Reinsurance (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
Effect of reinsurance transactions on our premium, loss and loss adjustment expense and policy acquisition costs
Direct written premium									$ 2,422,517	$ 2,305,190	$ 2,269,858
Reinsurance assumed									361,555	343,936	309,050
Reinsurance ceded									(530,676)	(466,968)	(552,711)
Net written premium									2,253,396	2,182,158	2,026,197
Direct earned premium									2,396,756	2,308,810	2,284,396
Reinsurance assumed									351,611	340,745	298,475
Reinsurance ceded									(505,742)	(522,385)	(540,947)
Net earned premium	566,503	563,650	565,331	547,141	550,183	544,256	524,251	508,480	2,242,625	2,127,170	2,041,924
Direct loss and loss adjustment expense									1,434,830	1,535,270	1,360,761
Reinsurance assumed									162,534	224,655	176,096
Reinsurance ceded									(291,853)	(360,678)	(323,828)
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
Policy acquisition costs									398,453	392,172	373,500
Ceding commissions									(117,252)	(126,047)	(118,364)
Net policy acquisition costs									$ 281,201	$ 266,125	$ 255,136