Summary of Quarterly Performance (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
Quarterly Financial Information [Line Items]
Net premiums earned	$ 109,472	$ 119,553	$ 117,804	$ 121,636	$ 123,222	$ 124,707	$ 128,877	$ 127,337	$ 468,465	$ 504,143	$ 268,519
Net investment income	8,880	8,795	6,562	9,746	8,375	8,852	9,141	8,241	33,983	34,609	28,821
Net realized investment gains (losses)	30,778	1,928	(3,492)	(7,493)	3,842	(10,778)	6,532	(2,970)	21,721	(3,374)	35,860
Net losses and loss adjustment expenses	48,946	72,162	78,111	64,784	48,498	77,691	79,560	69,619	264,003	275,368	137,561
Acquisition costs and other underwriting expenses	47,889	48,129	48,542	52,090	51,185	50,934	50,926	48,258	196,650	201,303	109,619
Income (loss) before income taxes	46,535	10,598	(11,468)	1,953	29,463	(9,727)	9,772	3,238	47,618	32,746	71,194
Net income (loss)	$ 38,373	$ 9,535	$ (5,165)	$ 7,125	$ 27,304	$ (3,746)	$ 11,117	$ 6,794	$ 49,868	$ 41,469	$ 62,856
Per share data-Diluted:
Net income (loss)	$ 2.18	$ 0.54	$ (0.30)	$ 0.41	$ 1.30	$ (0.15)	$ 0.43	$ 0.26	$ 2.84	$ 1.69	$ 2.48