Debt and Financing Arrangements (Narrative) (Details) In Thousands, unless otherwise specified	12 Months Ended										12 Months Ended					12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2014 Allied World Bermuda (Subsidiary Issuer) USD ($)	Dec. 31, 2014 7.5% Senior notes due 2016 Allied World Bermuda (Subsidiary Issuer) USD ($)	Dec. 31, 2014 5.5% Senior notes due 2020 Allied World Bermuda (Subsidiary Issuer) USD ($)	Dec. 31, 2014 $1,000,000 secured letter of credit facility - uncommitted USD ($)	Dec. 31, 2013 $1,000,000 secured letter of credit facility - uncommitted USD ($)	Dec. 31, 2014 $1,000,000 secured letter of credit facility - uncommitted Allied World Bermuda (Subsidiary Issuer) USD ($)	Dec. 31, 2013 $1,000,000 secured letter of credit facility - uncommitted Allied World Bermuda (Subsidiary Issuer) USD ($)	Dec. 31, 2014 $800,000 secured letter of credit facility [Member] Allied World Bermuda (Subsidiary Issuer) USD ($)	Dec. 31, 2014 $400,000 secured letter of credit facility (pre-amendment) Allied World Bermuda (Subsidiary Issuer) USD ($)	Dec. 31, 2014 $400,000 unsecured letter of credit facility Allied World Bermuda (Subsidiary Issuer) USD ($)	Dec. 31, 2014 $150,000 secured letter of credit facility USD ($)	Dec. 31, 2013 $150,000 secured letter of credit facility USD ($)	Dec. 31, 2014 $150,000 secured letter of credit facility Allied World Bermuda (Subsidiary Issuer) USD ($)	Dec. 31, 2014 Switzerland	Dec. 31, 2014 Switzerland Swiss office building mortgage USD ($)	Dec. 31, 2014 Switzerland Swiss office building mortgage CHF	Dec. 31, 2014 Switzerland Swiss office building credit facility USD ($)	Dec. 31, 2014 Switzerland Swiss office building credit facility CHF
Debt Instrument [Line Items]
Senior Notes, face amount					$ 500,000	$ 300,000
Debt Instrument, interest rate					7.50%	5.50%												3.20%	3.20%	2.50%	2.50%
Senior Notes, maturity date					Aug. 01, 2016	Nov. 15, 2020
Senior Notes, frequency of periodic payment					semi-annually	semi-annually
Senior Notes, date of first required payment					Feb. 01, 2007	May 15, 2011
Proceeds from other long-term debt	19,213	0	0	0														14,157	14,000
Additional borrowing capacity																		4,044	4,000
Annual principal payment																		303	300
Number of annual principal payments required																	19 years
Line of Credit Facility, Initiation Date											Nov. 27, 2007					Jun. 07, 2012
Line of Credit Facility, Maximum Borrowing Capacity							1,000,000	1,000,000	1,000,000	1,000,000	800,000	400,000	400,000	150,000	450,000	450,000				5,056	5,000
Debt instrument term											5 years					4 years		20 years	20 years	3 years	3 years
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases																150,000
Additional commitments subject to lender approval																$ 150,000