Income Taxes - Significant Components of our Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current Federal, State and Local, Tax Expense (Benefit) [Abstract]
Total provision for income taxes	$ 204	$ 116	$ 166
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	37,690	0
Geographic Distribution, Domestic [Member]
Current Federal, State and Local, Tax Expense (Benefit) [Abstract]
Total provision for income taxes	$ 0	$ 0	$ 0