Commitments and Contingencies - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($) ft²	Dec. 31, 2016 USD ($)
Commitments and Contingencies [Line Items]
Date of dismissal	Oct. 20, 2017
Northern Virginia Office Space
Commitments and Contingencies [Line Items]
Office space area leased under the agreement | ft²	214,000
Lease expiration date	2020-12
Deferred rent included in other long-term liabilities	$ 9.5	$ 12.3
Deferred rent included in current accrued expenses	$ 3.7	$ 3.5