Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) segment Subsidiary State	Mar. 31, 2019 USD ($)
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 premiums earned	$ 922,574	$ 870,245
Losses and loss adjustment expenses	651,670	630,416
Underwriting expenses	233,100	215,900
Underwriting gain (loss)	37,800	23,900
Net investment income	34,495	34,174
Net realized investment gains (losses)	(251,320)	111,074
Other	2,562	2,250
Interest expense	(4,256)	(4,256)
Pre-tax income	(180,705)	167,169
Net income	(139,204)	135,867
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	915,400	863,100
Losses and loss adjustment expenses	648,200	626,800
Underwriting expenses	229,400	212,500
Underwriting gain (loss)	37,800	23,800
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,200	7,100
Losses and loss adjustment expenses	3,500	3,600
Underwriting expenses	3,700	3,400
Underwriting gain (loss)	$ 0	$ 100