Supplemental Financial Statement Information - Summary of Cash Equivalents and Short-Term Investments (Details) - USD ($) $ in Thousands		Mar. 31, 2019		Dec. 31, 2018
Cash Equivalents and Short-Term Investments [Line Items]
Unrestricted cash equivalents and short-term investments, Cost or Amortized Cost		$ 706,324	[1]	$ 1,790,411	[2]
Unrestricted cash equivalents and short-term investments, Unrealized Gains		486	[1]	76	[2]
Unrestricted cash equivalents and short-term investments, Unrealized Losses		(33)	[1]	(645)	[2]
Unrestricted cash equivalents and short-term investments, Estimated Fair Value		706,777	[1]	1,789,842	[2]
Restricted cash equivalents and investments, Cost or Amortized Cost		1,109,284	[3],[4]	1,002,605	[5],[6]
Restricted cash equivalents and investments, Unrealized Gains		598	[3],[4]	64	[5],[6]
Restricted cash equivalents and investments, Unrealized Losses		(100)	[3],[4]	(350)	[5],[6]
Restricted cash equivalents and investments, Estimated Fair Value		1,109,782	[3],[4]	1,002,319	[5],[6]
Unrestricted and restricted cash equivalents and investments, Cost or Amortized Cost		1,815,608		2,793,016
Unrestricted and restricted cash equivalents and investments, Unrealized Gains		1,084		140
Unrestricted and restricted cash equivalents and investments, Unrealized Losses		(133)		(995)
Unrestricted and restricted cash equivalents and investments, Estimated Fair Value		1,816,559		2,792,161
Corporate Bonds
Cash Equivalents and Short-Term Investments [Line Items]
Unrestricted cash equivalents and short-term investments, Cost or Amortized Cost		94,415	[1]	119,043	[2]
Unrestricted cash equivalents and short-term investments, Unrealized Gains		93	[1]	19	[2]
Unrestricted cash equivalents and short-term investments, Unrealized Losses		(1)	[1]	(23)	[2]
Unrestricted cash equivalents and short-term investments, Estimated Fair Value		94,507	[1]	119,039	[2]
Restricted cash equivalents and investments, Cost or Amortized Cost		62,204	[3],[4]	65,616	[5],[6]
Restricted cash equivalents and investments, Unrealized Gains		58	[3],[4]	6	[5],[6]
Restricted cash equivalents and investments, Unrealized Losses		(8)	[3],[4]	(36)	[5],[6]
Restricted cash equivalents and investments, Estimated Fair Value		62,254	[3],[4]	65,586	[5],[6]
Money Market Funds
Cash Equivalents and Short-Term Investments [Line Items]
Unrestricted cash equivalents and short-term investments, Cost or Amortized Cost		1,406	[1]	38,528	[2]
Unrestricted cash equivalents and short-term investments, Estimated Fair Value		1,406	[1]	38,528	[2]
Restricted cash equivalents and investments, Cost or Amortized Cost		2,822	[3],[4]	4,620	[5],[6]
Restricted cash equivalents and investments, Estimated Fair Value		2,822	[3],[4]	4,620	[5],[6]
Certificates of Deposit
Cash Equivalents and Short-Term Investments [Line Items]
Unrestricted cash equivalents and short-term investments, Cost or Amortized Cost		168,700	[1]	497,748	[2]
Unrestricted cash equivalents and short-term investments, Unrealized Gains		203	[1]	19	[2]
Unrestricted cash equivalents and short-term investments, Unrealized Losses	[2]			(213)
Unrestricted cash equivalents and short-term investments, Estimated Fair Value		168,903	[1]	497,554	[2]
Restricted cash equivalents and investments, Cost or Amortized Cost		348,007	[3],[4]	307,650	[5],[6]
Restricted cash equivalents and investments, Unrealized Gains		364	[3],[4]	41	[5],[6]
Restricted cash equivalents and investments, Unrealized Losses		(2)	[3],[4]	(87)	[5],[6]
Restricted cash equivalents and investments, Estimated Fair Value		348,369	[3],[4]	307,604	[5],[6]
Commercial Paper
Cash Equivalents and Short-Term Investments [Line Items]
Unrestricted cash equivalents and short-term investments, Cost or Amortized Cost		441,803	[1]	1,135,092	[2]
Unrestricted cash equivalents and short-term investments, Unrealized Gains		190	[1]	38	[2]
Unrestricted cash equivalents and short-term investments, Unrealized Losses		(32)	[1]	(409)	[2]
Unrestricted cash equivalents and short-term investments, Estimated Fair Value		441,961	[1]	1,134,721	[2]
Restricted cash equivalents and investments, Cost or Amortized Cost		696,251	[3],[4]	624,719	[5],[6]
Restricted cash equivalents and investments, Unrealized Gains		176	[3],[4]	17	[5],[6]
Restricted cash equivalents and investments, Unrealized Losses		(90)	[3],[4]	(227)	[5],[6]
Restricted cash equivalents and investments, Estimated Fair Value		$ 696,337	[3],[4]	$ 624,509	[5],[6]

[1] Included in cash and cash equivalents and short-term investments in the Company’s condensed consolidated balance sheet as of March 31, 2019 in addition to $328.1 million of cash.

[2] Included in cash and cash equivalents and short-term investments in the Company’s condensed consolidated balance sheet as of December 31, 2018 in addition to $248.0 million of cash.

[3] Included in prepaid expenses and other current assets in the Company’s condensed consolidated balance sheet as of March 31, 2019 is $1.9 million of restricted cash.

[4] Included in restricted cash and cash equivalents and restricted investments in the Company’s condensed consolidated balance sheet as of March 31, 2019 in addition to $58.0 million of restricted cash.

[5] Included in prepaid expenses and other current assets in the Company’s condensed consolidated balance sheet as of December 31, 2018 is $1.4 million of restricted cash.

[6] Included in restricted cash and cash equivalents and restricted investments in the Company’s condensed consolidated balance sheet as of December 31, 2018 in addition to $50.2 million of restricted cash.