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Summary of Significant Accounting Policies Allowance for Uncollectible Accounts (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Financing Receivable, Net $ 124 $ 123  
Allowance for Notes, Loans and Financing Receivable, Noncurrent 1    
Carrying Value      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Financing Receivable, Net $ 139 $ 138 $ 139