Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) Subsidiary State	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Subsidiary State	Sep. 30, 2016 USD ($)
Segment Reporting [Abstract]
Number of insurance companies | Subsidiary	14		14
Number of states in which company operates | State	11		11
Segment Reporting Information [Line Items]
Net premiums earned	$ 801,200	$ 790,900	$ 2,388,600	$ 2,337,300
Losses and loss adjustment expenses	595,290	576,316	1,790,550	1,765,484
Underwriting expenses	200,600	199,300	599,600	599,700
Underwriting gain	5,300	15,300	(1,600)	(27,900)
Net investment income	30,988	30,371	94,058	91,440
Net realized investment gains (losses)	20,718	(15,465)	66,334	54,973
Other	5,446	2,406	9,675	6,416
Interest Expense	(4,191)	(1,012)	(10,873)	(2,922)
Pre-tax income	58,247	31,625	157,598	121,994
Net income	46,485	26,930	125,098	99,126
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	792,700	780,600	2,362,400	2,305,000
Losses and loss adjustment expenses	591,100	570,800	1,777,100	1,748,200
Underwriting expenses	196,700	194,800	587,700	586,100
Underwriting gain	4,900	15,000	(2,400)	(29,300)
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	8,500	10,300	26,200	32,300
Losses and loss adjustment expenses	4,200	5,500	13,500	17,300
Underwriting expenses	3,900	4,500	11,900	13,600
Underwriting gain	$ 400	$ 300	$ 800	$ 1,400