Summary of Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Gross written premiums	$ 323,509	$ 354,062	$ 379,471	$ 396,460	$ 333,299	$ 327,469	$ 348,795	$ 422,790	$ 1,453,502	$ 1,432,353	$ 1,370,517
Revenues:
Net written premiums	244,719	251,939	258,244	288,958	228,007	228,417	231,864	311,850	1,043,860	1,000,138	887,922
Change in unearned premiums	6,826	2,143	(15,916)	(52,826)	6,165	16,950	(780)	(86,578)	(59,773)	(64,243)	(45,983)
Net earned premiums	251,545	254,082	242,328	236,132	234,172	245,367	231,084	225,272	984,087	935,895	841,939
Net investment income	18,499	17,371	16,595	16,253	16,071	15,839	15,648	16,610	68,718	64,168	56,251
Total other-than-temporary impairment losses	(123)	(1,298)	(423)		(21)	158			(1,870)	137	(2,393)
Portion of loss recognized in other comprehensive income (before tax)	123	23			21	(158)			172	(137)
Net other-than-temporary impairment losses recognized in earnings		(1,275)	(423)						(1,698)	0	(2,393)
Net realized gains (losses)	(2,080)	518	4,339	5,596	788	6,718	4,473	833
Other income (expense)	4,147	(2,518)	(4,362)	2,242	586	1,336	(1,665)	10,399	(491)	10,656	(1,172)
Total revenues	272,111	268,178	258,477	260,223	251,617	269,260	249,540	253,114	1,058,989	1,023,531	917,564
Expenses:
Net losses and LAE	153,881	146,546	141,973	130,198	134,658	135,284	140,220	135,067	572,598	545,229	518,961
Commission expenses	31,339	34,253	31,480	32,905	33,708	33,943	32,150	25,727	129,977	125,528	113,494
Other operating expenses	59,219	56,599	52,789	54,909	51,299	50,388	47,992	47,146
Interest expense	3,857	3,856	3,856	3,855	3,854	3,388	4,319	3,852	15,424	15,413	10,507
Total expenses	248,296	241,254	230,098	221,867	223,519	223,003	224,681	211,792	941,515	882,995	825,291
Income before income taxes	23,815	26,924	28,379	38,356	28,098	46,257	24,859	41,322	117,474	140,536	92,273
Income tax expense	6,072	8,723	9,195	12,427	8,823	15,032	7,998	13,354	36,417	45,207	28,807
Net income	17,743	18,201	19,184	25,929	19,275	31,225	16,861	27,968	81,057	95,329	63,466
Comprehensive income (loss)	$ 15,990	$ 18,849	$ (3,789)	$ 27,052	$ 27,015	$ 20,628	$ 32,965	$ 32,920	$ 58,102	$ 113,528	$ 16,618
Combined ratio	97.20%	93.40%	93.40%	92.30%	93.80%	89.50%	95.30%	92.20%	94.10%	92.60%	94.80%
Net income per share:
Basic	$ 1.23	$ 1.26	$ 1.33	$ 1.81	$ 1.35	$ 2.19	$ 1.18	$ 1.96	$ 5.63	$ 6.69	$ 4.49
Diluted	$ 1.19	$ 1.23	$ 1.30	$ 1.77	$ 1.31	$ 2.14	$ 1.17	$ 1.94	$ 5.47	$ 6.51	$ 4.42