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Fair Value Measurements (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Financial Instruments |  |
Cash and cash equivalents | $ 509 | $ 352
Short-term investments | $ 0 | $ 50
Institutional and Other | Minimum |  |
Financial Instruments |  |
Fair value assumption, interest rate | 4.58% | 4.74%
Institutional and Other | Maximum |  |
Financial Instruments |  |
Fair value assumption, interest rate | 6.34% | 7.13%
Fair Value | Level 1 |  |
Financial Instruments |  |
Cash and cash equivalents | $ 509 | $ 352
Short-term investments | 0 | 50
Fair Value | Level 2 | Retail |  |
Financial Instruments |  |
Long-term debt | 944 | 939
Fair Value | Level 2 | Institutional |  |
Financial Instruments |  |
Long-term debt | 555 | 522
Fair Value | Level 2 | Other |  |
Financial Instruments |  |
Long-term debt | 188 | 191
Book Value |  |
Financial Instruments |  |
Cash and cash equivalents | 509 | 352
Short-term investments | 0 | 50
Book Value | Retail |  |
Financial Instruments |  |
Long-term debt | 917 | 917
Book Value | Institutional |  |
Financial Instruments |  |
Long-term debt | 534 | 534
Book Value | Other |  |
Financial Instruments |  |
Long-term debt | $ 188 | $ 191

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