Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Convertible preferred stock warrant liability				$ 93,000
Change in fair value of Level 3 financial liabilities		$ (16,598,000)	$ 3,000	$ 45,000
2016 Placement Warrant
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Placement warrants exercised	25,014
Aggregate fair value of warrants upon exercise		$ 200,000
Placement warrants outstanding	1,355,641	1,355,641
2017 Placement Warrants
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Placement warrants exercised	9,389
Aggregate fair value of warrants upon exercise		$ 74,000
Placement warrants outstanding	1,711,123	1,711,123
Two Thousand Sixteen And Two Thousand Seventeen Private Placement Warrants
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Aggregate fair value of warrants upon exercise	$ 274,000
Change in fair value of Level 3 financial liabilities		$ (16,600,000)	$ 3,000
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