Fair Value Measurements - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019		Dec. 31, 2018
Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Unrestricted and restricted cash equivalents and investments, Estimated Fair Value	$ 4,203,942		$ 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 at fair value	2,866,130	[1]	1,789,842	[2]
Total restricted cash equivalents and investments at fair value	1,232,812	[3]	1,002,319	[4]
Unrestricted and restricted cash equivalents and investments, Estimated Fair Value	4,098,942		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 at fair value	67,232	[1]	38,528	[2]
Total restricted cash equivalents and investments at fair value	1,438	[3]	4,620	[4]
Unrestricted and restricted cash equivalents and investments, Estimated Fair Value	68,670		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 at fair value	2,798,898	[1]	1,751,314	[2]
Total restricted cash equivalents and investments at fair value	1,231,374	[3]	997,699	[4]
Unrestricted and restricted cash equivalents and investments, Estimated Fair Value	4,030,272		2,749,013
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 at fair value	67,232	[1]	38,528	[2]
Total restricted cash equivalents and investments at fair value	1,438	[3]	4,620	[4]
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 at fair value	67,232	[1]	38,528	[2]
Total restricted cash equivalents and investments at fair value	1,438	[3]	4,620	[4]
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 at fair value	1,488,456	[1]	497,554	[2]
Total restricted cash equivalents and investments at fair value	434,463	[3]	307,604	[4]
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 at fair value	1,488,456	[1]	497,554	[2]
Total restricted cash equivalents and investments at fair value	434,463	[3]	307,604	[4]
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 at fair value	1,025,765	[1]	1,134,721	[2]
Total restricted cash equivalents and investments at fair value	734,380	[3]	624,509	[4]
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 at fair value	1,025,765	[1]	1,134,721	[2]
Total restricted cash equivalents and investments at fair value	734,380	[3]	624,509	[4]
Corporate Bonds | 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 at fair value	284,677	[1]	119,039	[2]
Total restricted cash equivalents and investments at fair value	62,531	[3]	65,586	[4]
Corporate Bonds | 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 at fair value	284,677	[1]	119,039	[2]
Total restricted cash equivalents and investments at fair value	$ 62,531	[3]	$ 65,586	[4]

[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 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.

[4] 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.