Liability for Unpaid Loss and Loss Adjustment Expense (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNetAbstract
Net reserves for loss and loss adjustment expense payable at beginning of period	$ 2,811,021	$ 2,748,995	$ 2,749,803	$ 2,683,483
Net reserve additions from acquired businesses	0	0	0	14,705
Foreign currency adjustment	22,331	15,717	(2,296)	11,261
Net loss and loss adjustment expense	320,376	304,014	992,547	969,767
Net loss and loss adjustment expense payments	(297,696)	(333,713)	(884,022)	(944,203)
Net reserves for loss and loss adjustment expense payable at end of period	$ 2,856,032	$ 2,735,013	$ 2,856,032	$ 2,735,013