Income Taxes - Components of Income Tax Expense (Benefit) (Parenthetical) (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Deferred Federal AMT expense (benefit)	$ 8,000	$ (8,000)	$ (27,000)	$ (149,000)