Reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Written premiums:
Written premiums, direct	$ 1,883,032	$ 1,785,266	$ 3,712,847	$ 3,572,501
Written premiums, assumed	206,828	162,808	423,243	354,995
Written premiums, ceded	(346,396)	(323,970)	(683,025)	(638,054)
Total net premiums written	1,743,464	1,624,104	3,453,065	3,289,442
Earned premiums:
Earned premiums, direct	1,773,437	1,705,277	3,497,047	3,379,827
Earned premiums, assumed	198,661	180,509	386,852	369,375
Earned premiums, ceded	(325,257)	(304,733)	(644,202)	(600,741)
Net premiums earned	1,646,841	1,581,053	3,239,697	3,148,461
Ceded losses and loss expenses incurred	239,267	161,073	412,314	400,068
Ceded commissions earned	73,092	$ 65,922	144,109	$ 132,278
Uncollectible reinsurance	$ 1,000		$ 1,000		$ 1,000