Supplemental Financial Statement Information - Summary of Cash Equivalents and Short-Term Investments (Details) - USD ($) $ in Thousands		Jun. 30, 2019		Dec. 31, 2018
Cash Equivalents and Short-Term Investments [Line Items]
Unrestricted cash equivalents and short-term investments, Cost or Amortized Cost		$ 2,968,117	[1]	$ 1,790,411	[2]
Unrestricted cash equivalents and short-term investments, Unrealized Gains		3,098	[1]	76	[2]
Unrestricted cash equivalents and short-term investments, Unrealized Losses		(85)	[1]	(645)	[2]
Unrestricted cash equivalents and short-term investments, Estimated Fair Value		2,971,130	[1]	1,789,842	[2]
Restricted cash equivalents and investments, Cost or Amortized Cost		1,231,987	[3],[4]	1,002,605	[5],[6]
Restricted cash equivalents and investments, Unrealized Gains		901	[3],[4]	64	[5],[6]
Restricted cash equivalents and investments, Unrealized Losses		(76)	[3],[4]	(350)	[5],[6]
Restricted cash equivalents and investments, Estimated Fair Value		1,232,812	[3],[4]	1,002,319	[5],[6]
Unrestricted and restricted cash equivalents and investments, Cost or Amortized Cost		4,200,104		2,793,016
Unrestricted and restricted cash equivalents and investments, Unrealized Gains		3,999		140
Unrestricted and restricted cash equivalents and investments, Unrealized Losses		(161)		(995)
Unrestricted and restricted cash equivalents and investments, Estimated Fair Value		4,203,942		2,792,161
Corporate Bonds
Cash Equivalents and Short-Term Investments [Line Items]
Unrestricted cash equivalents and short-term investments, Cost or Amortized Cost		284,195	[1]	119,043	[2]
Unrestricted cash equivalents and short-term investments, Unrealized Gains		482	[1]	19	[2]
Unrestricted cash equivalents and short-term investments, Unrealized Losses	[2]			(23)
Unrestricted cash equivalents and short-term investments, Estimated Fair Value		284,677	[1]	119,039	[2]
Restricted cash equivalents and investments, Cost or Amortized Cost		62,450	[3],[4]	65,616	[5],[6]
Restricted cash equivalents and investments, Unrealized Gains		82	[3],[4]	6	[5],[6]
Restricted cash equivalents and investments, Unrealized Losses		(1)	[3],[4]	(36)	[5],[6]
Restricted cash equivalents and investments, Estimated Fair Value		62,531	[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		67,232	[1]	38,528	[2]
Unrestricted cash equivalents and short-term investments, Estimated Fair Value		67,232	[1]	38,528	[2]
Restricted cash equivalents and investments, Cost or Amortized Cost		1,438	[3],[4]	4,620	[5],[6]
Restricted cash equivalents and investments, Estimated Fair Value		1,438	[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		1,486,173	[1]	497,748	[2]
Unrestricted cash equivalents and short-term investments, Unrealized Gains		2,284	[1]	19	[2]
Unrestricted cash equivalents and short-term investments, Unrealized Losses		(1)	[1]	(213)	[2]
Unrestricted cash equivalents and short-term investments, Estimated Fair Value		1,488,456	[1]	497,554	[2]
Restricted cash equivalents and investments, Cost or Amortized Cost		433,859	[3],[4]	307,650	[5],[6]
Restricted cash equivalents and investments, Unrealized Gains		608	[3],[4]	41	[5],[6]
Restricted cash equivalents and investments, Unrealized Losses		(4)	[3],[4]	(87)	[5],[6]
Restricted cash equivalents and investments, Estimated Fair Value		434,463	[3],[4]	307,604	[5],[6]
Term Deposits
Cash Equivalents and Short-Term Investments [Line Items]
Unrestricted cash equivalents and short-term investments, Cost or Amortized Cost	[2]	105,000
Unrestricted cash equivalents and short-term investments, Estimated Fair Value	[2]	105,000
Commercial Paper
Cash Equivalents and Short-Term Investments [Line Items]
Unrestricted cash equivalents and short-term investments, Cost or Amortized Cost		1,025,517	[1]	1,135,092	[2]
Unrestricted cash equivalents and short-term investments, Unrealized Gains		332	[1]	38	[2]
Unrestricted cash equivalents and short-term investments, Unrealized Losses		(84)	[1]	(409)	[2]
Unrestricted cash equivalents and short-term investments, Estimated Fair Value		1,025,765	[1]	1,134,721	[2]
Restricted cash equivalents and investments, Cost or Amortized Cost		734,240	[3],[4]	624,719	[5],[6]
Restricted cash equivalents and investments, Unrealized Gains		211	[3],[4]	17	[5],[6]
Restricted cash equivalents and investments, Unrealized Losses		(71)	[3],[4]	(227)	[5],[6]
Restricted cash equivalents and investments, Estimated Fair Value		$ 734,380	[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 June 30, 2019 in addition to $339.2 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 June 30, 2019 is $2.1 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 June 30, 2019 in addition to $57.6 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.