Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Warrants exercised	385,627
Change in fair value of Level 3 financial liabilities	$ 9,379,000	$ (16,598,000)	$ 3,000
Two Thousand Sixteen And Two Thousand Seventeen Private Placement Warrants
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Warrants exercised	226,617	34,493	0
Aggregate fair value of warrants upon exercise	$ 1,200,000	$ 300,000
Change in fair value of Level 3 financial liabilities	9,400,000	(16,600,000)
Transfers between Level 1 to Level 2	0	0
Transfers between Level 2 to Level 1	0	0
Transfers into or out of Level 3	$ 0	$ 0