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, 2019	Dec. 31, 2018
Fair Value, Financial Instruments [Line Items]
Total unrestricted and restricted cash equivalents and investments	$ 1,284	$ 1,546
Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Cash equivalents	89	5
Investments	193	189
Restricted cash equivalents	61	68
Restricted investments	6	8
Total unrestricted and restricted cash equivalents and investments	349	270
Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Cash equivalents	89	4
Investments	0	0
Restricted cash equivalents	61	68
Restricted investments	1	1
Total unrestricted and restricted cash equivalents and investments	151	73
Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Cash equivalents	0	1
Investments	193	189
Restricted cash equivalents	0	0
Restricted investments	5	7
Total unrestricted and restricted cash equivalents and investments	198	197
Asset-backed securities \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	35	33
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	35	33
Corporate bonds \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	95	99
Corporate bonds \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Investments	0	0
Corporate bonds \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Investments	95	99
U.S. government agencies and government- sponsored agencies \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	5	7
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
U.S. government agencies and government- sponsored agencies \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Investments	5	7
U.S. treasuries \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	54	41
Restricted investments	5	5
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	54	41
Restricted investments	5	5
Other debt securities \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	4	9
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	4	9
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 unrestricted and restricted cash equivalents and investments		2
Certificate of deposit \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Restricted investments	0	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	0	2
Money market mutual funds \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Cash equivalents	89	4
Restricted cash equivalents	43	48
Money market mutual funds \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Cash equivalents	89	4
Restricted cash equivalents	43	48
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
U.S. treasuries \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Cash equivalents		1
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		1
Commercial paper \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	18	20
Commercial paper \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	18	20
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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The aggregate value of all restricted investments.

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