Reinsurance (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
Effect of reinsurance transactions on our premium, loss and loss adjustment expense and policy acquisition costs
Direct written premium									$ 2,305,190	$ 2,269,858	$ 2,308,667
Reinsurance assumed									343,936	309,050	251,124
Reinsurance ceded									(466,968)	(552,711)	(513,502)
Net written premium									2,182,158	2,026,197	2,046,289
Direct earned premium									2,308,810	2,284,396	2,265,500
Reinsurance assumed									340,745	298,475	250,133
Reinsurance ceded									(522,385)	(540,947)	(478,398)
Net earned premium	550,183	544,256	524,251	508,480	509,786	516,166	506,385	509,587	2,127,170	2,041,924	2,037,235
Direct loss and loss adjustment expense									1,535,270	1,360,761	1,335,571
Reinsurance assumed									224,655	176,096	153,325
Reinsurance ceded									(360,678)	(323,828)	(273,137)
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
Policy acquisition costs									454,181	440,410	422,254
Ceding commissions									(126,047)	(118,364)	(113,700)
Net policy acquisition costs									$ 328,134	$ 322,046	$ 308,554