Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) State	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) Subsidiary State	Jun. 30, 2015 USD ($)
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	$ 779,300	$ 731,600	$ 1,546,400	$ 1,452,300
Losses and loss adjustment expenses	595,086	521,214	1,189,168	1,035,614
Underwriting expenses	197,600	199,700	400,400	399,200
Underwriting gain	(13,400)	10,700	(43,200)	17,500
Net investment income	31,414	31,697	61,069	63,203
Net realized investment gains (losses)	45,381	(39,348)	70,438	(49,309)
Other	1,887	2,276	4,010	4,542
Interest Expense	(960)	(769)	(1,910)	(1,519)
Pre-tax income	64,335	4,511	90,369	34,370
Net income	48,873	9,639	72,196	35,804
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	768,600	718,500	1,524,400	1,425,700
Losses and loss adjustment expenses	589,400	514,100	1,177,400	1,020,900
Underwriting expenses	193,400	194,900	391,300	388,800
Underwriting gain	(14,200)	9,500	(44,300)	16,000
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	10,700	13,100	22,000	26,600
Losses and loss adjustment expenses	5,700	7,100	11,800	14,700
Underwriting expenses	4,200	4,800	9,100	10,400
Underwriting gain	$ 800	$ 1,200	$ 1,100	$ 1,500