Fair Value Measurements - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 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	$ 1,816,559		$ 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	706,777	[1]	1,789,842	[2]
Total restricted cash equivalents and investments at fair value	1,109,782	[3]	1,002,319	[4]
Unrestricted and restricted cash equivalents and investments, Estimated Fair Value	1,816,559		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	1,406	[1]	38,528	[2]
Total restricted cash equivalents and investments at fair value	2,822	[3]	4,620	[4]
Unrestricted and restricted cash equivalents and investments, Estimated Fair Value	4,228		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	705,371	[1]	1,751,314	[2]
Total restricted cash equivalents and investments at fair value	1,106,960	[3]	997,699	[4]
Unrestricted and restricted cash equivalents and investments, Estimated Fair Value	1,812,331		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	1,406	[1]	38,528	[2]
Total restricted cash equivalents and investments at fair value	2,822	[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	1,406	[1]	38,528	[2]
Total restricted cash equivalents and investments at fair value	2,822	[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	168,903	[1]	497,554	[2]
Total restricted cash equivalents and investments at fair value	348,369	[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	168,903	[1]	497,554	[2]
Total restricted cash equivalents and investments at fair value	348,369	[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	441,961	[1]	1,134,721	[2]
Total restricted cash equivalents and investments at fair value	696,337	[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	441,961	[1]	1,134,721	[2]
Total restricted cash equivalents and investments at fair value	696,337	[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	94,507	[1]	119,039	[2]
Total restricted cash equivalents and investments at fair value	62,254	[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	94,507	[1]	119,039	[2]
Total restricted cash equivalents and investments at fair value	$ 62,254	[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 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 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.

[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.