Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($) State	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) State Subsidiary	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Segment Reporting [Abstract]
Number of insurance companies | Subsidiary									14
Number of states in which company operates | State	13								13
Segment Reporting Information [Line Items]
Net premiums earned	$ 760,094	$ 745,520	$ 731,546	$ 720,737	$ 709,368	$ 705,237	$ 697,889	$ 683,701	$ 2,957,897	$ 2,796,195	$ 2,698,187
Losses and loss adjustment expenses									2,145,495	1,986,122	1,962,690
Underwriting expenses									790,000	775,600	724,900
Underwriting gain									22,400	34,500	10,600
Net investment income									126,299	125,723	124,538
Net realized investment (losses) gains	(8,200)				(21,600)				(83,807)	81,184	(11,422)
Other									8,911	8,671	9,738
Interest Expense									(3,168)	(2,637)	(1,260)
Pre-tax income	23,930	12,267	4,511	29,859	(28,161)	37,120	136,436	102,030	70,567	247,425	132,096
Net income	$ 23,405	$ 15,270	$ 9,639	$ 26,165	$ (20,956)	$ 31,296	$ 94,960	$ 72,649	74,479	177,949	112,143
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,906,600	2,737,300	2,638,400
Losses and loss adjustment expenses									2,117,300	1,951,400	1,926,400
Underwriting expenses									770,000	749,700	701,000
Underwriting gain									19,300	36,200	11,000
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned									51,300	58,900	59,800
Losses and loss adjustment expenses									28,200	34,700	36,300
Underwriting expenses									20,000	25,900	23,900
Underwriting gain									$ 3,100	$ (1,700)	$ (400)