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Commitments and Contingencies (Letters of credit and surety bonds) (Details) - Letters of credit and surety bonds - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Letters of credit and surety bonds    
Outstanding letters of credit $ 19 $ 31
Aggregate cash collateral 10 5
Surety bonds outstanding $ 113 $ 84