Commitments and Contingencies (Energy Purchase Commitments) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2015	Sep. 30, 2015
Fuel \| Loss on Long-term Purchase Contracts
Loss Contingencies [Line Items]
Coal contract modification charges	$ 41	$ 41

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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