Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Balance Sheet) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010
ASSETS:
Investments	$ 8,210,065		$ 7,406,594
Cash and cash equivalents	639,517		633,996	869,320		756,995
Insurance balances receivable	754,978		652,158
Reinsurance recoverable	1,077,522	1,073,612	1,002,919	1,009,643	1,013,951	927,588
Net deferred acquisition costs	127,527		100,334
Goodwill and intangible assets	320,374		322,274
Balances receivable on sale of investments	756,570		580,443
Investments in subsidiaries	0		0
Due (to) from subsidiaries	0		0
Other assets	425,436		423,440
Total assets	12,311,989		11,122,158
LIABILITIES
Reserve for losses and loss expenses	5,450,787	5,377,518	5,225,143	5,245,553	5,251,304	4,879,188
Unearned premiums	1,316,399		1,078,412
Reinsurance balances payable	120,432		124,539
Balances due on purchases of investments	1,075,069		616,728
Senior notes	798,147		797,949
Other liabilities	115,369		130,365
Total liabilities	8,876,203		7,973,136
Total shareholders' equity	3,435,786		3,149,022	3,003,074		3,075,820
Total liabilities and shareholders' equity	12,311,989		11,122,158
Allied World Switzerland (Parent Guarantor) [Member]
ASSETS:
Investments	0		0
Cash and cash equivalents	24,450		112,672	98,941		138,488
Insurance balances receivable	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	3,427,446		3,064,066
Due (to) from subsidiaries	(4,536)		(4,853)
Other assets	1,813		1,504
Total assets	3,449,173		3,173,389
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	13,387		24,367
Total liabilities	13,387		24,367
Total shareholders' equity	3,435,786		3,149,022
Total liabilities and shareholders' equity	3,449,173		3,173,389
Allied World Bermuda (Subsidiary Issuer) [Member]
ASSETS:
Investments	0		0
Cash and cash equivalents	9,573		8,886	18,545		125,663
Insurance balances receivable	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	4,716,399		3,964,585
Due (to) from subsidiaries	(6,869)		(6,769)
Other assets	6,450		6,367
Total assets	4,725,553		3,973,069
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	798,147		797,949
Other liabilities	12,493		17,688
Total liabilities	810,640		815,637
Total shareholders' equity	3,914,913		3,157,432
Total liabilities and shareholders' equity	4,725,553		3,973,069
Other Allied World Subsidiaries [Member]
ASSETS:
Investments	8,210,065		7,406,594
Cash and cash equivalents	605,494		512,438	751,834		492,844
Insurance balances receivable	754,978		652,158
Reinsurance recoverable	1,077,522		1,002,919
Net deferred acquisition costs	127,527		100,334
Goodwill and intangible assets	320,374		322,274
Balances receivable on sale of investments	756,570		580,443
Investments in subsidiaries	0		0
Due (to) from subsidiaries	11,405		11,622
Other assets	417,173		415,569
Total assets	12,281,108		11,004,351
LIABILITIES
Reserve for losses and loss expenses	5,450,787		5,225,143
Unearned premiums	1,316,399		1,078,412
Reinsurance balances payable	120,432		124,539
Balances due on purchases of investments	1,075,069		616,728
Senior notes	0		0
Other liabilities	89,489		88,310
Total liabilities	8,052,176		7,133,132
Total shareholders' equity	4,228,932		3,871,219
Total liabilities and shareholders' equity	12,281,108		11,004,351
Consolidating Adjustments [Member]
ASSETS:
Investments	0		0
Cash and cash equivalents	0		0
Insurance balances receivable	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	(8,143,845)		(7,028,651)
Due (to) from subsidiaries	0		0
Other assets	0		0
Total assets	(8,143,845)		(7,028,651)
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	(8,143,845)		(7,028,651)
Total liabilities and shareholders' equity	$ (8,143,845)		$ (7,028,651)