Derivative Instruments, Net Notional Volumes and Additional Disclosures (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2011 megawatt-hours
Commodity contract - Power [Member]
Derivative [Line Items]
Non-monetary net notional volumes	(115,000,000)
Commodity contract - Natural gas [Member]
Derivative [Line Items]
Non-monetary net notional volumes	1,183,000,000
Commodity contract - Oil [Member]
Derivative [Line Items]
Non-monetary net notional volumes	(2,000,000)
Interest rate swaps [Member]
Derivative [Line Items]
Number of instruments held	28
Notional amount	6,600
Foreign currency swaps [Member]
Derivative [Line Items]
Number of instruments held	2
Notional amount	544