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Fair Value Measurements - Investments Held in Trusts (Details) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Short-term cash investments $ 15 $ 9
Debt securities    
Debt Securities, Available-for-sale [Line Items]    
Cost Basis 272 275
Unrealized Gains 4 7
Unrealized Losses (7) (6)
Fair Value $ 269 $ 276