Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues:
Net premiums written	$ 760,845	$ 685,287	$ 570,186
Decrease (increase) in unearned premiums	16,580	(35,795)	(40,056)
Net premiums earned	777,425	649,492	530,130
Net investment income	83,542	64,091	40,226
Realized investment gains, net	3,229	1,318	2,015
Other income	3,371	4,452	4,140
Total revenues	867,567	719,353	576,511
Losses and expenses:
Provision for losses and LAE	32,986	11,575	27,232
Other underwriting and operating expenses	165,369	150,900	145,533
Interest expense	10,151	10,179	5,178
Total losses and expenses	208,506	172,654	177,943
Income before income taxes	659,061	546,699	398,568
Income tax expense	103,348	79,336	18,821
Net income	$ 555,713	$ 467,363	$ 379,747
Earnings per share:
Basic (in dollars per share)	$ 5.68	$ 4.80	$ 4.07
Diluted (in dollars per share)	$ 5.66	$ 4.77	$ 3.99
Weighted average shares outstanding:
Basic (in shares)	97,762	97,403	93,330
Diluted (in shares)	98,227	97,974	95,211
Net income	$ 555,713	$ 467,363	$ 379,747
Other comprehensive income (loss):
Change in unrealized appreciation (depreciation) of investments, net of tax expense (benefit) of $17,497 in 2019, ($5,686) in 2018 and $4,519 in 2017	85,180	(25,741)	8,068
Total other comprehensive income (loss)	85,180	(25,741)	8,068
Comprehensive income	$ 640,893	$ 441,622	$ 387,815