Fair Value Measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	$ 155,069	$ 174,360
Total liabilities	25,290	23,509
Common stock warrants
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	25,290	23,509
(Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	82,160	67,056
Total liabilities	0	0
(Level 1) | Common stock warrants
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	0	0
(Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	72,909	107,304
Total liabilities	0	0
(Level 2) | Common stock warrants
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	0	0
(Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	0	0
Total liabilities	25,290	23,509
(Level 3) | Common stock warrants
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities	25,290	23,509
Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	69,346	49,077
Cash equivalents | (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	69,346	49,077
Cash equivalents | (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	0	0
Cash equivalents | (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	0	0
Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	17,363	24,157
Commercial paper | (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	0	0
Commercial paper | (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	17,363	24,157
Commercial paper | (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	0	0
U.S. Government-sponsored enterprise
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	20,318	33,099
U.S. Government-sponsored enterprise | (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	0	0
U.S. Government-sponsored enterprise | (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	20,318	33,099
U.S. Government-sponsored enterprise | (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	0	0
U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	12,814	17,979
U.S. Treasury securities | (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	12,814	17,979
U.S. Treasury securities | (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	0	0
U.S. Treasury securities | (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	0	0
Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	35,228	50,048
Corporate debt securities | (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	0	0
Corporate debt securities | (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	35,228	50,048
Corporate debt securities | (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	$ 0	$ 0