Quarterly report pursuant to Section 13 or 15(d)

Financial Instruments - Summary of Estimated Fair Value and Related Valuation Input Hierarchy of Available-for-Sale Securities (Detail)

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Financial Instruments - Summary of Estimated Fair Value and Related Valuation Input Hierarchy of Available-for-Sale Securities (Detail) - Fair Value, Measurements, Recurring - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost $ 412,818 $ 164,793
Total Unrealized Gain 15 2
Total Unrealized Loss (520) (153)
Total Fair Value 412,313 164,642
Money Market Funds | Level 1    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 58,908 68,730
Total Unrealized Gain 0 0
Total Unrealized Loss 0 0
Total Fair Value 58,908 68,730
U.S. Treasury Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 170,458 39,068
Total Unrealized Gain 5 0
Total Unrealized Loss (146) (28)
Total Fair Value 170,317 39,040
Government Agency Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 16,868 4,749
Total Unrealized Gain 0 0
Total Unrealized Loss (33) (21)
Total Fair Value 16,835 4,728
Corporate Debt Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 133,960 46,532
Total Unrealized Gain 9 2
Total Unrealized Loss (308) (98)
Total Fair Value 133,661 46,436
Commercial Paper | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 17,463 1,592
Total Unrealized Gain 0 0
Total Unrealized Loss 0 0
Total Fair Value 17,463 1,592
Asset-Backed Securities | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 13,661 4,122
Total Unrealized Gain 1 0
Total Unrealized Loss (33) (6)
Total Fair Value 13,629 4,116
Certificate of Deposit | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 1,500  
Total Unrealized Gain 0  
Total Unrealized Loss 0  
Total Fair Value 1,500  
Amounts Classified As Cash Equivalents    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 98,499 77,769
Total Unrealized Gain 2 0
Total Unrealized Loss 0 0
Total Fair Value 98,501 77,769
Amounts Classified As Short-Term Investments    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 314,319 87,024
Total Unrealized Gain 13 2
Total Unrealized Loss (520) (153)
Total Fair Value $ 313,812 $ 86,873