Fair Value Measurements - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Total unrestricted and restricted cash equivalents and investments	$ 4,264,026	$ 2,792,161
Fair Value Measurements on a Recurring Basis
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Total unrestricted cash equivalents and short-term investments	2,400,667	1,789,842
Total restricted cash equivalents and investments	1,495,141	1,002,319
Total unrestricted and restricted cash equivalents and investments	3,895,808	2,792,161
Fair Value Measurements on a Recurring Basis | Level 1
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Total unrestricted cash equivalents and short-term investments	0	38,528
Total restricted cash equivalents and investments	19,250	4,620
Total unrestricted and restricted cash equivalents and investments	19,250	43,148
Fair Value Measurements on a Recurring Basis | Level 2
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Total unrestricted cash equivalents and short-term investments	2,400,667	1,751,314
Total restricted cash equivalents and investments	1,475,891	997,699
Total unrestricted and restricted cash equivalents and investments	3,876,558	2,749,013
Fair Value Measurements on a Recurring Basis | Level 3
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
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 [Line Items]
Total unrestricted cash equivalents and short-term investments		38,528
Total restricted cash equivalents and investments	19,250	4,620
Fair Value Measurements on a Recurring Basis | Money market funds | Level 1
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Total unrestricted cash equivalents and short-term investments		38,528
Total restricted cash equivalents and investments	19,250	4,620
Fair Value Measurements on a Recurring Basis | Money market funds | Level 2
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Total unrestricted cash equivalents and short-term investments		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 [Line Items]
Total unrestricted cash equivalents and short-term investments		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 [Line Items]
Total unrestricted cash equivalents and short-term investments	1,276,594	497,554
Total restricted cash equivalents and investments	608,828	307,604
Fair Value Measurements on a Recurring Basis | Certificates of deposit | Level 1
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
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 [Line Items]
Total unrestricted cash equivalents and short-term investments	1,276,594	497,554
Total restricted cash equivalents and investments	608,828	307,604
Fair Value Measurements on a Recurring Basis | Certificates of deposit | Level 3
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
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 [Line Items]
Total unrestricted cash equivalents and short-term investments	876,495	1,134,721
Total restricted cash equivalents and investments	791,155	624,509
Fair Value Measurements on a Recurring Basis | Commercial paper | Level 1
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
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 [Line Items]
Total unrestricted cash equivalents and short-term investments	876,495	1,134,721
Total restricted cash equivalents and investments	791,155	624,509
Fair Value Measurements on a Recurring Basis | Commercial paper | Level 3
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
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 [Line Items]
Total unrestricted cash equivalents and short-term investments	247,578	119,039
Total restricted cash equivalents and investments	75,908	65,586
Fair Value Measurements on a Recurring Basis | Corporate bonds | Level 1
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
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 [Line Items]
Total unrestricted cash equivalents and short-term investments	247,578	119,039
Total restricted cash equivalents and investments	75,908	65,586
Fair Value Measurements on a Recurring Basis | Corporate bonds | Level 3
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Total unrestricted cash equivalents and short-term investments	0	0
Total restricted cash equivalents and investments	$ 0	$ 0