Financial Instruments and Fair Value Measurements - Summary of Financial Instruments by Significant Categories and Fair Value Measurement on a Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Fair Value, Financial Instruments [Line Items]
Total investments	$ 565	$ 451
Total cash equivalents and investments and restricted cash equivalents and investments	1,926	2,094
Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	3	13
Total investments	325	195
Total restricted cash equivalents	94	99
Total restricted investments	240	256
Total cash equivalents and investments and restricted cash equivalents and investments	662	563
Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	3	2
Total investments	0	0
Total restricted cash equivalents	94	99
Total restricted investments	0	0
Total cash equivalents and investments and restricted cash equivalents and investments	97	101
Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	0	11
Total investments	325	195
Total restricted cash equivalents	0	0
Total restricted investments	240	256
Total cash equivalents and investments and restricted cash equivalents and investments	565	462
Money market mutual funds \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	3	2
Total restricted cash equivalents	94	99
Money market mutual funds \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	3	2
Total restricted cash equivalents	94	99
Money market mutual funds \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	0	0
Total restricted cash equivalents	0	0
U.S. treasuries \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total cash equivalents		11
U.S. treasuries \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Total cash equivalents		0
U.S. treasuries \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Total cash equivalents		11
Certificate of deposits \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total restricted cash equivalents	0
Certificate of deposits \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Total restricted cash equivalents	0
Certificate of deposits \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Total restricted cash equivalents	0
Commercial paper \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total restricted cash equivalents	0
Commercial paper \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Total restricted cash equivalents	0
Commercial paper \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Total restricted cash equivalents	0
Asset-backed securities
Fair Value, Financial Instruments [Line Items]
Total investments	44	24
Asset-backed securities \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total investments	44	24
Asset-backed securities \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Total investments	0	0
Asset-backed securities \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Total investments	44	24
Corporate bonds
Fair Value, Financial Instruments [Line Items]
Total investments	169	128
Corporate bonds \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total investments	132	93
Total restricted investments	37	35
Corporate bonds \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Total investments	0	0
Total restricted investments	0	0
Corporate bonds \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Total investments	132	93
Total restricted investments	37	35
U.S. government agencies and government- sponsored agencies
Fair Value, Financial Instruments [Line Items]
Total investments	19	28
U.S. government agencies and government- sponsored agencies \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total investments	3	5
Total restricted investments	16	23
U.S. government agencies and government- sponsored agencies \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Total investments	0	0
Total restricted investments	0	0
U.S. government agencies and government- sponsored agencies \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Total investments	3	5
Total restricted investments	16	23
U.S. treasuries
Fair Value, Financial Instruments [Line Items]
Total investments	315	265
U.S. treasuries \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total investments	138	67
Total restricted investments	177	198
U.S. treasuries \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Total investments	0	0
Total restricted investments	0	0
U.S. treasuries \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Total investments	138	67
Total restricted investments	177	198
Other debt securities
Fair Value, Financial Instruments [Line Items]
Total investments	8	6
Other debt securities \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total investments	8	6
Other debt securities \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Total investments	0	0
Other debt securities \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Total investments	8	6
Certificate of deposits
Fair Value, Financial Instruments [Line Items]
Total investments	10	$ 0
Certificate of deposits \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total restricted investments	10
Certificate of deposits \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Total restricted investments	0
Certificate of deposits \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Total restricted investments	$ 10

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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