Reinsurance - Effects of 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
Premiums Written
Direct									$ 3,100,946	$ 2,754,582	$ 2,673,864
Assumed									1,739,670	1,442,041	1,195,297
Ceded									(948,678)	(845,256)	(816,926)
Net	895,481	959,539	971,928	1,064,990	748,921	839,135	810,535	952,776	3,891,938	3,351,367	3,052,235
Premiums Earned
Direct									2,914,755	2,666,104	2,572,078
Assumed									1,574,049	1,296,048	1,165,371
Ceded									(895,056)	(816,200)	(802,309)
Net premiums earned	923,165	903,651	907,152	859,780	839,366	795,000	758,816	752,770	3,593,748	3,145,952	2,935,140
Losses and Loss Adjustment Expenses
Direct									1,763,492	1,603,369	1,725,707
Assumed									621,346	450,618	578,081
Ceded									(465,588)	(374,563)	(442,511)
Net									$ 1,919,250	$ 1,679,424	$ 1,861,277