Liability for Unpaid Loss and Loss Adjustment Expense (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNetAbstract
Reserves for loss and loss adjustment expense payable at beginning of year				$ 3,658,317				$ 3,471,858	$ 3,658,317	$ 3,471,858	$ 3,492,309
Reinsurance Recoverables, Beginning Balance				974,834				934,086	974,834	934,086	936,469
Net Reserves at beginning of year				2,683,483				2,537,772	2,683,483	2,537,772	2,555,840
Net reserve additions from acquired businesses	14,705				6,261				14,705	6,261	8,110
Foreign currency adjustment	18,449				(6,108)				18,449	(6,108)	(21,127)
Net loss and loss adjustment expense:
Provision for loss and loss adjustment expense for claims occurring in current year									1,375,522	1,389,100	1,235,692
Increase (decrease) in estimated loss and loss adjustment expense for claims occurring in prior years									(70,011)	10,147	(22,663)
Net loss and loss adjustment expense	335,744	304,014	336,825	328,928	337,007	380,372	334,282	347,586	1,305,511	1,399,247	1,213,029
Net loss and loss adjustment expense payments for claims occurring during:
Current year									543,010	527,244	454,940
Prior years									729,335	726,445	763,140
Net loss and loss adjustment expense payments									1,272,345	1,253,689	1,218,080
Net reserves at end of year	2,749,803				2,683,483				2,749,803	2,683,483	2,537,772
Reinsurance Recoverables, Ending Balance	1,018,047				974,834				1,018,047	974,834	934,086
Reserves for loss and loss adjustment expense payable at end of year	$ 3,767,850				$ 3,658,317				$ 3,767,850	$ 3,658,317	$ 3,471,858