Price Risk Management Price Risk Management assets and liabilities subject to master netting agreements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Electricity [Member] | Energy Related Derivative [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	$ 91	$ 55
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	91	55
Electricity [Member] | Derivative [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	91	55
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] | Energy Related Derivative [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	16	17
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	16	17
Natural Gas [Member] | Derivative [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	16	17
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] | Energy Related Derivative [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	107	72
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	107	72
Liabilities, Total [Member] | Derivative [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	107	72
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