Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) Subsidiary State	Mar. 31, 2017 USD ($)
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	$ 808,100	$ 789,800
Losses and loss adjustment expenses	632,234	606,665
Underwriting expenses	206,400	207,800
Underwriting gain	(30,500)	(24,700)
Net investment income	31,510	31,169
Net realized investment (losses) gains	(58,735)	24,460
Other	2,325	2,105
Interest Expense	(4,266)	(2,453)
Pre-tax income	(59,699)	30,599
Net (loss) income	(42,607)	26,980
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	800,300	780,800
Losses and loss adjustment expenses	628,300	601,800
Underwriting expenses	202,600	203,800
Underwriting gain	(30,600)	(24,800)
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,800	9,000
Losses and loss adjustment expenses	3,900	4,900
Underwriting expenses	3,800	4,000
Underwriting gain	$ 100	$ 100