Price Risk Management Price Risk Management assets and liabilities subject to master netting agreements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Electricity [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Asset, Fair Value, Net	$ 1
Derivative Liability, Fair Value, Net	12	20
Electricity [Member] | Gross amounts offset [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Asset, Fair Value, Net	0
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 Asset, Fair Value, Net	1
Derivative Liability, Fair Value, Net	12	20
Electricity [Member] | Derivative [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Asset, Fair Value, Net	(1)
Derivative Liability, Fair Value, Net	(12)	(20)
Electricity [Member] | Securities Pledged as Collateral [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Asset, Fair Value, Net	0
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 Asset, Fair Value, Net	0
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 Asset, Fair Value, Net	1
Derivative Liability, Fair Value, Net	5	7
Natural Gas [Member] | Gross amounts offset [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Asset, Fair Value, Net	0
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 Asset, Fair Value, Net	1
Derivative Liability, Fair Value, Net	5	7
Natural Gas [Member] | Derivative [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Asset, Fair Value, Net	(1)
Derivative Liability, Fair Value, Net	(5)	(7)
Natural Gas [Member] | Securities Pledged as Collateral [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Asset, Fair Value, Net	0
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 Asset, Fair Value, Net	0
Derivative Liability, Fair Value, Net	0	0
Assets, Total [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Asset, Fair Value, Net	2
Assets, Total [Member] | Gross amounts offset [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Asset, Fair Value, Net	0
Assets, Total [Member] | net amount presented [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Asset, Fair Value, Net	2
Assets, Total [Member] | Derivative [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Asset, Fair Value, Net	(2)
Assets, Total [Member] | Securities Pledged as Collateral [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Asset, Fair Value, Net	0
Assets, Total [Member] | Commodity Contract [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Asset, Fair Value, Net	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	17	27
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	17	27
Liabilities, Total [Member] | Derivative [Member]
Price Risk Management assets and liabilities subject to master netting agreements [Line Items]
Derivative Liability, Fair Value, Net	(17)	(27)
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