Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes And Tax Related [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 37,690	$ 0
Accrued interest and penalties recorded	0	$ 0	$ 0
Federal and State
Income Taxes And Tax Related [Line Items]
Net operating loss carryforwards	232,800
Tax credit carryforwards	$ 7,000