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Summary of Significant Accounting Policies Operations and Maintenance (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Operating Expense [Member]      
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items]      
Results of Operations, Expense from Oil and Gas Producing Activities $ 363 $ 342 $ 349