Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) State	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) State Subsidiary segment	Sep. 30, 2020 USD ($)
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	$ 940,941	$ 899,304	$ 2,783,682	$ 2,633,775
Losses and loss adjustment expenses	697,947	618,657	1,981,519	1,765,627
Underwriting expenses	233,400	229,100	685,500	683,100
Underwriting gain (loss)	9,600	51,500	116,700	185,100
Net investment income	32,334	32,140	95,566	100,801
Net realized investment (losses) gains	(43,543)	64,436	56,953	(28,458)
Other income	2,481	2,894	7,883	6,810
Interest expense	(4,267)	(4,262)	(12,845)	(12,786)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(3,381)	146,631	264,441	251,392
Net income	1,288	118,857	217,464	207,864
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	933,605	892,012	2,761,934	2,612,153
Losses and loss adjustment expenses	694,200	614,900	1,970,800	1,755,200
Underwriting expenses	229,700	225,400	675,100	672,200
Underwriting gain (loss)	9,700	51,700	116,000	184,800
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,300	7,300	21,800	21,600
Losses and loss adjustment expenses	3,700	3,800	10,700	10,400
Underwriting expenses	3,700	3,700	10,400	10,900
Underwriting gain (loss)	$ (100)	$ (200)	$ 700	$ 300