Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) State	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) State Subsidiary segment	Jun. 30, 2019 USD ($)
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	$ 811,898	$ 888,776	$ 1,734,471	$ 1,759,021
Losses and loss adjustment expenses	495,300	656,577	1,146,970	1,286,993
Underwriting expenses	220,800	217,000	453,900	432,900
Underwriting gain (loss)	95,800	15,200	133,600	39,100
Net investment income	34,166	35,032	68,661	69,206
Net realized investment gains (losses)	158,426	53,329	(92,894)	164,403
Other	1,353	2,350	3,915	4,600
Interest expense	(4,268)	(4,266)	(8,523)	(8,522)
Pre-tax income	285,466	101,595	104,760	268,764
Net income	228,211	83,250	89,007	219,117
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	804,800	881,800	1,720,200	1,744,900
Losses and loss adjustment expenses	492,100	653,300	1,140,300	1,280,100
Underwriting expenses	217,400	213,500	446,800	426,000
Underwriting gain (loss)	95,300	15,000	133,100	38,800
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,100	7,000	14,300	14,100
Losses and loss adjustment expenses	3,200	3,300	6,700	6,900
Underwriting expenses	3,400	3,500	7,100	6,900
Underwriting gain (loss)	$ 500	$ 200	$ 500	$ 300