Schedule VI - Supplemental Information Concerning Property-Casualty Insurance Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Abstract]
Deferred policy acquisition costs	$ 44,760	$ 41,114	$ 40,939
Reserves for losses and settlement expenses	777,190	732,612	690,532
Discount, if any, deducted from reserves	0	0	0
Unearned premiums	268,511	257,797	244,885
Earned premiums	645,183	607,158	592,408
Net investment income	47,593	45,441	47,477
Losses and settlement expenses incurred related to current year	475,843	441,588	427,838
Losses and settlement expenses incurred related to prior years	(18,684)	(19,619)	(40,941)
Amortization of deferred policy acquisition costs	115,803	108,910	108,403
Paid losses and settlement expenses	417,135	392,586	372,888
Premiums written	$ 661,043	$ 616,301	$ 594,703