Fair Value of Financial Instruments - Schedule of Fair Value of Financial Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	$ 3,194	$ 11,844
Fair Value Measurement on Recurring Basis
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	3,194	11,844
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	2,408	7,115
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	786	4,729
Level 1 | Fair Value Measurement on Recurring Basis
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	0	0
Level 1 | 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	0	0
Level 1 | 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	0	0
Level 2 | Fair Value Measurement on Recurring Basis
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	0	0
Level 2 | 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	0	0
Level 2 | 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	0	0
Level 3 | Fair Value Measurement on Recurring Basis
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Warrant Liability	3,194	11,844
Level 3 | 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	2,408	7,115
Level 3 | 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	$ 786	$ 4,729