Price Risk Management Price Risk Management assets and liabilities subject to master netting agreements (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Electricity [Member] \| Carrying (Reported) Amount, Fair Value Disclosure [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	$ 52	$ 91
Electricity [Member] \| Gross amounts offset [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	0	0
Electricity [Member] \| net amount presented [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	52	91
Electricity [Member] \| Derivative [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	52	91
Electricity [Member] \| Securities Pledged as Collateral [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	0	0
Electricity [Member] \| Commodity Contract [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	0	0
Natural Gas [Member] \| Carrying (Reported) Amount, Fair Value Disclosure [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	1	1
Natural Gas [Member] \| Gross amounts offset [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	0	0
Natural Gas [Member] \| net amount presented [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	1	1
Natural Gas [Member] \| Derivative [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	1	1
Natural Gas [Member] \| Securities Pledged as Collateral [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	0	0
Natural Gas [Member] \| Commodity Contract [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	0	0
Liabilities, Total [Member] \| Carrying (Reported) Amount, Fair Value Disclosure [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	53	92
Liabilities, Total [Member] \| Gross amounts offset [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	0	0
Liabilities, Total [Member] \| net amount presented [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	53	92
Liabilities, Total [Member] \| Derivative [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	53	92
Liabilities, Total [Member] \| Securities Pledged as Collateral [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	0	0
Liabilities, Total [Member] \| Commodity Contract [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	$ 0	$ 0

Fair value, after offset of derivative asset, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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