Schedule VI. Supplemental Information Concerning Property and Casualty Insurance Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Net realized investment gains	$ 4,055	$ 4,947	$ 1,124
Claims and Claim Adjustment Expenses Incurred Related to: Current Year	779,966	683,662	560,482
Claims and Claim Adjustment Expenses Incurred Related to: Prior Years	(54,253)	(31,229)	(40,395)
Consolidated Property and Casualty Insurance Entity
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Policy Acquisition Costs	88,102	93,362	90,547
Reserves for Unpaid Claims and Claim Adjustment Expenses	1,224,183	1,123,896	1,003,895
Unearned Premiums	465,391	443,802	414,971
Earned Premiums	997,492	936,131	851,695
Net realized investment gains	4,055	4,947	1,124
Net Investment Income	51,190	55,284	46,559
Claims and Claim Adjustment Expenses Incurred Related to: Current Year	(779,966)	683,662	560,482
Claims and Claim Adjustment Expenses Incurred Related to: Prior Years	(54,253)	(31,229)	(40,395)
Amortization of Deferred Policy Acquisition Costs	207,746	202,892	180,183
Paid Claims and Claim Adjustment Expenses	625,503	537,573	476,525
Premiums Written	$ 1,019,113	$ 964,970	$ 887,874