Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues:
Net premiums written	$ 834,113	$ 760,845	$ 685,287
Decrease (increase) in unearned premiums	28,451	16,580	(35,795)
Net premiums earned	862,564	777,425	649,492
Net investment income	80,087	83,542	64,091
Realized investment gains, net	2,697	3,229	1,318
Other income	9,806	3,371	4,452
Total revenues	955,154	867,567	719,353
Losses and expenses:
Provision for losses and LAE	301,293	32,986	11,575
Other underwriting and operating expenses	154,691	165,369	150,900
Interest expense	9,074	10,151	10,179
Total losses and expenses	465,058	208,506	172,654
Income before income taxes	490,096	659,061	546,699
Income tax expense	77,055	103,348	79,336
Net income	$ 413,041	$ 555,713	$ 467,363
Earnings per share:
Basic (in dollars per share)	$ 3.89	$ 5.68	$ 4.80
Diluted (in dollars per share)	$ 3.88	$ 5.66	$ 4.77
Weighted average shares outstanding:
Basic (in shares)	106,098	97,762	97,403
Diluted (in shares)	106,376	98,227	97,974
Net income	$ 413,041	$ 555,713	$ 467,363
Other comprehensive income (loss):
Change in unrealized appreciation (depreciation) of investments, net of tax expense (benefit) of $16,836 in 2020, $17,497 in 2019 and $(5,686) in 2018	82,087	85,180	(25,741)
Total other comprehensive income (loss)	82,087	85,180	(25,741)
Comprehensive income	$ 495,128	$ 640,893	$ 441,622