Reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Written premiums:
Written premiums, direct	$ 1,727,014	$ 1,708,535	$ 3,448,076	$ 3,417,664
Written premiums, assumed	160,139	230,830	375,283	477,398
Written premiums, ceded	(322,902)	(296,796)	(612,270)	(588,771)
Total net premiums written	1,564,251	1,642,569	3,211,089	3,306,291
Earned premiums:
Earned premiums, direct	1,666,938	1,609,644	3,280,302	3,177,712
Earned premiums, assumed	193,683	225,643	402,310	444,009
Earned premiums, ceded	(291,918)	(275,494)	(543,868)	(534,593)
Net premiums earned	1,568,703	1,559,793	3,138,744	3,087,128
Ceded losses and loss expenses incurred	142,609	167,265	177,801	294,194
Ceded commissions earned	57,752	$ 50,172	114,302	$ 96,494
Uncollectible reinsurance	$ 1,000		$ 1,000		$ 1,000