Reinsurance - Effect on Net Premiums Written and Earned (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net premiums written:
Direct									$ 922,851	$ 796,344	$ 696,055
Ceded									(88,738)	(35,499)	(10,768)
Net premiums written									834,113	760,845	685,287
Net premiums earned:
Direct									951,302	812,924	660,260
Ceded									(88,738)	(35,499)	(10,768)
Net premiums earned	$ 222,339	$ 222,258	$ 211,471	$ 206,496	$ 207,671	$ 203,473	$ 188,490	$ 177,791	$ 862,564	$ 777,425	$ 649,492