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)

v3.19.3
Financial Instruments - Summary of Estimated Fair Value and Related Valuation Input Hierarchy of Available-for-Sale Securities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Total Fair Value $ 280,661 $ 310,584
Fair Value, Measurements, Recurring    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 280,428 310,924
Total Unrealized Gain 235 20
Total Unrealized Loss (2) (360)
Total Fair Value 280,661 310,584
Fair Value, Measurements, Recurring | Money Market Funds | Level 1    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 43,210 38,708
Total Unrealized Gain 0 0
Total Unrealized Loss 0 0
Total Fair Value 43,210 38,708
Fair Value, Measurements, Recurring | U.S. Treasury Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 117,946 111,164
Total Unrealized Gain 38 4
Total Unrealized Loss 0 (80)
Total Fair Value 117,984 111,088
Fair Value, Measurements, Recurring | Government Agency Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 4,524 15,206
Total Unrealized Gain 2 1
Total Unrealized Loss (1) (32)
Total Fair Value 4,525 15,175
Fair Value, Measurements, Recurring | Corporate Debt Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 70,385 121,017
Total Unrealized Gain 190 15
Total Unrealized Loss (1) (217)
Total Fair Value 70,574 120,815
Fair Value, Measurements, Recurring | Commercial Paper | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 36,522 12,935
Total Unrealized Gain 0 0
Total Unrealized Loss 0 0
Total Fair Value 36,522 12,935
Fair Value, Measurements, Recurring | Asset-Backed Securities | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 7,341 11,894
Total Unrealized Gain 5 0
Total Unrealized Loss 0 (31)
Total Fair Value 7,346 11,863
Fair Value, Measurements, Recurring | Certificate of Deposit | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 500  
Total Unrealized Gain 0  
Total Unrealized Loss 0  
Total Fair Value 500  
Fair Value, Measurements, Recurring | Amounts Classified As Cash Equivalents    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost (92,167) (61,651)
Total Unrealized Gain (3) 0
Total Unrealized Loss 0 0
Total Fair Value (92,170) (61,651)
Fair Value, Measurements, Recurring | Amounts Classified As Short-Term Investments    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 188,261 249,273
Total Unrealized Gain 232 20
Total Unrealized Loss (2) (360)
Total Fair Value $ 188,491 $ 248,933