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Supplemental Financial Statement Information - Summary of Cash Equivalents and Short-Term Investments (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Cash Equivalents and Short-Term Investments [Line Items]    
Unrestricted cash equivalents and investments, Cost or Amortized Cost $ 2,193,558 $ 2,117,165
Unrestricted cash equivalents and investments, Unrealized Gains 256 331
Unrestricted cash equivalents and investments, Unrealized Losses (52) (65)
Unrestricted cash equivalents and investments, Estimated Fair Value 2,193,762 2,117,431
Restricted cash equivalents and investments, Cost or Amortized Cost 1,066,913 1,166,332
Restricted cash equivalents and investments, Unrealized Gains 182 337
Restricted cash equivalents and investments, Unrealized Losses (31) (39)
Restricted cash equivalents and investments, Estimated Fair Value 1,067,064 1,166,630
Unrestricted and restricted cash equivalents and investments, Cost or Amortized Cost 3,260,471 3,283,497
Unrestricted and restricted cash equivalents and investments, Unrealized Gains 438 668
Unrestricted and restricted cash equivalents and investments, Unrealized Losses (83) (104)
Total unrestricted and restricted cash equivalents and investments 3,260,826 3,284,061
Money market deposit accounts    
Cash Equivalents and Short-Term Investments [Line Items]    
Unrestricted cash equivalents and investments, Cost or Amortized Cost 180,041 174,347
Unrestricted cash equivalents and investments, Unrealized Gains 0 0
Unrestricted cash equivalents and investments, Unrealized Losses 0 0
Unrestricted cash equivalents and investments, Estimated Fair Value 180,041 174,347
Restricted cash equivalents and investments, Cost or Amortized Cost 0 162
Restricted cash equivalents and investments, Unrealized Gains 0 0
Restricted cash equivalents and investments, Unrealized Losses 0 0
Restricted cash equivalents and investments, Estimated Fair Value 0 162
Money market funds    
Cash Equivalents and Short-Term Investments [Line Items]    
Unrestricted cash equivalents and investments, Cost or Amortized Cost 88,630  
Unrestricted cash equivalents and investments, Unrealized Gains 0  
Unrestricted cash equivalents and investments, Unrealized Losses 0  
Unrestricted cash equivalents and investments, Estimated Fair Value 88,630  
Restricted cash equivalents and investments, Cost or Amortized Cost 126,649 24,757
Restricted cash equivalents and investments, Unrealized Gains 0 0
Restricted cash equivalents and investments, Unrealized Losses 0 0
Restricted cash equivalents and investments, Estimated Fair Value 126,649 24,757
Term deposits    
Cash Equivalents and Short-Term Investments [Line Items]    
Unrestricted cash equivalents and investments, Cost or Amortized Cost 640,000 601,000
Unrestricted cash equivalents and investments, Unrealized Gains 0 0
Unrestricted cash equivalents and investments, Unrealized Losses 0 0
Unrestricted cash equivalents and investments, Estimated Fair Value 640,000 601,000
Restricted cash equivalents and investments, Cost or Amortized Cost 6,506 6,506
Restricted cash equivalents and investments, Unrealized Gains 0 0
Restricted cash equivalents and investments, Unrealized Losses 0 0
Restricted cash equivalents and investments, Estimated Fair Value 6,506 6,506
Certificates of deposit    
Cash Equivalents and Short-Term Investments [Line Items]    
Unrestricted cash equivalents and investments, Cost or Amortized Cost 864,376 677,602
Unrestricted cash equivalents and investments, Unrealized Gains 238 178
Unrestricted cash equivalents and investments, Unrealized Losses (16) (4)
Unrestricted cash equivalents and investments, Estimated Fair Value 864,598 677,776
Restricted cash equivalents and investments, Cost or Amortized Cost 606,178 481,154
Restricted cash equivalents and investments, Unrealized Gains 164 213
Restricted cash equivalents and investments, Unrealized Losses (11) (3)
Restricted cash equivalents and investments, Estimated Fair Value 606,331 481,364
Commercial paper    
Cash Equivalents and Short-Term Investments [Line Items]    
Unrestricted cash equivalents and investments, Cost or Amortized Cost 312,869 376,771
Unrestricted cash equivalents and investments, Unrealized Gains 18 38
Unrestricted cash equivalents and investments, Unrealized Losses (14) (20)
Unrestricted cash equivalents and investments, Estimated Fair Value 312,873 376,789
Restricted cash equivalents and investments, Cost or Amortized Cost 258,627 469,193
Restricted cash equivalents and investments, Unrealized Gains 18 57
Restricted cash equivalents and investments, Unrealized Losses (8) (10)
Restricted cash equivalents and investments, Estimated Fair Value 258,637 469,240
Corporate bonds    
Cash Equivalents and Short-Term Investments [Line Items]    
Unrestricted cash equivalents and investments, Cost or Amortized Cost 107,642 287,445
Unrestricted cash equivalents and investments, Unrealized Gains 0 115
Unrestricted cash equivalents and investments, Unrealized Losses (22) (41)
Unrestricted cash equivalents and investments, Estimated Fair Value 107,620 287,519
Restricted cash equivalents and investments, Cost or Amortized Cost 68,953 184,560
Restricted cash equivalents and investments, Unrealized Gains 0 67
Restricted cash equivalents and investments, Unrealized Losses (12) (26)
Restricted cash equivalents and investments, Estimated Fair Value $ 68,941 $ 184,601