Reinsurance (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Effect of reinsurance transactions on our premium, loss and loss adjustment expense and policy acquisition costs
Direct written premium									$ 2,689,478	$ 2,545,054	$ 2,422,517
Reinsurance assumed									312,140	335,195	361,556
Reinsurance ceded									(628,373)	(624,926)	(530,677)
Net written premium									2,373,245	2,255,323	2,253,396
Direct earned premium									2,633,433	2,482,527	2,396,756
Reinsurance assumed									306,896	333,344	351,611
Reinsurance ceded									(616,702)	(576,631)	(505,742)
Net earned premium	601,832	586,935	572,248	562,612	560,030	556,668	561,356	561,186	2,323,627	2,239,240	2,242,625
Direct loss and loss adjustment expense									1,551,700	1,616,097	1,467,393
Reinsurance assumed									119,980	151,145	162,534
Reinsurance ceded									(344,845)	(444,788)	(291,853)
Net loss and loss adjustment expense	339,434	320,758	335,897	330,746	305,477	328,530	347,353	341,094	1,326,835	1,322,454	1,338,074
Policy acquisition costs									445,794	413,782	398,453
Ceding commissions									(151,124)	(134,343)	(117,252)
Net policy acquisition costs									$ 294,670	$ 279,439	$ 281,201