Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest income	$ 900,000	$ 400,000	$ 1,700,000	$ 700,000
Gross realized gain (loss)	$ (200,000)	$ 0	$ (200,000.0)	$ 0
Warrants exercise price (in dollars per share)	$ 0.05		$ 0.05
Long-term Receivable | Accounting Standards Update 2016-16
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax effects deferred from past intercompany transactions					$ 2,100,000