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Fair Value Measurements - Investments Held in Trusts (Details) - USD ($) $ in Millions	Jun. 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	750	747
Debt securities held for sale	132	135
Debt securities
Debt Securities, Available-for-sale [Line Items]
Cost Basis	407	403
Unrealized Gains	8	9
Unrealized Losses	(8)	(11)
Fair Value	$ 407	$ 401

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