Schedule VI - Supplemental Information Concerning Property-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 [Abstract]
Deferred policy acquisition costs	$ 41,114	$ 40,939	$ 40,720
Reserves for losses and settlement expenses	732,612	690,532	678,774
Discount, if any, deducted from reserves	0	0	0
Unearned premiums	257,797	244,885	239,435
Earned premiums	607,158	592,408	570,266
Net investment income	45,441	47,477	45,591
Losses and settlement expenses incurred related to current year	441,588	427,838	405,850
Losses and settlement expenses incurred related to prior years	(19,619)	(40,941)	(35,114)
Amortization of deferred policy acquisition costs	108,910	108,403	102,184
Paid losses and settlement expenses	392,586	372,888	348,081
Premiums written	$ 616,301	$ 594,703	$ 578,938