Shareholders' Equity (Narrative) (Details) In Thousands, except Share data, unless otherwise specified	12 Months Ended					1 Months Ended			12 Months Ended				0 Months Ended		0 Months Ended		12 Months Ended	0 Months Ended
	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2012 CHF	Dec. 31, 2011 CHF	Nov. 30, 2010 Goldman Sachs USD ($)	Aug. 31, 2010 Goldman Sachs USD ($)	Dec. 31, 2010 Goldman Sachs	Dec. 31, 2012 Allied World Switzerland (Parent Guarantor)	Dec. 01, 2010 Allied World Switzerland (Parent Guarantor) Redomestication [Member] CHF	Dec. 31, 2012 Allied World Bermuda [Member] Redomestication [Member] USD ($)	Dec. 01, 2010 Allied World Bermuda [Member] Redomestication [Member] USD ($)	Jul. 30, 2012 Nonvoting common stock	Dec. 31, 2012 Nonvoting common stock	Jul. 30, 2012 Voting common stock	Dec. 31, 2012 Voting common stock	Dec. 31, 2012 Constructive retirement	Dec. 01, 2010 Treasury Shares Redomestication [Member] USD ($)
Class of Stock [Line Items]
Common shares, outstanding	34,797,781	37,742,131												0		34,797,781
Shares cancelled, shares	3,633,774												173,100		2,326,900		1,133,774	10,879,106
Maximum percent increase in share capital authorized	20.00%
Maximum authorized value increase of registered share capital				94,809
Maximum authorized increased voting shares																7,500,728
Maximum number of conditional capital voting shares																5,200,000
Total share capital	454,980	557,153								600,055	600,055
Common shares, par value				12.64	14.03					15.00		$ 0.03
Maximum percentage of registered capital in treasury shares	10.00%
Shares cancelled, value																		561,851
Shares repurchased, shares	3,655,959	1,419,163				3,159,793	5,000,000
Shares repurchased, average price per share	$ 72.20	$ 61.09				$ 58.69	$ 50.00
Cost of shares repurchased	$ 263,942	$ 86,700	$ 674,662			$ 185,448	$ 250,000
Percentage discount of weighted average trading price of common share								0.50%
Threshold for dividend restrictions									20.00%
Minimum Annual Profit Percentage Required									5.00%