Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($) Subsidiary State	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting [Abstract]
Number of insurance companies | Subsidiary	14
Number of states in which company operates | State	13
Segment Reporting Information [Line Items]
Net premiums earned	$ 767,100	$ 720,700	$ 767,100	$ 720,700
Losses and loss adjustment expenses	594,082	514,400
Underwriting expenses	202,800	199,500
Underwriting gain	(29,800)	6,800
Net investment income	29,655	31,506
Net realized investment gains (losses)	25,057	(9,961)
Other	2,123	2,266
Interest Expense	(950)	(750)
Pre-tax income	26,034	29,859
Net income	23,323	26,165
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	755,800	707,200	755,800	707,200
Losses and loss adjustment expenses	588,000	506,800
Underwriting expenses	197,900	193,900
Underwriting gain	(30,100)	6,500
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	11,300	13,500	$ 11,300	$ 13,500
Losses and loss adjustment expenses	6,100	7,600
Underwriting expenses	4,900	5,600
Underwriting gain	$ 300	$ 300