9,746
Other	(104,091)	8,442	(16,382)
Net cash (used in) provided by investing activities	(259,733)	(504,628)	502,067
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Dividends paid			(750,000)
Capital contribution from parent	110,000
Other		1,202	(2,781)
Net cash used in financing activities	110,000	1,202	(752,781)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	138,120	19,594	254,505
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	512,438	492,844	238,339
CASH AND CASH EQUIVALENTS, END OF PERIOD	650,558	512,438	492,844
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:			(750,000)
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Capital contribution to subsidiary	110,000
Net cash (used in) provided by investing activities	110,000
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Dividends paid			750,000
Capital contribution from parent	(110,000)
Net cash used in financing activities	(110,000)		750,000
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	0
CASH AND CASH EQUIVALENTS, END OF PERIOD	$ 0