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Summary of Significant Accounting Policies Significant Accounting Policies - Allowance for Doubtful Accounts (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Beginning balance $ 8us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent $ 9us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent $ 9us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent
Charged to operating costs and expenses, net 11us-gaap_ProvisionForDoubtfulAccounts 13us-gaap_ProvisionForDoubtfulAccounts 14us-gaap_ProvisionForDoubtfulAccounts
Write-offs, net (12)us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs (14)us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs (14)us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs
Ending balance $ 7us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent $ 8us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent $ 9us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent