Unaudited Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Gross written premiums	$ 375,170	$ 388,228	$ 380,003	$ 327,334	$ 295,310	$ 279,969	$ 293,378	$ 298,116	$ 1,470,735	$ 1,166,773	$ 1,081,905
Total revenues	241,150	231,474	220,374	214,127	214,522	224,692	228,041	218,344	907,125	885,599	817,625
Net income (loss)	20,471	(25,167)	20,307	22,728	11,632	19,581	16,984	15,633	38,339	63,830	43,566
Comprehensive income (loss)	$ 16,964	$ (15,710)	$ 41,039	$ 42,988	$ 18,923	$ 15,192	$ 10,426	$ (2,914)	$ 85,281	$ 41,627	$ 52,785
Earnings (loss) per share:
Basic (in dollars per share)	$ 0.67	$ (0.83)	$ 0.67	$ 0.76	$ 0.39	$ 0.65	$ 0.57	$ 0.53	$ 1.27	$ 2.14	$ 1.48
Diluted (in dollars per share)	$ 0.67	$ (0.83)	$ 0.66	$ 0.75	$ 0.38	$ 0.64	$ 0.56	$ 0.52	$ 1.25	$ 2.11	$ 1.44