Reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Written premiums:
Written premiums, direct	$ 1,785,266	$ 1,727,014	$ 3,572,501	$ 3,448,076
Written premiums, assumed	162,808	160,139	354,995	375,283
Written premiums, ceded	(323,970)	(322,902)	(638,054)	(612,270)
Total net premiums written	1,624,104	1,564,251	3,289,442	3,211,089
Earned premiums:
Earned premiums, direct	1,705,277	1,666,938	3,379,827	3,280,302
Earned premiums, assumed	180,509	193,683	369,375	402,310
Earned premiums, ceded	(304,733)	(291,918)	(600,741)	(543,868)
Net premiums earned	1,581,053	1,568,703	3,148,461	3,138,744
Ceded losses and loss expenses incurred	161,073	142,609	400,068	177,801
Ceded commissions earned	65,922	$ 57,752	132,278	$ 114,302
Uncollectible reinsurance	$ 1,000		$ 1,000		$ 1,000