Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) State	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) Subsidiary State	Jun. 30, 2016 USD ($)
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	$ 797,700	$ 779,300	$ 1,587,400	$ 1,546,400
Losses and loss adjustment expenses	588,595	595,086	1,195,260	1,189,168
Underwriting expenses	191,300	197,600	399,000	400,400
Underwriting gain	17,800	(13,400)	(6,900)	(43,200)
Net investment income	31,901	31,414	63,070	61,069
Net realized investment gains	21,156	45,381	45,616	70,438
Other	2,124	1,887	4,229	4,010
Interest Expense	(4,229)	(960)	(6,682)	(1,910)
Pre-tax income	68,752	64,335	99,351	90,369
Net income	51,633	48,873	78,613	72,196
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	789,000	768,600	1,569,700	1,524,400
Losses and loss adjustment expenses	584,200	589,400	1,186,000	1,177,400
Underwriting expenses	187,300	193,400	391,100	391,300
Underwriting gain	17,500	(14,200)	(7,400)	(44,300)
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	8,700	10,700	17,700	22,000
Losses and loss adjustment expenses	4,400	5,700	9,300	11,800
Underwriting expenses	4,000	4,200	7,900	9,100
Underwriting gain	$ 300	$ 800	$ 500	$ 1,100