Reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Written premiums:
Written premiums, direct	$ 1,679,389	$ 1,643,870	$ 5,127,465	$ 5,061,646
Written premiums, assumed	194,769	224,979	570,052	702,265
Written premiums, ceded	(302,975)	(261,484)	(915,245)	(850,255)
Total net premiums written	1,571,183	1,607,365	4,782,272	4,913,656
Earned premiums:
Earned premiums, direct	1,692,453	1,647,033	4,972,755	4,824,768
Earned premiums, assumed	202,972	216,758	605,281	635,443
Earned premiums, ceded	(313,925)	(277,847)	(857,792)	(787,139)
Net premiums earned	1,581,500	1,585,944	4,720,244	4,673,072
Ceded losses and loss expenses incurred	247,104	213,065	424,905	507,258
Ceded commissions earned	63,222	$ 47,315	177,524	$ 143,809
Uncollectible reinsurance	$ 1,000		$ 1,000		$ 1,000