Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($) State	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) State segment Subsidiary	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting [Abstract]
Number of insurance companies | Subsidiary									14
Number of states in which company operates | State	11								11
Number of reportable segments | segment									1
Segment Reporting Information [Line Items]
Net premiums earned	$ 921,860	$ 899,304	$ 811,897	$ 922,574	$ 925,384	$ 915,012	$ 888,776	$ 870,245	$ 3,555,635	$ 3,599,418	$ 3,368,411
Losses and loss adjustment expenses									2,395,343	2,706,024	2,576,789
Underwriting expenses									913,700	871,400	816,800
Underwriting gain (loss)									246,600	22,000	(25,200)
Net investment income									134,858	141,263	135,838
Net realized investment gains (losses)									85,731	222,793	(133,520)
Other income									8,287	9,044	9,275
Interest expense									(17,048)	(17,035)	(17,036)
Income (loss) before income taxes	207,110	146,631	285,466	(180,705)	28,465	80,840	101,595	167,169	458,501	378,069	(30,615)
Net income (loss)	$ 166,743	$ 118,857	$ 228,211	$ (139,204)	$ 31,688	$ 69,282	$ 83,250	$ 135,867	374,607	320,087	(5,728)
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned									3,526,753	3,571,042	3,337,916
Losses and loss adjustment expenses									2,381,500	2,692,700	2,562,000
Underwriting expenses									899,100	857,300	802,700
Underwriting gain (loss)									246,200	21,000	(26,800)
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned									28,800	28,400	30,500
Losses and loss adjustment expenses									13,800	13,300	14,800
Underwriting expenses									14,600	14,100	14,100
Underwriting gain (loss)									$ 400	$ 1,000	$ 1,600