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	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Financial Instruments [Line Items]
Investments	$ 434	$ 342
Total cash equivalents and investments and restricted cash equivalents and investments	1,699	1,798
Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Cash equivalents	328	92
Investments	195	194
Restricted cash equivalents	105	70
Restricted investments	239	148
Total cash equivalents and investments and restricted cash equivalents and investments	867	504
Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Cash equivalents	328	89
Investments	0	0
Restricted cash equivalents	104	56
Restricted investments	0	0
Total cash equivalents and investments and restricted cash equivalents and investments	432	145
Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Cash equivalents	0	3
Investments	195	194
Restricted cash equivalents	1	14
Restricted investments	239	148
Total cash equivalents and investments and restricted cash equivalents and investments	435	359
Asset-backed securities \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	30	30
Asset-backed securities \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Investments	0	0
Asset-backed securities \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Investments	30	30
Corporate bonds \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	95	96
Restricted investments	31	28
Corporate bonds \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Investments	0	0
Restricted investments	0	0
Corporate bonds \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Investments	95	96
Restricted investments	31	28
U.S. government agencies and government- sponsored agencies \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	7	5
Restricted investments	18	9
U.S. government agencies and government- sponsored agencies \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Investments	0	0
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]
Investments	7	5
Restricted investments	18	9
U.S. treasuries \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	55	53
Restricted investments	189	110
U.S. treasuries \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Investments	0	0
Restricted investments	0	0
U.S. treasuries \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Investments	55	53
Restricted investments	189	110
Other debt securities \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	8	10
Other debt securities \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Investments	0	0
Other debt securities \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Investments	8	10
Certificate of deposit
Fair Value, Financial Instruments [Line Items]
Total cash equivalents and investments and restricted cash equivalents and investments	1	1
Certificate of deposit \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Restricted investments	1	1
Certificate of deposit \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Restricted investments	0	0
Certificate of deposit \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Restricted investments	1	1
Money market mutual funds \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Cash equivalents	328	89
Restricted cash equivalents	90	42
Money market mutual funds \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Cash equivalents	328	89
Restricted cash equivalents	90	42
Money market mutual funds \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Cash equivalents	0	0
Restricted cash equivalents	0	0
Certificate of deposit \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	1	2
Certificate of deposit \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	0	0
Certificate of deposit \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	1	2
U.S. treasuries \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Cash equivalents		3
Restricted cash equivalents		12
U.S. treasuries \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Cash equivalents		0
Restricted cash equivalents		0
U.S. treasuries \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Cash equivalents		3
Restricted cash equivalents		12
Commercial paper \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	14	14
Commercial paper \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	14	14
Commercial paper \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	$ 0	$ 0

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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Amount of short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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