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Financial Instruments And Fair Value Measurements - Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Total	$ 1,546	$ 1,778
Certificate of deposit
Debt Securities, Available-for-sale [Line Items]
Total	2	2
Fair Value Measurements on a Recurring Basis
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	5
Investments	189
Restricted cash equivalents	68	220
Restricted investments	8	40
Total	270	260
Fair Value Measurements on a Recurring Basis | U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
Investments	41
Restricted investments	5	37
Fair Value Measurements on a Recurring Basis | Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Investments	33
Fair Value Measurements on a Recurring Basis | Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Investments	99
Fair Value Measurements on a Recurring Basis | U.S. government agencies and government- sponsored agencies
Debt Securities, Available-for-sale [Line Items]
Investments	7
Fair Value Measurements on a Recurring Basis | Other debt securities
Debt Securities, Available-for-sale [Line Items]
Investments	9
Fair Value Measurements on a Recurring Basis | Exchange traded fund
Debt Securities, Available-for-sale [Line Items]
Restricted investments	1	1
Fair Value Measurements on a Recurring Basis | Certificate of deposit
Debt Securities, Available-for-sale [Line Items]
Restricted investments	2	2
Fair Value Measurements on a Recurring Basis | Level 1
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	4
Investments	0
Restricted cash equivalents	68	220
Restricted investments	1	38
Total	73	258
Fair Value Measurements on a Recurring Basis | Level 1 | U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
Investments	0
Restricted investments	0	37
Fair Value Measurements on a Recurring Basis | Level 1 | Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Investments	0
Fair Value Measurements on a Recurring Basis | Level 1 | Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Investments	0
Fair Value Measurements on a Recurring Basis | Level 1 | U.S. government agencies and government- sponsored agencies
Debt Securities, Available-for-sale [Line Items]
Investments	0
Fair Value Measurements on a Recurring Basis | Level 1 | Other debt securities
Debt Securities, Available-for-sale [Line Items]
Investments	0
Fair Value Measurements on a Recurring Basis | Level 1 | Exchange traded fund
Debt Securities, Available-for-sale [Line Items]
Restricted investments	1	1
Fair Value Measurements on a Recurring Basis | Level 1 | Certificate of deposit
Debt Securities, Available-for-sale [Line Items]
Restricted investments	0	0
Fair Value Measurements on a Recurring Basis | Level 2
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	1
Investments	189
Restricted cash equivalents	0	0
Restricted investments	7	2
Total	197	2
Fair Value Measurements on a Recurring Basis | Level 2 | U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
Investments	41
Restricted investments	5	0
Fair Value Measurements on a Recurring Basis | Level 2 | Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Investments	33
Fair Value Measurements on a Recurring Basis | Level 2 | Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Investments	99
Fair Value Measurements on a Recurring Basis | Level 2 | U.S. government agencies and government- sponsored agencies
Debt Securities, Available-for-sale [Line Items]
Investments	7
Fair Value Measurements on a Recurring Basis | Level 2 | Other debt securities
Debt Securities, Available-for-sale [Line Items]
Investments	9
Fair Value Measurements on a Recurring Basis | Level 2 | Exchange traded fund
Debt Securities, Available-for-sale [Line Items]
Restricted investments	0	0
Fair Value Measurements on a Recurring Basis | Level 2 | Certificate of deposit
Debt Securities, Available-for-sale [Line Items]
Restricted investments	2	2
Money market mutual funds | Fair Value Measurements on a Recurring Basis
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	4	199
Restricted cash equivalents	48
Money market mutual funds | Fair Value Measurements on a Recurring Basis | Level 1
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	4	199
Restricted cash equivalents	48
Money market mutual funds | Fair Value Measurements on a Recurring Basis | Level 2
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	0	0
Restricted cash equivalents	0
Commercial paper | Fair Value Measurements on a Recurring Basis
Debt Securities, Available-for-sale [Line Items]
Restricted cash equivalents	20	21
Commercial paper | Fair Value Measurements on a Recurring Basis | Level 1
Debt Securities, Available-for-sale [Line Items]
Restricted cash equivalents	20	21
Commercial paper | Fair Value Measurements on a Recurring Basis | Level 2
Debt Securities, Available-for-sale [Line Items]
Restricted cash equivalents	0	$ 0
U.S. treasuries | Fair Value Measurements on a Recurring Basis
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	1
U.S. treasuries | Fair Value Measurements on a Recurring Basis | Level 1
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	0
U.S. treasuries | Fair Value Measurements on a Recurring Basis | Level 2
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	$ 1

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