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Revenue (Accounts Receivable, Contract Assets and Liabilities) (Details) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Revenue from External Customer [Line Items]    
Accounts receivable $ 934 $ 899
Unbilled    
Revenue from External Customer [Line Items]    
Accounts receivable 479 476
Trade & other    
Revenue from External Customer [Line Items]    
Accounts receivable $ 455 $ 423