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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

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