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)

v3.19.2
Financial Instruments - Summary of Estimated Fair Value and Related Valuation Input Hierarchy of Available-for-Sale Securities (Detail) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Total Fair Value $ 187,101 $ 310,584
Fair Value, Measurements, Recurring    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 186,928 310,924
Total Unrealized Gain 185 20
Total Unrealized Loss (12) (360)
Total Fair Value 187,101 310,584
Fair Value, Measurements, Recurring | Money Market Funds | Level 1    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 36,621 38,708
Total Unrealized Gain 0 0
Total Unrealized Loss 0 0
Total Fair Value 36,621 38,708
Fair Value, Measurements, Recurring | U.S. Treasury Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 69,324 111,164
Total Unrealized Gain 49 4
Total Unrealized Loss (1) (80)
Total Fair Value 69,372 111,088
Fair Value, Measurements, Recurring | Government Agency Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 9,079 15,206
Total Unrealized Gain 1 1
Total Unrealized Loss (4) (32)
Total Fair Value 9,076 15,175
Fair Value, Measurements, Recurring | Corporate Debt Obligations | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 63,788 121,017
Total Unrealized Gain 135 15
Total Unrealized Loss (6) (217)
Total Fair Value 63,917 120,815
Fair Value, Measurements, Recurring | Commercial Paper | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 2,846 12,935
Total Unrealized Gain 0 0
Total Unrealized Loss 0 0
Total Fair Value 2,846 12,935
Fair Value, Measurements, Recurring | Asset-Backed Securities | Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 5,270 11,894
Total Unrealized Gain 0 0
Total Unrealized Loss (1) (31)
Total Fair Value 5,269 11,863
Fair Value, Measurements, Recurring | Amounts Classified As Cash Equivalents    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost (56,124) (61,651)
Total Unrealized Gain (1) 0
Total Unrealized Loss 0 0
Total Fair Value (56,125) (61,651)
Fair Value, Measurements, Recurring | Amounts Classified As Short-Term Investments    
Schedule of Available-for-sale Securities [Line Items]    
Total Amortized Cost 130,804 249,273
Total Unrealized Gain 184 20
Total Unrealized Loss (12) (360)
Total Fair Value $ 130,976 $ 248,933