Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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 Disclosure [Abstract]
Net premiums earned	$ 116,792	$ 110,801	$ 100,711	$ 94,403	$ 89,378	$ 83,694	$ 78,361	$ 75,038	$ 422,707	$ 326,471	$ 223,229
Other revenues	9,581	10,453	7,454	8,063	8,098	8,042	5,706	4,973
Provision for losses and LAE	3,865	4,965	2,964	3,731	4,199	3,393	2,314	1,999	15,525	11,905	6,308
Other underwriting and operating expenses	35,206	32,848	31,409	31,388	29,627	28,714	27,148	27,498	130,851	112,987	97,232
Income before income taxes	87,302	83,441	73,792	67,347	63,650	59,629	54,605	50,514	311,882	228,398	135,927
Net income	$ 62,686	$ 59,711	$ 52,258	$ 47,951	$ 44,479	$ 40,821	$ 37,193	$ 34,838	$ 222,606	$ 157,331	$ 88,497
Basic earnings per share (in dollars per share)	$ 0.69	$ 0.66	$ 0.57	$ 0.53	$ 0.49	$ 0.45	$ 0.41	$ 0.39	$ 2.45	$ 1.74	$ 1.05
Diluted earnings per share (in dollars per share)	$ 0.68	$ 0.65	$ 0.57	$ 0.52	$ 0.48	$ 0.44	$ 0.41	$ 0.38	$ 2.41	$ 1.72	$ 1.03
Basic weighted average shares outstanding (in shares)	90,991	90,961	90,912	90,785	90,454	90,418	90,344	90,185	90,913	90,351	83,986
Diluted weighted average shares outstanding (in shares)	92,577	92,399	92,138	91,859	91,918	91,841	91,674	91,514	92,245	91,738	85,602