Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)
Summary Of Significant Accounting Policy [Line Items]
Net foreign exchange transaction loss	$ 0.1	$ 0.1
Deposits in excess of federally-insured limits	$ 100.0	108.7
Number of operating segment | Segment	1
Minimum [Member] | Property Plant and Equipment Other than Leasehold Improvement [Member]
Summary Of Significant Accounting Policy [Line Items]
Estimated useful lives of assets	3 years
Maximum [Member] | Property Plant and Equipment Other than Leasehold Improvement [Member]
Summary Of Significant Accounting Policy [Line Items]
Estimated useful lives of assets	5 years
Other Assets [Member]
Summary Of Significant Accounting Policy [Line Items]
Long-term portion of restricted cash	$ 0.2	$ 0.2