Investments and Fair Value of Financial Instruments - Summary of Changes in Fair Value and Other Adjustments of Liabilities (Detail) - Level 3 [Member] $ in Thousands	6 Months Ended
	Jun. 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	2,435
Reclassification of common stock adjustment feature liability balance to equity	(8,378)
Balance as of June 30, 2018	17,079
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	10,200
Change in fair value and other adjustments	(2,185)
Balance as of June 30, 2018	8,015
Common Stock Adjustment Feature 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	42
Balance as of June 30, 2018	9,064
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)