Statutory Capital And Surplus (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Allied World Assurance Company, Ltd (Bermuda) [Member]
Minimum solvency margin required to maintain	$ 100,000
Net premiums written, percentage	50.00%
Maximum gross premiums considered as ceded premiums, percentage	25.00%
Reserve for losses and loss expenses, percentage	15.00%
Statutory capital and surplus	3,184,294	2,944,351
Maximum amount of dividends that could be paid without statutory notification	796,074	736,088
Statutory net income	197,624	666,372	570,306
Minimum solvency margin	591,050	551,203
U.S. Subsidiaries [Member]
Statutory capital and surplus	1,054,441	1,004,616
Maximum amount of dividends that could be paid without statutory notification	80,900	74,726
Statutory net income	44,974	63,304	79,622
Allied World Assurance Company (Europe) Limited [Member]
Statutory capital and surplus	62,349	59,300
Statutory net income	3,049	4,514	1,681
Minimum solvency margin	12,907	13,235
Allied World Assurance Company (Reinsurance) Limited [Member]
Statutory capital and surplus	340,807	329,266
Statutory net income	11,541	4,864	12,227
Minimum solvency margin	9,150	10,712
Allied World Assurance Company, AG [Member]
Statutory capital and surplus	57,195
Statutory net income	$ 1,221