Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) State	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) Subsidiary State	Sep. 30, 2015 USD ($)
Segment Reporting [Abstract]
Number of insurance companies | Subsidiary			14
Number of states in which company operates | State	11		11
Segment Reporting Information [Line Items]
Net premiums earned	$ 790,900	$ 745,500	$ 2,337,300	$ 2,197,800
Losses and loss adjustment expenses	576,316	545,692	1,765,484	1,581,306
Underwriting expenses	199,300	193,700	599,700	592,900
Underwriting gain	15,300	6,100	(27,900)	23,600
Net investment income	30,371	30,898	91,440	94,101
Net realized investment losses	(15,465)	(26,286)	54,973	(75,595)
Other	2,406	2,281	6,416	6,823
Interest Expense	(1,012)	(785)	(2,922)	(2,304)
Pre-tax income	31,625	12,267	121,994	46,637
Net income	26,930	15,270	99,126	51,074
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	780,600	732,900	2,305,000	2,158,600
Losses and loss adjustment expenses	570,800	538,600	1,748,200	1,559,500
Underwriting expenses	194,800	188,600	586,100	577,400
Underwriting gain	15,000	5,700	(29,300)	21,700
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	10,300	12,600	32,300	39,200
Losses and loss adjustment expenses	5,500	7,100	17,300	21,800
Underwriting expenses	4,500	5,100	13,600	15,500
Underwriting gain	$ 300	$ 400	$ 1,400	$ 1,900