Commitments and Contingencies - Amounts of Assets Under Capitalized Lease Agreements and Related Obligations are Included in Debt (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Capital Leased Assets [Line Items]
Accumulated depreciation	$ (2,717)	$ (2,429)
Net assets under capital leases	1,049	1,337
Property and Equipment [Member]
Capital Leased Assets [Line Items]
Other long-term assets	3,636	3,636
Prepaid Expenses and Other Assets [Member]
Capital Leased Assets [Line Items]
Other long-term assets	$ 130	$ 130