Reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Written premiums:
Written premiums, direct	$ 1,787,235	$ 1,721,062
Written premiums, assumed	192,187	215,145
Written premiums, ceded	(314,084)	(289,369)
Total net premiums written	1,665,338	1,646,838
Earned premiums:
Earned premiums, direct	1,674,550	1,613,364
Earned premiums, assumed	188,866	208,626
Earned premiums, ceded	(296,008)	(251,948)
Net premiums earned	1,567,408	1,570,042
Ceded losses and loss expenses incurred	238,995	35,192
Ceded commissions earned	66,356	$ 56,550
Uncollectible reinsurance	$ 1,000		$ 1,000