Investments and Fair Value of Financial Instruments - Summary of Changes in Fair Value and Other Adjustments of Liabilities (Detail) - Level 3 [Member] $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Common stock and warrants issued in conjunction with securities purchase agreements	$ 23,022
Change in fair value and other adjustments	(5,986)
Reclassification of common stock adjustment feature liability balance to equity	(8,378)
Balance as of September 30, 2018	8,658
Common Stock Warrant Liability [Member] | Private Placement [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Common stock and warrants issued in conjunction with securities purchase agreements	10,200
Change in fair value and other adjustments	(6,192)
Balance as of September 30, 2018	4,008
Common Stock Warrant Liability [Member] | June Offering [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Common stock and warrants issued in conjunction with securities purchase agreements	9,022
Change in fair value and other adjustments	(4,372)
Balance as of September 30, 2018	4,650
Common Stock Adjustment Feature Liability [Member] | Private Placement [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Common stock and warrants issued in conjunction with securities purchase agreements	3,800
Change in fair value and other adjustments	4,578
Reclassification of common stock adjustment feature liability balance to equity	$ (8,378)