Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($) Subsidiary segment State	Mar. 31, 2020 USD ($)
Segment Reporting [Abstract]
Number of insurance companies | Subsidiary	14
Number of states in which company operates | State	11
Number of reportable segments | segment	1
Segment Reporting Information [Line Items]
Net	$ 915,922	$ 922,574
Losses and loss adjustment expenses	626,344	651,670
Underwriting expenses	230,100	233,100
Underwriting gain (loss)	59,500	37,800
Net investment income	32,279	34,495
Net realized investment gains (losses)	41,691	(251,320)
Other income	3,204	2,562
Interest expense	(4,343)	(4,256)
Pre-tax income (loss)	132,421	(180,705)
Net income (loss)	106,995	(139,204)
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net	908,815	915,345
Losses and loss adjustment expenses	622,900	648,200
Underwriting expenses	227,300	229,400
Underwriting gain (loss)	58,600	37,700
Other Segments [Member]
Segment Reporting Information [Line Items]
Net	7,100	7,300
Losses and loss adjustment expenses	3,400	3,500
Underwriting expenses	2,800	3,700
Underwriting gain (loss)	$ 900	$ 100