Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Gross unrecognized tax benefits, beginning of year			$ 9
Increase in tax position in current year	$ 0	$ 0	0
Settlement/Expiration of statute	$ 0	$ 0	0
De-recognition through administrative policy			$ (9)