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
Sep. 30, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost $ 363,721 $ 164,793
Total Unrealized Gain 6 2
Total Unrealized Loss (456) (153)
Total Fair Value 363,271 164,642
Money Market Funds | Level 1    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 40,424 68,730
Total Unrealized Gain 0 0
Total Unrealized Loss 0 0
Total Fair Value 40,424 68,730
U.S. Treasury Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 138,332 39,068
Total Unrealized Gain   0
Total Unrealized Loss (180) (28)
Total Fair Value 138,152 39,040
Government Agency Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 18,990 4,749
Total Unrealized Gain 0 0
Total Unrealized Loss (39) (21)
Total Fair Value 18,951 4,728
Corporate Debt Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 137,678 46,532
Total Unrealized Gain 5 2
Total Unrealized Loss (206) (98)
Total Fair Value 137,477 46,436
Commercial Paper | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 15,488 1,592
Total Unrealized Gain 0 0
Total Unrealized Loss 0 0
Total Fair Value 15,488 1,592
Asset-Backed Securities | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 12,809 4,122
Total Unrealized Gain 1 0
Total Unrealized Loss (31) (6)
Total Fair Value 12,779 4,116
Amounts Classified As Cash Equivalents    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 64,761 77,769
Total Unrealized Gain 0 0
Total Unrealized Loss (5) 0
Total Fair Value 64,756 77,769
Amounts Classified As Short-Term Investments    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 298,960 87,024
Total Unrealized Gain 6 2
Total Unrealized Loss (451) (153)
Total Fair Value $ 298,515 $ 86,873