Derivative Instruments (Net Notional Volumes and Additional Disclosures) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012 megawatt-hours
Commodity contract - Power [Member]
Derivative [Line Items]
Non-monetary net notional volumes	(86,000,000)
Commodity contract - Natural gas [Member]
Derivative [Line Items]
Non-monetary net notional volumes	1,481,000,000
Commodity contract - Oil [Member]
Derivative [Line Items]
Non-monetary net notional volumes	(3,000,000)
Interest rate swaps [Member]
Derivative [Line Items]
Notional amount	6,600
Currency Swap [Member]
Derivative [Line Items]
Notional amount	544