Income Taxes - Components of Income Tax Expense (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Current
Federal	$ 76
State	138	$ 42
Foreign	0	0
Total current	214	42
Deferred expense
Federal	0	0
State	39	(41)
Foreign	0	0
Total deferred	39	(41)
Total income tax expense	$ 253	$ 1