Summary of Condensed Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Income Statement [Abstract]
Gross written premiums	$ 367,539		$ 374,930		$ 412,565		$ 413,877		$ 323,509		$ 354,062		$ 379,471		$ 396,460		$ 1,568,911		$ 1,453,502		$ 1,432,353
Revenues:
Net written premiums	282,868		277,001		306,535		319,820		244,719		251,939		258,244		288,958		1,186,224		1,043,860		1,000,138
Change in unearned premiums	1,117		7,009		(38,543)		(55,462)		6,826		2,143		(15,916)		(52,826)		(85,879)		(59,773)		(64,243)
Net earned premiums	283,985		284,010		267,992		264,358		251,545		254,082		242,328		236,132		1,100,345		984,087		935,895
Net investment income	20,107		19,875		19,875		19,594		18,499		17,371		16,595		16,253		79,451		68,718		64,168
Total other-than-temporary impairment losses	21		23		(162)		(109)		(123)		(1,298)		(423)				(227)		(1,870)		137
Portion of loss recognized in other comprehensive income (before tax)	(21)		(23)		12		109		123		23						77		172		(137)
Net other-than-temporary impairment losses recognized in earnings					(150)						(1,275)		(423)				(150)		(1,698)		0
Net realized gains (losses)	4,043		1,586		1,960		1,597		(2,080)		518		4,339		5,596
Other income (expense)	1,905		(183)		4,430		2,549		4,147		(2,518)		(4,362)		2,242		8,701		(491)		10,656
Total revenues	310,040		305,288		294,107		288,098		272,111		268,178		258,477		260,223		1,197,533		1,058,989		1,023,531
Expenses:
Net losses and LAE	172,493		172,793		167,206		152,956		153,881		146,546		141,973		130,198		665,448		572,598		545,229
Commission expenses	44,154		42,611		40,726		37,554		31,339		34,253		31,480		32,905		165,045		129,977		125,528
Other operating expenses	58,076		56,137		59,074		60,809		59,219		56,599		52,789		54,909
Interest expense	3,860		3,859		3,858		3,858		3,857		3,856		3,856		3,855		15,435		15,424		15,413
Total expenses	278,583		275,400		270,864		255,177		248,296		241,254		230,098		221,867		1,080,024		941,515		882,995
Income before income taxes	31,457		29,888		23,243		32,921		23,815		26,924		28,379		38,356		117,509		117,474		140,536
Income tax expense	9,866		7,875		7,053		9,989		6,072		8,723		9,195		12,427		34,783		36,417		45,207
Net income	21,591		22,013		16,190		22,932		17,743		18,201		19,184		25,929		82,726		81,057		95,329
Comprehensive income (loss)	$ (31,712)		$ 18,760		$ 35,228		$ 45,721		$ 15,990		$ 18,849		$ (3,789)		$ 27,052		$ 67,997		$ 58,102		$ 113,528
Combined ratio	96.70%		95.60%		99.60%		95.10%		97.20%		93.40%		93.40%		92.30%		96.70%		94.10%		92.60%
Net income per share:
Basic	$ 0.74	[1]	$ 0.76	[1]	$ 0.56	[1]	$ 0.79	[1]	$ 0.62	[2]	$ 0.63	[2]	$ 0.67	[2]	$ 0.90	[2]	$ 2.85	[1]	$ 2.82	[1]	$ 3.34	[1]
Diluted	$ 0.71	[1]	$ 0.73	[1]	$ 0.54	[1]	$ 0.77	[1]	$ 0.59	[2]	$ 0.61	[2]	$ 0.65	[2]	$ 0.89	[2]	$ 2.75	[1]	$ 2.73	[1]	$ 3.25	[1]

[1] We completed a two-for-one stock split on January 20, 2017. All share and per share data has been retroactively restated on a post-split basis.

[2] We completed a two-for-one stock split on January 20, 2017. All per share data has been retroactively restated on a post-split basis.