Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) Subsidiary State	Mar. 31, 2016 USD ($)
Segment Reporting [Abstract]
Number of insurance companies | Subsidiary	14
Number of states in which company operates | State	11
Segment Reporting Information [Line Items]
Net premiums earned	$ 789,800	$ 767,100
Losses and loss adjustment expenses	606,665	594,082
Underwriting expenses	207,800	202,800
Underwriting gain	(24,700)	(29,800)
Net investment income	31,169	29,655
Net realized investment gains	24,460	25,057
Other	2,105	2,123
Interest Expense	(2,453)	(950)
Pre-tax income	30,599	26,034
Net income	26,980	23,323
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	780,800	755,800
Losses and loss adjustment expenses	601,800	588,000
Underwriting expenses	203,800	197,900
Underwriting gain	(24,800)	(30,100)
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	9,000	11,300
Losses and loss adjustment expenses	4,900	6,100
Underwriting expenses	4,000	4,900
Underwriting gain	$ 100	$ 300