Price Risk Management Price risk management assets and liabilities subject to master netting agreements (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Electricity [Member] | Energy Related Derivative [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	$ (55)	$ (91)
Electricity [Member] | Gross amounts offset [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0	0
Electricity [Member] | net amount presented [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(55)	(91)
Electricity [Member] | Derivative [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(55)	(91)
Electricity [Member] | Securities Pledged as Collateral [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0	0
Electricity [Member] | Commodity Contract [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0	0
Natural Gas [Member] | Energy Related Derivative [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(17)	(1)
Natural Gas [Member] | Gross amounts offset [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0	0
Natural Gas [Member] | net amount presented [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(17)	(1)
Natural Gas [Member] | Derivative [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(17)	(1)
Natural Gas [Member] | Securities Pledged as Collateral [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0	0
Natural Gas [Member] | Commodity Contract [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0	0
Liabilities, Total [Member] | Energy Related Derivative [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(72)	(92)
Liabilities, Total [Member] | Gross amounts offset [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0	0
Liabilities, Total [Member] | net amount presented [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(72)	(92)
Liabilities, Total [Member] | Derivative [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(72)	(92)
Liabilities, Total [Member] | Securities Pledged as Collateral [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0	0
Liabilities, Total [Member] | Commodity Contract [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	$ 0	$ 0