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Fair Value Measurements - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents $ 17,907 $ 29,251
Total assets measured at fair value 78,523 67,012
Available-for-sale [Member] | Short-term Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value 60,616 33,759
Available-for-sale [Member] | Short-term Investments [Member] | Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value 21,193 7,843
Available-for-sale [Member] | Short-term Investments [Member] | Corporate Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value 21,594 20,913
Available-for-sale [Member] | Short-term Investments [Member] | United States Treasury Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value 10,427 5,003
Available-for-sale [Member] | Short-term Investments [Member] | Asset-backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value 7,402  
Available-for-sale [Member] | Long-term Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value   4,002
Available-for-sale [Member] | Long-term Investments [Member] | Asset-backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value   4,002
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 17,907 29,251
Total assets measured at fair value 28,334 34,254
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Available-for-sale [Member] | Short-term Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value 10,427 5,003
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Available-for-sale [Member] | Short-term Investments [Member] | United States Treasury Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value 10,427 5,003
Significant Other Observable Inputs (Level 2) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value 50,189 32,758
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Short-term Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value 50,189 28,756
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Short-term Investments [Member] | Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value 21,193 7,843
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Short-term Investments [Member] | Corporate Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value 21,594 20,913
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Short-term Investments [Member] | Asset-backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value $ 7,402  
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Long-term Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value   4,002
Significant Other Observable Inputs (Level 2) [Member] | Available-for-sale [Member] | Long-term Investments [Member] | Asset-backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value   $ 4,002