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Fair Value Measurements - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Thousands
Mar. 31, 2022
Dec. 31, 2021
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted and restricted cash equivalents and investments $ 3,073,433 $ 3,214,049
Fair Value Measurements on a Recurring Basis    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 1,863,288 1,433,781
Total restricted cash equivalents and investments 889,830 1,059,969
Total unrestricted and restricted cash equivalents and investments 2,753,118 2,493,750
Fair Value Measurements on a Recurring Basis | Level 1    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 20,654 22,250
Total restricted cash equivalents and investments 11,868 20,161
Total unrestricted and restricted cash equivalents and investments 32,522 42,411
Fair Value Measurements on a Recurring Basis | Level 2    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 1,842,634 1,411,531
Total restricted cash equivalents and investments 877,962 1,039,808
Total unrestricted and restricted cash equivalents and investments 2,720,596 2,451,339
Fair Value Measurements on a Recurring Basis | Level 3    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 0 0
Total restricted cash equivalents and investments 0 0
Total unrestricted and restricted cash equivalents and investments 0 0
Fair Value Measurements on a Recurring Basis | Money market funds    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 10,824 22,250
Total restricted cash equivalents and investments 11,868 20,161
Fair Value Measurements on a Recurring Basis | Money market funds | Level 1    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 10,824 22,250
Total restricted cash equivalents and investments 11,868 20,161
Fair Value Measurements on a Recurring Basis | Money market funds | Level 2    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 0 0
Total restricted cash equivalents and investments 0 0
Fair Value Measurements on a Recurring Basis | Money market funds | Level 3    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 0 0
Total restricted cash equivalents and investments 0 0
Fair Value Measurements on a Recurring Basis | Certificates of deposit    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 480,128 505,438
Total restricted cash equivalents and investments 281,080 421,144
Fair Value Measurements on a Recurring Basis | Certificates of deposit | Level 1    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 0 0
Total restricted cash equivalents and investments 0 0
Fair Value Measurements on a Recurring Basis | Certificates of deposit | Level 2    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 480,128 505,438
Total restricted cash equivalents and investments 281,080 421,144
Fair Value Measurements on a Recurring Basis | Certificates of deposit | Level 3    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 0 0
Total restricted cash equivalents and investments 0 0
Fair Value Measurements on a Recurring Basis | Commercial paper    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 1,194,329 806,388
Total restricted cash equivalents and investments 485,042 523,489
Fair Value Measurements on a Recurring Basis | Commercial paper | Level 1    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 0 0
Total restricted cash equivalents and investments 0 0
Fair Value Measurements on a Recurring Basis | Commercial paper | Level 2    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 1,194,329 806,388
Total restricted cash equivalents and investments 485,042 523,489
Fair Value Measurements on a Recurring Basis | Commercial paper | Level 3    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 0 0
Total restricted cash equivalents and investments 0 0
Fair Value Measurements on a Recurring Basis | Corporate bonds    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 165,082 99,705
Total restricted cash equivalents and investments 65,742 63,458
Fair Value Measurements on a Recurring Basis | Corporate bonds | Level 1    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 0 0
Total restricted cash equivalents and investments 0 0
Fair Value Measurements on a Recurring Basis | Corporate bonds | Level 2    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 165,082 99,705
Total restricted cash equivalents and investments 65,742 63,458
Fair Value Measurements on a Recurring Basis | Corporate bonds | Level 3    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 0 0
Total restricted cash equivalents and investments 0 0
Fair Value Measurements on a Recurring Basis | Marketable equity securities    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 9,830  
Fair Value Measurements on a Recurring Basis | Marketable equity securities | Level 1    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 9,830  
Fair Value Measurements on a Recurring Basis | Marketable equity securities | Level 2    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 0  
Fair Value Measurements on a Recurring Basis | Marketable equity securities | Level 3    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 0  
Fair Value Measurements on a Recurring Basis | U.S. government securities    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 3,095  
Total restricted cash equivalents and investments 46,098 31,717
Fair Value Measurements on a Recurring Basis | U.S. government securities | Level 1    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 0  
Total restricted cash equivalents and investments 0 0
Fair Value Measurements on a Recurring Basis | U.S. government securities | Level 2    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 3,095  
Total restricted cash equivalents and investments 46,098 31,717
Fair Value Measurements on a Recurring Basis | U.S. government securities | Level 3    
Financial Instruments Measured at Fair Value on a Recurring Basis    
Total unrestricted cash equivalents and short-term investments 0  
Total restricted cash equivalents and investments $ 0 $ 0