Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Cash Flows) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Condensed Financial Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	$ 125,906	$ 299,508
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of trading securities	(3,186,162)	(4,005,352)
Purchases of other invested assets	(141,805)	(17,778)
Sales of available for sale securities	0	214,015
Sales of trading securities	3,171,977	3,959,204
Sales of other invested assets	126,491	108,759
Other	(13,924)	(157,647)
Net cash (used in) provided by investing activities	(43,423)	101,201
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Partial par value reduction	(12,981)	(28,003)
Proceeds from the exercise of stock options	5,293	6,697
Share repurchases	(82,571)	(148,949)
Net cash used in financing activities	(90,259)	(170,255)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(7,776)	230,454
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	681,879	633,996
CASH AND CASH EQUIVALENTS, END OF PERIOD	674,103	864,450
Allied World Switzerland (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	73,938	81,902
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Other		0
Net cash (used in) provided by investing activities		0
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Partial par value reduction	(12,981)	(28,003)
Proceeds from the exercise of stock options	5,293	6,697
Share repurchases	(82,571)	(148,949)
Other		0
Net cash used in financing activities	(90,259)	(170,255)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(16,321)	(88,353)
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	19,997	112,672
CASH AND CASH EQUIVALENTS, END OF PERIOD	3,676	24,319
Allied World Bermuda (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	(3,025)	10,326
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Other		0
Net cash used in financing activities		0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(3,025)	10,326
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	11,324	8,886
CASH AND CASH EQUIVALENTS, END OF PERIOD	8,299	19,212
Other Allied World Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	54,993	207,280
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of trading securities	(3,186,162)	(4,005,352)
Purchases of other invested assets	(141,805)	(17,778)
Sales of available for sale securities	0	214,015
Sales of trading securities	3,171,977	3,959,204
Sales of other invested assets	126,491	108,759
Other	(13,924)	(157,647)
Net cash (used in) provided by investing activities	(43,423)	101,201
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Other		0
Net cash used in financing activities		0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	11,570	308,481
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	650,558	512,438
CASH AND CASH EQUIVALENTS, END OF PERIOD	662,128	820,919
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 (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	0
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	0
CASH AND CASH EQUIVALENTS, END OF PERIOD	$ 0