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)

v3.19.1
Financial Instruments - Summary of Estimated Fair Value and Related Valuation Input Hierarchy of Available-for-Sale Securities (Detail) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Total Fair Value $ 231,834 $ 310,584
Fair Value, Measurements, Recurring    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 231,796 310,924
Total Unrealized Gain 123 20
Total Unrealized Loss (85) (360)
Total Fair Value 231,834 310,584
Fair Value, Measurements, Recurring | Money Market Funds | Level 1    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 25,338 38,708
Total Unrealized Gain 0 0
Total Unrealized Loss 0 0
Total Fair Value 25,338 38,708
Fair Value, Measurements, Recurring | U.S. Treasury Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 94,616 111,164
Total Unrealized Gain 42 4
Total Unrealized Loss (17) (80)
Total Fair Value 94,641 111,088
Fair Value, Measurements, Recurring | Government Agency Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 8,351 15,206
Total Unrealized Gain 1 1
Total Unrealized Loss (14) (32)
Total Fair Value 8,338 15,175
Fair Value, Measurements, Recurring | Corporate Debt Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 86,373 121,017
Total Unrealized Gain 80 15
Total Unrealized Loss (45) (217)
Total Fair Value 86,408 120,815
Fair Value, Measurements, Recurring | Commercial Paper | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 7,439 12,935
Total Unrealized Gain 0 0
Total Unrealized Loss 0 0
Total Fair Value 7,439 12,935
Fair Value, Measurements, Recurring | Asset-Backed Securities | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 9,679 11,894
Total Unrealized Gain 0 0
Total Unrealized Loss (9) (31)
Total Fair Value 9,670 11,863
Fair Value, Measurements, Recurring | Amounts Classified As Cash Equivalents    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 50,457 61,651
Total Unrealized Gain 0 0
Total Unrealized Loss 0 0
Total Fair Value 50,457 61,651
Fair Value, Measurements, Recurring | Amounts Classified As Short-Term Investments    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 181,339 249,273
Total Unrealized Gain 123 20
Total Unrealized Loss (85) (360)
Total Fair Value $ 181,377 $ 248,933