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Balance Sheet Components Allowance for Uncollectible Accounts (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Allowance for Uncollectible Accounts    
Balance as of begining of period $ 5 $ 6
Provision, net 2 1
Amounts written off, less recoveries (1) (1)
Balance as of end of period $ 6 $ 6