Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Balance Sheet) (Details) (USD $) In Thousands	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Dec. 31, 2009
Condensed Financial Statements, Captions [Line Items]
Investments	$ 7,540,759		$ 7,183,554
Cash and cash equivalents	869,320		756,995	461,286		292,188
Insurance balances receivable	607,270		529,927
Prepaid reinsurance	218,514		187,287
Reinsurance recoverable	1,009,643	1,013,951	927,588	939,956	932,435	919,991
Net deferred acquisition costs	115,070		96,803
Goodwill	268,376		268,376
Intangible assets	54,576		56,876
Balances receivable on sale of investments	349,327		188,408
Other assets	150,292		231,817
Total assets	11,183,147		10,427,631
Reserve for losses and loss expenses	5,245,553	5,251,304	4,879,188	4,889,825	4,920,435	4,761,772
Unearned premiums	1,158,842		962,203
Reinsurance balances payable	95,463		99,732
Balances due on purchases of investments	754,451		506,978
Senior notes	797,885		797,700
Accounts payable and accrued liabilities	127,879		106,010
Total liabilities	8,180,073		7,351,811
Total shareholders' equity	3,003,074		3,075,820
Total liabilities and shareholders' equity	11,183,147		10,427,631
Allied World Switzerland (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
Cash and cash equivalents	98,941		138,488
Investments in subsidiaries	2,935,337		2,944,975
Due (to) from subsidiaries	(4,800)		(7,143)
Other assets	1,672
Total assets	3,031,150		3,076,320
Accounts payable and accrued liabilities	28,075		500
Total liabilities	28,075		500
Total shareholders' equity	3,003,075		3,075,820
Total liabilities and shareholders' equity	3,031,150		3,076,320
Allied World Bermuda (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
Cash and cash equivalents	18,545		125,663	73,269		53,849
Investments in subsidiaries	3,767,463		3,635,741
Due (to) from subsidiaries	(6,886)		(9,419)
Other assets	60,574		8,801
Total assets	3,839,696		3,760,786
Senior notes	797,885		797,700
Accounts payable and accrued liabilities	13,601		18,111
Total liabilities	811,486		815,811
Total shareholders' equity	3,028,210		2,944,975
Total liabilities and shareholders' equity	3,839,696		3,760,786
Other Allied World Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Investments	7,540,759		7,183,554
Cash and cash equivalents	751,834		492,844	388,017		238,339
Insurance balances receivable	607,270		529,927
Prepaid reinsurance	218,514		187,287
Reinsurance recoverable	1,009,643		927,588
Net deferred acquisition costs	115,070		96,803
Goodwill	268,376		268,376
Intangible assets	54,576		56,876
Balances receivable on sale of investments	349,327		188,408
Due (to) from subsidiaries	11,686		16,562
Other assets	141,666		223,016
Total assets	11,068,721		10,171,241
Reserve for losses and loss expenses	5,245,553		4,879,188
Unearned premiums	1,158,842		962,203
Reinsurance balances payable	95,463		99,732
Balances due on purchases of investments	754,451		506,978
Accounts payable and accrued liabilities	86,203		87,399
Total liabilities	7,340,512		6,535,500
Total shareholders' equity	3,728,209		3,635,741
Total liabilities and shareholders' equity	11,068,721		10,171,241
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Investments in subsidiaries	(6,702,800)		(6,580,716)
Other assets	(53,620)
Total assets	(6,756,420)		(6,580,716)
Total shareholders' equity	(6,756,420)		(6,580,716)
Total liabilities and shareholders' equity	$ (6,756,420)		$ (6,580,716)