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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 | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Fair Value, Financial Instruments [Line Items] |  |
Total restricted cash equivalents and investments | $ 1,211 | $ 1,778
Certificate of deposit |  |
Fair Value, Financial Instruments [Line Items] |  |
Total restricted cash equivalents and investments | 2 | 2
Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents | 244 | 220
Restricted investments | 26 | 40
Total restricted cash equivalents and investments | 270 | 260
Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents | 244 | 220
Restricted investments | 24 | 38
Total restricted cash equivalents and investments | 268 | 258
Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents | 0 | 0
Restricted investments | 2 | 2
Total restricted cash equivalents and investments | 2 | 2
Fair Value Measurement on a Recurring Basis | U.S. treasuries |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted investments | 23 | 37
Fair Value Measurement on a Recurring Basis | U.S. treasuries | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted investments | 23 | 37
Fair Value Measurement on a Recurring Basis | U.S. treasuries | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted investments | 0 | 0
Fair Value Measurement on a Recurring Basis | Exchange traded fund |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted investments | 1 | 1
Fair Value Measurement on a Recurring Basis | Exchange traded fund | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted investments | 1 | 1
Fair Value Measurement on a Recurring Basis | Exchange traded fund | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted investments | 0 | 0
Fair Value Measurement on a Recurring Basis | Certificate of deposit |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted investments | 2 | 2
Fair Value Measurement on a Recurring Basis | Certificate of deposit | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted investments | 0 | 0
Fair Value Measurement on a Recurring Basis | Certificate of deposit | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted investments | 2 | 2
Fair Value Measurement on a Recurring Basis | Money market mutual funds |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents | 224 | 199
Fair Value Measurement on a Recurring Basis | Money market mutual funds | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents | 224 | 199
Fair Value Measurement on a Recurring Basis | Money market mutual funds | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents | 0 | 0
Fair Value Measurement on a Recurring Basis | Commercial paper |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents | 20 | 21
Fair Value Measurement on a Recurring Basis | Commercial paper | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents | 20 | 21
Fair Value Measurement on a Recurring Basis | Commercial paper | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents | $ 0 | $ 0

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