Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest income	$ 800,000	$ 600,000	$ 2,500,000	$ 1,300,000
Gross realized loss	$ 0	$ 0	$ 200,000	$ 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