Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Quarterly Financial Information Disclosure [Abstract]
Net premiums earned	$ 89,378	$ 83,694	$ 78,361	$ 75,038	$ 67,814	$ 60,323	$ 50,342	$ 44,750	$ 326,471	$ 223,229	$ 123,371
Other revenues	8,098	8,042	5,706	4,973	4,928	4,298	3,941	3,071
Provision for losses and LAE	4,199	3,393	2,314	1,999	3,049	1,391	966	902	11,905	6,308	2,321
Other underwriting and operating expenses	29,627	28,714	27,148	27,498	25,656	24,469	23,648	23,459	112,987	97,232	71,055
Income before income taxes	63,650	59,629	54,605	50,514	44,037	38,761	29,669	23,460	228,398	135,927	58,027
Net income	$ 44,479	$ 40,821	$ 37,193	$ 34,838	$ 28,866	$ 25,070	$ 19,555	$ 15,006	$ 157,331	$ 88,497	$ 65,413
Basic earnings (loss) per share (in dollars per share)	$ 0.49	$ 0.45	$ 0.41	$ 0.39	$ 0.34	$ 0.30	$ 0.23	$ 0.18	$ 1.74	$ 1.05	$ 0.90
Diluted earnings (loss) per share (in dollars per share)	$ 0.48	$ 0.44	$ 0.41	$ 0.38	$ 0.33	$ 0.29	$ 0.23	$ 0.18	$ 1.72	$ 1.03	$ 0.70
Basic weighted average shares outstanding (in shares)	90,454	90,418	90,344	90,185	86,134	83,640	83,276	82,864	90,351	83,986	14,044
Diluted weighted average shares outstanding (in shares)	91,918	91,841	91,674	91,514	87,950	85,028	84,706	84,696	91,738	85,602	18,103