Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($) State	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) State Subsidiary segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 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	$ 925,384	$ 915,012	$ 888,776	$ 870,245	$ 868,233	$ 858,135	$ 833,959	$ 808,084	$ 3,599,418	$ 3,368,411	$ 3,195,437
Losses and loss adjustment expenses									2,706,024	2,576,789	2,444,884
Underwriting expenses									871,400	816,800	788,700
Underwriting gain (loss)									22,000	(25,200)	(38,200)
Net investment income									141,263	135,838	124,930
Net realized investment gains (losses)									222,793	(133,520)	83,650
Other income									9,044	9,275	11,945
Interest expense									(17,035)	(17,036)	(15,168)
Income (loss) before income taxes	28,465	80,840	101,595	167,169	(114,448)	70,286	73,246	(59,699)	378,069	(30,615)	167,085
Net income (loss)	$ 31,688	$ 69,282	$ 83,250	$ 135,867	$ (81,879)	$ 58,578	$ 60,180	$ (42,607)	320,087	(5,728)	144,877
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned									3,571,000	3,337,900	3,160,900
Losses and loss adjustment expenses									2,692,700	2,562,000	2,427,800
Underwriting expenses									857,300	802,700	773,100
Underwriting gain (loss)									21,000	(26,800)	(40,000)
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned									28,400	30,500	34,500
Losses and loss adjustment expenses									13,300	14,800	17,100
Underwriting expenses									14,100	14,100	15,600
Underwriting gain (loss)									$ 1,000	$ 1,600	$ 1,800