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
Mar. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost $ 403,833 $ 164,793
Total Unrealized Gain 7 2
Total Unrealized Loss (531) (153)
Total Fair Value 403,309 164,642
Money Market Funds | Level 1    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 47,411 68,730
Total Unrealized Gain 0 0
Total Unrealized Loss 0 0
Total Fair Value 47,411 68,730
U.S. Treasury Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 157,156 39,068
Total Unrealized Gain 1 0
Total Unrealized Loss (125) (28)
Total Fair Value 157,032 39,040
Government Agency Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 8,481 4,749
Total Unrealized Gain 0 0
Total Unrealized Loss (38) (21)
Total Fair Value 8,443 4,728
Corporate Debt Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 142,148 46,532
Total Unrealized Gain 5 2
Total Unrealized Loss (340) (98)
Total Fair Value 141,813 46,436
Commercial Paper | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 31,828 1,592
Total Unrealized Gain 0 0
Total Unrealized Loss 0 0
Total Fair Value 31,828 1,592
Asset-Backed Securities | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 15,309 4,122
Total Unrealized Gain 1 0
Total Unrealized Loss (28) (6)
Total Fair Value 15,282 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 86,485 77,769
Total Unrealized Gain 0 0
Total Unrealized Loss (2) 0
Total Fair Value 86,483 77,769
Amounts Classified As Short-Term Investments    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 317,348 87,024
Total Unrealized Gain 7 2
Total Unrealized Loss (529) (153)
Total Fair Value $ 316,826 $ 86,873