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Balance Sheet Components Allowance for Uncollectible accounts activity (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Balance as of beginning of year $ 6 $ 6 $ 6
Increase in provision 6 5 6
Amounts written off, less recoveries (6) (5) (6)
Balance as of end of year $ 6 $ 6 $ 6