Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis (Detail)

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Summary of Significant Accounting Policies - Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value $ 175,619 $ 93,013
Total liabilities measured at fair value   6,091
Warrant Liability [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total liabilities measured at fair value   6,091
Cash Equivalents [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 45,950 18,857
Cash Equivalents [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 39,481 12,822
Cash Equivalents [Member] | U.S. and Foreign Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 6,469 6,035
Short-term Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 129,669 74,156
Short-term Investments [Member] | U.S. and Foreign Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 51,627 35,886
Short-term Investments [Member] | U.S. and Foreign Corporate Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 34,634 19,760
Short-term Investments [Member] | Asset-backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 25,472 11,060
Short-term Investments [Member] | U.S. Treasury Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 17,936 7,450
Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 39,481 12,822
Level 1 [Member] | Cash Equivalents [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 39,481 12,822
Level 1 [Member] | Cash Equivalents [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 39,481 12,822
Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 136,138 80,191
Level 2 [Member] | Cash Equivalents [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 6,469 6,035
Level 2 [Member] | Cash Equivalents [Member] | U.S. and Foreign Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 6,469 6,035
Level 2 [Member] | Short-term Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 129,669 74,156
Level 2 [Member] | Short-term Investments [Member] | U.S. and Foreign Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 51,627 35,886
Level 2 [Member] | Short-term Investments [Member] | U.S. and Foreign Corporate Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 34,634 19,760
Level 2 [Member] | Short-term Investments [Member] | Asset-backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 25,472 11,060
Level 2 [Member] | Short-term Investments [Member] | U.S. Treasury Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value $ 17,936 7,450
Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total liabilities measured at fair value   6,091
Level 3 [Member] | Warrant Liability [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total liabilities measured at fair value   $ 6,091