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Financial Instruments - Schedule of Cash, Cash Equivalents and Available-for-Sale Securities by Significant Investment Category (Detail) - USD ($)
$ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 1,544,506 $ 1,345,801
Gross Unrealized Gains 1,823 5,069
Gross Unrealized Losses (235) (11)
Aggregate Fair Value 1,546,094 1,350,859
Cash and Cash Equivalents 617,143 649,158 [1]
Short-term Marketable Securities 462,333 416,228 [1]
Long-term Marketable Securities 466,618 285,473 [1]
Level 1: | Cash    
Schedule of Available-for-sale Securities [Line Items]    
Cash 294,209 370,325
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Aggregate Fair Value 294,209 370,325
Cash and Cash Equivalents 294,209 370,325
Short-term Marketable Securities 0 0
Long-term Marketable Securities 0 0
Level 2:    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 1,250,297 975,476
Gross Unrealized Gains 1,823 5,069
Gross Unrealized Losses (235) (11)
Aggregate Fair Value 1,251,885 980,534
Cash and Cash Equivalents 322,934 278,833
Short-term Marketable Securities 462,333 416,228
Long-term Marketable Securities 466,618 285,473
Level 2: | Money market instruments    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 291,735 264,833
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Aggregate Fair Value 291,735 264,833
Cash and Cash Equivalents 291,735 264,833
Short-term Marketable Securities 0 0
Long-term Marketable Securities 0 0
Level 2: | Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 560,004 413,137
Gross Unrealized Gains 1,230 3,261
Gross Unrealized Losses (198) (8)
Aggregate Fair Value 561,036 416,390
Cash and Cash Equivalents 703 0
Short-term Marketable Securities 177,468 220,551
Long-term Marketable Securities 382,865 195,839
Level 2: | U.S. government agency securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 210,444 265,298
Gross Unrealized Gains 416 1,555
Gross Unrealized Losses (7) (1)
Aggregate Fair Value 210,853 266,852
Cash and Cash Equivalents 12,999 14,000
Short-term Marketable Securities 161,207 192,488
Long-term Marketable Securities 36,647 60,364
Level 2: | Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 129,818  
Gross Unrealized Gains 1  
Gross Unrealized Losses (1)  
Aggregate Fair Value 129,818  
Cash and Cash Equivalents 17,497  
Short-term Marketable Securities 112,321  
Long-term Marketable Securities 0  
Level 2: | Asset-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 55,182 31,659
Gross Unrealized Gains 26 85
Gross Unrealized Losses (23) (2)
Aggregate Fair Value 55,185 31,742
Cash and Cash Equivalents 0 0
Short-term Marketable Securities 8,272 3,189
Long-term Marketable Securities 46,913 28,553
Level 2: | Foreign and other    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 3,114 549
Gross Unrealized Gains 150 168
Gross Unrealized Losses (6) 0
Aggregate Fair Value 3,258 717
Cash and Cash Equivalents 0 0
Short-term Marketable Securities 3,065 0
Long-term Marketable Securities $ 193 $ 717
Minimum    
Schedule of Available-for-sale Securities [Line Items]    
Long term marketable securities maturity period 1 year 1 year
Maximum    
Schedule of Available-for-sale Securities [Line Items]    
Short term marketable securities maturity period 1 year 1 year
Long term marketable securities maturity period 5 years 5 years
[1] (1)December 31, 2020 balances were derived from the audited Consolidated Financial Statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020, filed with the SEC on February 26, 2021.