Income Taxes - Schedule of Company's Deferred Income Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred income tax assets:
Discounting of net unpaid loss and loss adjustment expenses	$ 8,830	$ 16,035
Unearned premiums	8,466	14,483
Accrued expenses and other	2,359	3,640
State income tax	782	617
Accrued policyholder dividends	1,918	2,024
Impaired securities	42	70
Capital loss carryforward	190	15
Accrued insurance-related assessments	3,256	5,323
Net unrealized loss on securities		265
Total deferred tax assets	25,843	42,472
Deferred income tax liabilities:
Deferred policy acquisition costs	(5,317)	(8,492)
Callable bond amortization	(8)	(10)
Unrealized gain on securities available-for-sale	(1,070)
Property and equipment and other	(3)	204
Salvage and subrogation	(183)	(363)
Total deferred income tax liabilities	(6,581)	(8,661)
Net deferred income taxes	$ 19,262	$ 33,811