Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues:
Net premiums written	$ 441,278	$ 370,568	$ 276,778
Increase in unearned premiums	(18,571)	(44,097)	(53,549)
Net premiums earned	422,707	326,471	223,229
Net investment income	27,890	19,885	12,285
Realized investment gains, net	1,934	2,554	925
Other income	5,727	4,380	3,028
Total revenues	458,258	353,290	239,467
Losses and expenses:
Provision for losses and LAE	15,525	11,905	6,308
Other underwriting and operating expenses	130,851	112,987	97,232
Total losses and expenses	146,376	124,892	103,540
Income before income taxes	311,882	228,398	135,927
Income tax expense	89,276	71,067	47,430
Net income	$ 222,606	$ 157,331	$ 88,497
Earnings per share:
Basic (in dollars per share)	$ 2.45	$ 1.74	$ 1.05
Diluted (in dollars per share)	$ 2.41	$ 1.72	$ 1.03
Weighted average common shares outstanding:
Basic (in shares)	90,913	90,351	83,986
Diluted (in shares)	92,245	91,738	85,602
Net income	$ 222,606	$ 157,331	$ 88,497
Other comprehensive income (loss):
Change in unrealized (depreciation) appreciation of investments, net of tax (benefit) expense of ($2,941) in 2016, ($1,285) in 2015 and $2,825 in 2014	(12,156)	(4,766)	6,114
Total other comprehensive (loss) income	(12,156)	(4,766)	6,114
Comprehensive income	$ 210,450	$ 152,565	$ 94,611