Debt And Financing Arrangements (Details)	3 Months Ended								12 Months Ended					1 Months Ended	12 Months Ended								12 Months Ended
	Dec. 31, 2011 USD ($)	Sep. 30, 2011 USD ($)	Jun. 30, 2011 USD ($)	Mar. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Sep. 30, 2010 USD ($)	Jun. 30, 2010 USD ($)	Mar. 31, 2010 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2011 Allied World Capital (Europe) Limited [Member] USD ($)	Dec. 31, 2011 Allied World Capital (Europe) Limited [Member] GBP (£)	Nov. 30, 2010 5.50% Senior Notes [Member] USD ($)	Dec. 31, 2011 5.50% Senior Notes [Member]	Dec. 31, 2011 7.50% Senior Notes [Member] USD ($)	Dec. 31, 2006 7.50% Senior Notes [Member] USD ($)	Dec. 31, 2010 7.50% Senior Notes [Member] USD ($)	Nov. 30, 2010 Senior Notes [Member] 5.50% Senior Notes [Member]	Dec. 31, 2006 Senior Notes [Member] 7.50% Senior Notes [Member]	Dec. 31, 2011 Secured Debt [Member] USD ($)	Dec. 31, 2011 Unsecured Debt [Member] USD ($)	Dec. 31, 2011 Five-Year Senior Credit Facility [Member] USD ($)	Dec. 31, 2011 Maximum [Member] Citibank Europe Plc [Member] USD ($)
Debt Instrument [Line Items]
Senior Notes														$ 300,000,000			$ 500,000,000
Senior Notes due														Nov. 10, 2020		Aug. 01, 2016							Nov. 27, 2012
Senior Notes, interest rate																			5.50%	7.50%
Senior Notes, interest payment commencement date														May 15, 2011			February 1, 2007
Senior Notes, frequency of periodic payment															May 15 and November 15	August 1 and February 1
Accrued interest associated with outstanding debt obligations									17,688,000	17,687,000
Interest expense	13,754,000	13,748,000	13,745,000	13,742,000	11,650,000	9,533,000	9,531,000	9,528,000	54,989,000	40,242,000	39,019,000
Interest payable																15,625,000		15,625,000
Underwriters price as a percentage of principal amount														98.89%			99.71%
Effective yield to investors														5.56%			7.54%
Letter of credit																					400,000,000	400,000,000	800,000,000	900,000,000
Option to increase commitments, maximum									200,000,000
Irrevocable standby letter of credit												$ 90,955,000	£ 59,000,000