Income Taxes (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operating Loss Carryforwards [Line Items]
Material tax positions minimum percentage									50.00%
Current tax receivable	$ 6,287				$ 15,614				$ 6,287	$ 15,614
Current tax payable	8,856								8,856
Effective tax rate									10.10%	3.90%	5.70%
Effective tax rate reconciliation									7.80%	7.80%	0.00%
Income tax expense	11,952	5,672	11,073	2,283	(207)	13,569	10,378	3,205	30,980	26,945	36,644
Allied World Switzerland (Parent Guarantor) [Member]
Operating Loss Carryforwards [Line Items]
Effective tax rate									9.00%
Income tax expense									$ 7,940
Bermuda [Member]
Operating Loss Carryforwards [Line Items]
Effective tax rate reconciliation											0.00%
Switzerland [Member]
Operating Loss Carryforwards [Line Items]
Effective tax rate reconciliation									7.80%	7.80%