Comprehensive Income and Accumulated Other Comprehensive Income - Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Unrealized gain (loss) on available-for-sale securities, Pre-tax Amount	$ 6,960	$ (4,908)	$ (2,572)
Change in unrealized losses on available-for-sale securities with OTTI, Pre-Tax Amount			176
Less amortization of differences between fair value and amortized cost for fixed maturity security transfer, Pre-tax Amount		(63)	(40)
Reclassification adjustment for gains (losses) realized in net income, Pre-Tax Amount	(1,106)	234	2,093
Net unrealized gain loss, Pre-Tax Amount	5,854	(4,737)	(343)
Other comprehensive income (loss), Pre-Tax Amount	5,854	(4,737)	(343)
Unrealized gain loss on available-for-sale securities, Tax Expense	2,137	(1,718)	(900)
Change in unrealized losses on available-for-sale securities with OTTI, Tax Expense			62
Less amortization of differences between fair value and amortized cost for fixed maturity security transfer, Tax Expense		(22)	(14)
Reclassification adjustment for gains (losses) realized in net income, Tax Expense	(387)	82	732
Net unrealized gain loss, Tax Expense	1,750	(1,658)	(120)
Other comprehensive income (loss), Tax Expense	1,750	(1,658)	(120)
Unrealized gain (loss) on available-for-sale securities, Net-of-Tax Amount	4,823	(3,190)	(1,672)
Change in unrealized losses on available-for-sale securities with OTTI, Net-of-Tax Amount			114
Less amortization of differences between fair value and amortized cost for fixed maturity security transfer, Net-of-Tax Amount		(41)	(26)
Reclassification adjustment for gains (losses) realized in net income, Net-of-Tax Amount	(719)	152	1,361
Net unrealized gain loss, Net-of-Tax Amount	4,104	(3,079)	(223)
Other comprehensive income (loss), Net-of-Tax Amount	$ 4,104	$ (3,079)	$ (223)