Schedule VI. Supplemental Information Concerning Property-Casualty Insurance Operations (Detail) - 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	$ 20,251	$ 19,300	$ 20,412
Reserves for Unpaid Loss and Loss Adjustment Expense	771,845	742,776	718,033
Unearned Premium	157,270	162,028	167,983
Net Premiums Earned	346,156	368,704	375,894
Net Investment Income	29,281	28,106	27,902
Loss and LAE Related to Current Period	244,094	250,337	262,387
Loss and LAE Related to Prior Periods	(34,770)	(51,306)	(47,814)
Amortization of Deferred Policy Acquisition Costs	(43,009)	(44,321)	(44,841)
Paid Claims and Claim Adjustment Expenses	186,888	187,686	189,666
Net Premiums Written	$ 341,398	$ 362,748	$ 375,301