Commitments and Contingencies - Additional Information (Detail) $ in Millions	1 Months Ended	3 Months Ended
	Jan. 31, 2018	Sep. 30, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)
Commitments and Contingencies [Line Items]
Transition tax			$ 40.3
Measurement period adjustment to reduce transition tax		$ 3.1
Other Long-term Liabilities
Commitments and Contingencies [Line Items]
Transition tax		$ 28.9	36.8
Northern Virginia Office Space
Commitments and Contingencies [Line Items]
Office space area leased under the agreement | ft²		214,000
Lease expiration date	2030-12
Northern Virginia Office Space | Other Long-term Liabilities
Commitments and Contingencies [Line Items]
Deferred rent included in other long-term liabilities		$ 27.1	$ 8.5