Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) segment Subsidiary State	Mar. 31, 2018 USD ($)
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	$ 870,245	$ 808,084
Losses and loss adjustment expenses	630,416	632,234
Underwriting expenses	215,900	206,400
Underwriting gain (loss)	23,900	(30,500)
Net investment income	34,174	31,510
Net realized investment gains (losses)	111,074	(58,735)
Other	2,250	2,325
Interest expense	(4,256)	(4,266)
Pre-tax income (loss)	167,169	(59,699)
Net income (loss)	135,867	(42,607)
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	863,100	800,300
Losses and loss adjustment expenses	626,800	628,300
Underwriting expenses	212,500	202,600
Underwriting gain (loss)	23,800	(30,600)
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,100	7,800
Losses and loss adjustment expenses	3,600	3,900
Underwriting expenses	3,400	3,800
Underwriting gain (loss)	$ 100	$ 100