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Summary of Significant Accounting Policies (Roll Forward of the Allowance For Doubtful Accounts) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Allowance for doubtful accounts beginning $ (16) $ (16) $ (16)
Bad debt expense (29) (10) (9)
Write-offs, net of recoveries 4 10 9
Allowance for doubtful accounts ending $ (41) $ (16) $ (16)