Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Schedule of Amortized Cost, Unrealized Gain and Loss, and Fair Value of Available for Sale Marketable Securities (Detail)

v3.19.1
Summary of Significant Accounting Policies - Schedule of Amortized Cost, Unrealized Gain and Loss, and Fair Value of Available for Sale Marketable Securities (Detail) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 139,137 $ 129,727
Gross Unrealized Gains 46 0
Gross Unrealized Losses (1) (58)
Estimated Fair Value 139,182 129,669
U.S. and Foreign Commercial Paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 52,158 51,627
Estimated Fair Value 52,158 51,627
U.S. and Foreign Corporate Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 38,289 34,668
Gross Unrealized Gains 29  
Gross Unrealized Losses   (34)
Estimated Fair Value 38,318 34,634
Asset-backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 25,395 25,494
Gross Unrealized Gains 6 0
Gross Unrealized Losses (1) (22)
Estimated Fair Value 25,400 25,472
U.S. Treasury Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 23,295 17,938
Gross Unrealized Gains 11 0
Gross Unrealized Losses   (2)
Estimated Fair Value $ 23,306 $ 17,936