Reinsurance - Summary of Premiums Earned and Losses Incurred (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Premiums earned:
Direct	$ 268,236	$ 261,180	$ 533,487	$ 520,608
Assumed	106	62	227	160
Ceded	(21,378)	(30,106)	(54,643)	(60,529)
Net premiums earned	246,964	231,136	479,071	460,239
Losses incurred:
Direct	(16,778)	31,396	14,723	63,809
Assumed	(100)	61	(10)	166
Ceded	3,423	(4,118)	(4,318)	(9,017)
Losses incurred, net	$ (13,455)	$ 27,339	$ 10,395	$ 54,958