Supplemental Financial Statement Information - Summary of Cash Equivalents and Short-Term Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Cash Equivalents and Short-Term Investments [Line Items]
Unrestricted cash equivalents and investments, Cost or Amortized Cost	$ 2,232,955	$ 2,752,014
Unrestricted cash equivalents and investments, Unrealized Gains	1,177	1,287
Unrestricted cash equivalents and investments, Unrealized Losses	(28)	(111)
Unrestricted cash equivalents and investments, Estimated Fair Value	2,234,104	2,753,190
Restricted cash equivalents and investments, Cost or Amortized Cost	1,251,906	1,510,438
Restricted cash equivalents and investments, Unrealized Gains	916	507
Restricted cash equivalents and investments, Unrealized Losses	(23)	(109)
Restricted cash equivalents and investments, Estimated Fair Value	1,252,799	1,510,836
Unrestricted and restricted cash equivalents and investments, Cost or Amortized Cost	3,484,861	4,262,452
Unrestricted and restricted cash equivalents and investments, Unrealized Gains	2,093	1,794
Unrestricted and restricted cash equivalents and investments, Unrealized Losses	(51)	(220)
Total unrestricted and restricted cash equivalents and investments	3,486,903	4,264,026
Money market funds
Cash Equivalents and Short-Term Investments [Line Items]
Unrestricted cash equivalents and investments, Cost or Amortized Cost	0
Unrestricted cash equivalents and investments, Unrealized Gains	0
Unrestricted cash equivalents and investments, Unrealized Losses	0
Unrestricted cash equivalents and investments, Estimated Fair Value	0
Restricted cash equivalents and investments, Cost or Amortized Cost	35,323	19,250
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	35,323	19,250
Money market deposit accounts
Cash Equivalents and Short-Term Investments [Line Items]
Unrestricted cash equivalents and investments, Cost or Amortized Cost	203,835	217,523
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	203,835	217,523
Restricted cash equivalents and investments, Cost or Amortized Cost	8,605	7,884
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	8,605	7,884
Term deposits
Cash Equivalents and Short-Term Investments [Line Items]
Unrestricted cash equivalents and investments, Cost or Amortized Cost	621,000	135,000
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	621,000	135,000
Restricted cash equivalents and investments, Cost or Amortized Cost	7,811	7,811
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	7,811	7,811
Certificates of deposit
Cash Equivalents and Short-Term Investments [Line Items]
Unrestricted cash equivalents and investments, Cost or Amortized Cost	627,248	1,275,750
Unrestricted cash equivalents and investments, Unrealized Gains	427	887
Unrestricted cash equivalents and investments, Unrealized Losses	(9)	(43)
Unrestricted cash equivalents and investments, Estimated Fair Value	627,666	1,276,594
Restricted cash equivalents and investments, Cost or Amortized Cost	535,800	608,578
Restricted cash equivalents and investments, Unrealized Gains	473	262
Restricted cash equivalents and investments, Unrealized Losses	(4)	(12)
Restricted cash equivalents and investments, Estimated Fair Value	536,269	608,828
Commercial paper
Cash Equivalents and Short-Term Investments [Line Items]
Unrestricted cash equivalents and investments, Cost or Amortized Cost	417,788	876,382
Unrestricted cash equivalents and investments, Unrealized Gains	114	181
Unrestricted cash equivalents and investments, Unrealized Losses	0	(68)
Unrestricted cash equivalents and investments, Estimated Fair Value	417,902	876,495
Restricted cash equivalents and investments, Cost or Amortized Cost	460,380	791,087
Restricted cash equivalents and investments, Unrealized Gains	102	165
Restricted cash equivalents and investments, Unrealized Losses	(7)	(97)
Restricted cash equivalents and investments, Estimated Fair Value	460,475	791,155
Corporate bonds
Cash Equivalents and Short-Term Investments [Line Items]
Unrestricted cash equivalents and investments, Cost or Amortized Cost	363,084	247,359
Unrestricted cash equivalents and investments, Unrealized Gains	636	219
Unrestricted cash equivalents and investments, Unrealized Losses	(19)	0
Unrestricted cash equivalents and investments, Estimated Fair Value	363,701	247,578
Restricted cash equivalents and investments, Cost or Amortized Cost	201,488	75,828
Restricted cash equivalents and investments, Unrealized Gains	341	80
Restricted cash equivalents and investments, Unrealized Losses	(12)	0
Restricted cash equivalents and investments, Estimated Fair Value	201,817	$ 75,908
U.S. treasury securities
Cash Equivalents and Short-Term Investments [Line Items]
Restricted cash equivalents and investments, Cost or Amortized Cost	2,499
Restricted cash equivalents and investments, Unrealized Gains	0
Restricted cash equivalents and investments, Unrealized Losses	0
Restricted cash equivalents and investments, Estimated Fair Value	$ 2,499