Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) State	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Subsidiary segment State	Sep. 30, 2018 USD ($)
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	$ 915,012	$ 858,135	$ 2,674,034	$ 2,500,178
Losses and loss adjustment expenses	680,928	614,069	1,967,921	1,851,850
Underwriting expenses	221,300	206,100	654,200	614,900
Underwriting gain (loss)	12,800	37,900	51,900	33,400
Net investment income	36,356	38,159	105,562	104,455
Net realized investment gains (losses)	33,324	(3,910)	197,726	(48,355)
Other	2,603	2,427	7,203	7,108
Interest expense	(4,257)	(4,257)	(12,779)	(12,779)
Pre-tax income	80,840	70,286	349,604	83,833
Net income	69,282	58,578	288,399	76,151
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	907,900	850,500	2,652,800	2,476,900
Losses and loss adjustment expenses	677,300	610,200	1,957,400	1,840,100
Underwriting expenses	217,800	202,800	643,800	604,000
Underwriting gain (loss)	12,800	37,500	51,600	32,800
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,100	7,600	21,200	23,300
Losses and loss adjustment expenses	3,600	3,900	10,500	11,800
Underwriting expenses	3,500	3,300	10,400	10,900
Underwriting gain (loss)	$ 0	$ 400	$ 300	$ 600