Quarterly Results for 2018 and 2017 - Summary of Quarterly Results for the Periods Presented (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Premiums earned, net	$ 204,595	$ 188,938	$ 192,272	$ 182,577	$ 183,708	$ 174,517	$ 169,009	$ 161,559	$ 768,382	$ 688,793	$ 632,416
Net investment income	7,603	6,642	5,786	4,785	4,448	3,085	3,223	2,704	24,816	13,460	9,540
Total revenues	216,373	206,155	209,788	191,500	201,312	190,243	185,487	174,874	823,816	751,916	685,289
Total expenses	227,614	154,988	148,540	139,801	142,721	173,644	137,564	127,503	670,943	581,432	522,406
Net income	$ (6,468)	$ 37,380	$ 46,084	$ 40,055	$ 36,396	$ 9,964	$ 29,376	$ 31,199	$ 117,051	$ 106,935	$ 99,410
Basic earnings per share (in dollars per share)	$ (0.19)	$ 1.07	$ 1.32	$ 1.15	$ 1.05	$ 0.29	$ 0.84	$ 0.89	$ 3.36	$ 3.07	$ 2.85
Diluted earnings per share (in dollars per share)	$ (0.18)	$ 1.04	$ 1.29	$ 1.12	$ 1.03	$ 0.28	$ 0.82	$ 0.86	$ 3.27	$ 2.99	$ 2.79