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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, 2018 | Dec. 31, 2017
--- | --- | ---
Fair Value, Financial Instruments [Line Items] |  |
Total investments and restricted cash equivalents and investments | $ 1,172 | $ 1,778
Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Cash equivalents | 8 |
Investments | 162 |
Restricted cash equivalents | 250 | 220
Restricted investments | 17 | 40
Total investments and restricted cash equivalents and investments | 437 | 260
Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Cash equivalents | 1 |
Investments | 0 |
Restricted cash equivalents | 250 | 220
Restricted investments | 1 | 38
Total investments and restricted cash equivalents and investments | 252 | 258
Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Cash equivalents | 7 |
Investments | 162 |
Restricted cash equivalents | 0 | 0
Restricted investments | 16 | 2
Total investments and restricted cash equivalents and investments | 185 | 2
Asset-backed securities | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 38 |
Asset-backed securities | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 0 |
Asset-backed securities | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 38 |
Corporate bonds | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 87 |
Corporate bonds | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 0 |
Corporate bonds | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 87 |
U.S. government agencies and government- sponsored agencies | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 6 |
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 |
U.S. government agencies and government- sponsored agencies | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 6 |
U.S. treasuries | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 20 |
Restricted investments | 14 | 37
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 0 |
Restricted investments | 0 | 37
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 20 |
Restricted investments | 14 | 0
Other debt securities | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 11 |
Other debt securities | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 0 |
Other debt securities | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 11 |
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 investments and restricted cash equivalents and investments | 2 | 2
Certificate of deposit | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted investments | 2 | 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 | 2 | 2
Money market mutual funds | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Cash equivalents | 1 |
Restricted cash equivalents | 229 | 199
Money market mutual funds | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Cash equivalents | 1 |
Restricted cash equivalents | 229 | 199
Money market mutual funds | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Cash equivalents | 0 |
Restricted cash equivalents | 0 | 0
U.S. treasuries | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Cash equivalents | 7 |
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 | 7 |
Commercial paper | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents | 21 | 21
Commercial paper | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents | 21 | 21
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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