Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) State	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment Subsidiary State	Sep. 30, 2019 USD ($)
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	$ 899,304	$ 915,012	$ 2,633,775	$ 2,674,034
Losses and loss adjustment expenses	618,657	680,928	1,765,627	1,967,921
Underwriting expenses	229,100	221,300	683,100	654,200
Underwriting gain (loss)	51,500	12,800	185,100	51,900
Net investment income	32,140	36,356	100,801	105,562
Net realized investment gains (losses)	64,436	33,324	(28,458)	197,726
Other	2,894	2,603	6,810	7,203
Interest expense	(4,262)	(4,257)	(12,786)	(12,779)
Pre-tax income	146,631	80,840	251,392	349,604
Net income	118,857	69,282	207,864	288,399
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	892,000	907,900	2,612,200	2,652,800
Losses and loss adjustment expenses	614,900	677,300	1,755,200	1,957,400
Underwriting expenses	225,400	217,800	672,200	643,800
Underwriting gain (loss)	51,700	12,800	184,800	51,600
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,300	7,100	21,600	21,200
Losses and loss adjustment expenses	3,800	3,600	10,400	10,500
Underwriting expenses	3,700	3,500	10,900	10,400
Underwriting gain (loss)	$ (200)	$ 0	$ 300	$ 300