Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Net premiums earned	$ 222,339	$ 222,258	$ 211,471	$ 206,496	$ 207,671	$ 203,473	$ 188,490	$ 177,791	$ 862,564	$ 777,425	$ 649,492
Other revenues	24,860	20,780	24,606	22,344	21,091	22,914	23,402	22,735
Provision for losses and LAE	62,073	55,280	175,877	8,063	10,929	9,990	4,960	7,107	301,293	32,986	11,575
Other underwriting and operating expenses	36,825	37,100	38,819	41,947	41,231	41,588	41,520	41,030	154,691	165,369	150,900
Interest expense	2,149	2,227	2,566	2,132	2,218	2,584	2,679	2,670	9,074	10,151	10,179
Income before income taxes	146,152	148,431	18,815	176,698	174,384	172,225	162,733	149,719	490,096	659,061	546,699
Net income	$ 123,602	$ 124,536	$ 15,380	$ 149,523	$ 146,958	$ 144,630	$ 136,405	$ 127,720	$ 413,041	$ 555,713	$ 467,363
Basic earnings per common share (in dollars per share)	$ 1.10	$ 1.11	$ 0.15	$ 1.53	$ 1.50	$ 1.48	$ 1.39	$ 1.31	$ 3.89	$ 5.68	$ 4.80
Diluted earnings per common share (in dollars per share)	$ 1.10	$ 1.11	$ 0.15	$ 1.52	$ 1.49	$ 1.47	$ 1.39	$ 1.30	$ 3.88	$ 5.66	$ 4.77
Basic weighted average shares outstanding (in shares)	111,908	111,908	102,500	97,949	97,830	97,822	97,798	97,595	106,098	97,762	97,403
Diluted weighted average shares outstanding (in shares)	112,310	112,134	102,605	98,326	98,376	98,257	98,170	98,104	106,376	98,227	97,974