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, 2018	Dec. 31, 2017
Fair Value, Financial Instruments [Line Items]
Total investments and restricted cash equivalents and investments	$ 1,172	$ 1,778
Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Cash equivalents	8
Investments	162
Restricted cash equivalents	250	220
Restricted investments	17	40
Total investments and restricted cash equivalents and investments	437	260
Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Cash equivalents	1
Investments	0
Restricted cash equivalents	250	220
Restricted investments	1	38
Total investments and restricted cash equivalents and investments	252	258
Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Cash equivalents	7
Investments	162
Restricted cash equivalents	0	0
Restricted investments	16	2
Total investments and restricted cash equivalents and investments	185	2
Asset-backed securities \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	38
Asset-backed securities \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Investments	0
Asset-backed securities \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Investments	38
Corporate bonds \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	87
Corporate bonds \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Investments	0
Corporate bonds \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Investments	87
U.S. government agencies and government- sponsored agencies \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	6
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
U.S. government agencies and government- sponsored agencies \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Investments	6
U.S. treasuries \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	20
Restricted investments	14	37
U.S. treasuries \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Investments	0
Restricted investments	0	37
U.S. treasuries \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Investments	20
Restricted investments	14	0
Other debt securities \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	11
Other debt securities \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Investments	0
Other debt securities \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Investments	11
Exchange traded fund \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Restricted investments	1	1
Exchange traded fund \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Restricted investments	1	1
Exchange traded fund \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Restricted investments	0	0
Certificate of deposit
Fair Value, Financial Instruments [Line Items]
Total investments and restricted cash equivalents and investments	2	2
Certificate of deposit \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Restricted investments	2	2
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	2	2
Money market mutual funds \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Cash equivalents	1
Restricted cash equivalents	229	199
Money market mutual funds \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Cash equivalents	1
Restricted cash equivalents	229	199
Money market mutual funds \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Cash equivalents	0
Restricted cash equivalents	0	0
U.S. treasuries \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Cash equivalents	7
U.S. treasuries \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Cash equivalents	0
U.S. treasuries \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Cash equivalents	7
Commercial paper \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	21	21
Commercial paper \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	21	21
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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