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Fair Value Measurements - Investments Held in Trusts (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Short-term cash investments | $ 753 | $ 751
Short-term cash investments held for sale | 746 | 747
Debt securities held for sale | 135 | 135
Debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost Basis | 406 | 403
Unrealized Gains | 11 | 9
Unrealized Losses | (8) | (11)
Fair Value | $ 409 | $ 401

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