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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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