Quarterly Results of Operations (Unaudited) - Summary of Quarterly Results (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								5 Months Ended	12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Net premiums earned Investment income, net of realized gains (losses) on investments Total revenues Total expenses Net Income Basic earnings per share Diluted earnings per share	$ 81,554	$ 55,527	$ 44,295	$ 42,236	$ 31,764	$ 21,805	$ 21,624	$ 19,966	$ 5,599	$ 223,612	$ 95,159
Investment income, including realized gains (losses) on investments	1,628	1,206	743	576	258	179	79	210
Total revenues	85,390	58,013	46,539	43,878	33,109	22,780	48,601	20,342	5,630	233,820	124,832
Total expenses	54,179	41,904	31,429	32,056	30,549	15,113	14,320	9,389	10,237	159,568	69,371
Net Income	$ 19,678	$ 9,965	$ 9,566	$ 7,888	$ 814	$ 5,327	$ 21,018	$ 7,054	$ (5,466)	$ 47,097	$ 34,213
Basic earnings per share	$ 0.67	$ 0.33	$ 0.43	$ 0.48	$ 0.05	$ 0.35	$ 1.38	$ 0.66	$ (0.87)	$ 1.92	$ 2.39
Diluted earnings per share	$ 0.67	$ 0.33	$ 0.39	$ 0.42	$ 0.05	$ 0.35	$ 1.38	$ 0.66	$ (0.87)	$ 1.82	$ 2.36