Liability for Unpaid Loss and Loss Adjustment Expense (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
Liability for Unpaid Loss and Loss Adjustment Expense
Reserves for loss and loss adjustment expense payable at beginning of year				$ 3,471,858				$ 3,492,309	$ 3,471,858	$ 3,492,309	$ 3,415,230
Less reinsurance recoverables on reserves at beginning of year				934,086				936,469	934,086	936,469	998,959
Net Reserves at beginning of year				2,537,772				2,555,840	2,537,772	2,555,840	2,416,271
Net reserve additions from acquired businesses	6,261				8,110				6,261	8,110	36,522
Foreign currency adjustment	(6,108)				(21,127)				(6,108)	(21,127)	25,067
Net loss and loss adjustment expense:
Provision for loss and loss adjustment expense for claims occuring in current years									1,389,100	1,235,692	1,269,283
Decrease in estimated loss and loss adjustment expense for claims occurring in prior years									10,147	(22,663)	(53,524)
Net loss and loss adjustment expense	337,007	380,372	334,282	347,586	290,384	297,138	298,986	326,521	1,399,247	1,213,029	1,215,759
Net loss and loss adjustment expense payments for claims occurring during:
Current year									527,244	454,940	519,080
Prior years									726,445	763,140	618,699
Net loss and loss adjustment expense payments									1,253,689	1,218,080	1,137,779
Net reserves at end of year	2,683,483				2,537,772				2,683,483	2,537,772	2,555,840
Plus reinsurance recoverables on reserves at end of year	974,834				934,086				974,834	934,086	936,469
Reserves for loss and loss adjustment expense payable at end of year	$ 3,658,317				$ 3,471,858				$ 3,658,317	$ 3,471,858	$ 3,492,309