Fair Value of Financial Instruments - Schedule of Fair Value of Financial Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	$ 11,844	$ 22,420
Fair Value Measurement on Recurring Basis
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	11,844	22,420
Fair Value Measurement on Recurring Basis | 2017 Placement Warrants
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	7,115	12,487
Fair Value Measurement on Recurring Basis | 2016 Placement Warrants
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	4,729	9,933
Fair Value Measurement on Recurring Basis | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	0	0
Fair Value Measurement on Recurring Basis | Level 1 | 2017 Placement Warrants
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	0	0
Fair Value Measurement on Recurring Basis | Level 1 | 2016 Placement Warrants
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	0	0
Fair Value Measurement on Recurring Basis | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	0	0
Fair Value Measurement on Recurring Basis | Level 2 | 2017 Placement Warrants
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	0	0
Fair Value Measurement on Recurring Basis | Level 2 | 2016 Placement Warrants
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	0	0
Fair Value Measurement on Recurring Basis | Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	11,844	22,420
Fair Value Measurement on Recurring Basis | Level 3 | 2017 Placement Warrants
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	7,115	12,487
Fair Value Measurement on Recurring Basis | Level 3 | 2016 Placement Warrants
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	$ 4,729	$ 9,933