Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Schedule of Amortized Cost, Unrealized Gain and Loss, and Fair Value (Detail)

v3.10.0.1
Summary of Significant Accounting Policies - Schedule of Amortized Cost, Unrealized Gain and Loss, and Fair Value (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Amortized Cost $ 167,420 $ 74,200
Gross Unrealized Gains 3  
Gross Unrealized Losses (70) (44)
Estimated Fair Value 167,353 74,156
Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Amortized Cost 76,745 35,886
Gross Unrealized Losses (2)  
Estimated Fair Value 76,743 35,886
Corporate Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Amortized Cost 47,198 19,785
Gross Unrealized Losses (55) (25)
Estimated Fair Value 47,143 19,760
Asset-backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Amortized Cost 31,008 11,070
Gross Unrealized Gains 3  
Gross Unrealized Losses (8) (10)
Estimated Fair Value 31,003 11,060
US Treasury Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Amortized Cost 12,469 7,459
Gross Unrealized Losses (5) (9)
Estimated Fair Value $ 12,464 $ 7,450