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)

v3.19.2
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
Jun. 30, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 212,898 $ 129,727
Gross Unrealized Gains 229  
Gross Unrealized Losses (1) (58)
Total Fair Value 213,126 129,669
U.S. and Foreign Commercial Paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 57,509 51,627
Total Fair Value 57,509 51,627
U.S. and Foreign Corporate Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 59,895 34,668
Gross Unrealized Gains 116  
Gross Unrealized Losses   (34)
Total Fair Value 60,011 34,634
Asset-backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 47,282 25,494
Gross Unrealized Gains 59  
Gross Unrealized Losses (1) (22)
Total Fair Value 47,340 25,472
U.S. Treasury Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 48,212 17,938
Gross Unrealized Gains 54  
Gross Unrealized Losses   (2)
Total Fair Value $ 48,266 $ 17,936