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Fair Value Measurements - Financial Instruments (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Financial Instruments |  |
Cash and cash equivalents | $ 580 | $ 352
Short-term investments | 17 | 50
Book Value |  |
Financial Instruments |  |
Cash and cash equivalents | 580 | 352
Short-term investments | 17 | 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 | 180 | 191
Fair Value | Level 1 |  |
Financial Instruments |  |
Cash and cash equivalents | 580 | 352
Short-term investments | 17 | 50
Fair Value | Level 2 | Retail |  |
Financial Instruments |  |
Long-term debt | 850 | 939
Fair Value | Level 2 | Institutional |  |
Financial Instruments |  |
Long-term debt | 531 | 522
Fair Value | Level 2 | Other |  |
Financial Instruments |  |
Long-term debt | $ 180 | $ 191

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