Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) State	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Subsidiary segment State	Sep. 30, 2017 USD ($)
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	$ 858,135	$ 801,205	$ 2,500,178	$ 2,388,641
Losses and loss adjustment expenses	614,069	595,290	1,851,850	1,790,550
Underwriting expenses	206,100	200,600	614,900	599,600
Underwriting gain (loss)	37,900	5,300	33,400	(1,600)
Net investment income	38,159	30,988	104,455	94,058
Net realized investment (losses) gains	(3,910)	20,718	(48,355)	66,334
Other	2,427	5,446	7,108	9,675
Interest expense	(4,257)	(4,191)	(12,779)	(10,873)
Pre-tax income	70,286	58,247	83,833	157,598
Net income	58,578	46,485	76,151	125,098
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	850,500	792,700	2,476,900	2,362,400
Losses and loss adjustment expenses	610,200	591,100	1,840,100	1,777,100
Underwriting expenses	202,800	196,700	604,000	587,700
Underwriting gain (loss)	37,500	4,900	32,800	(2,400)
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,600	8,500	23,300	26,200
Losses and loss adjustment expenses	3,900	4,200	11,800	13,500
Underwriting expenses	3,300	3,900	10,900	11,900
Underwriting gain (loss)	$ 400	$ 400	$ 600	$ 800