Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($) State	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) State Subsidiary segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 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	$ 868,233	$ 858,135	$ 833,959	$ 808,084	$ 806,796	$ 801,205	$ 797,666	$ 789,770	$ 3,368,411	$ 3,195,437	$ 3,131,773
Losses and loss adjustment expenses									2,576,789	2,444,884	2,355,138
Underwriting expenses									816,800	788,700	797,900
Underwriting (loss) gain									(25,200)	(38,200)	(21,200)
Net investment income									135,838	124,930	121,871
Net realized investment (losses) gains									(133,520)	83,650	(34,255)
Other income									9,275	11,945	8,294
Interest expense									(17,036)	(15,168)	(3,962)
(Loss) income before income taxes	(114,448)	70,286	73,246	(59,699)	9,487	58,247	68,752	30,599	(30,615)	167,085	70,724
Net (loss) income	$ (81,879)	$ 58,578	$ 60,180	$ (42,607)	$ 19,779	$ 46,485	$ 51,633	$ 26,980	(5,728)	144,877	73,044
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned									3,337,900	3,160,900	3,089,900
Losses and loss adjustment expenses									2,562,000	2,427,800	2,333,200
Underwriting expenses									802,700	773,100	780,400
Underwriting (loss) gain									(26,800)	(40,000)	(23,700)
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned									30,500	34,500	41,900
Losses and loss adjustment expenses									14,800	17,100	21,900
Underwriting expenses									14,100	15,600	17,500
Underwriting (loss) gain									$ 1,600	$ 1,800	$ 2,500