Derivative Instruments (Commodity Derivative Instruments) (Details)	Dec. 31, 2011 megawatt-hours
Power [Member]
Schedule of net notional volume of commodity derivative instruments [Abstract]
Non monetary net notional volumes	(121,000,000)	[1]
Natural gas [Member]
Schedule of net notional volume of commodity derivative instruments [Abstract]
Non monetary net notional volumes	1,132,000,000	[2]
Oil [Member]
Schedule of net notional volume of commodity derivative instruments [Abstract]
Non monetary net notional volumes	(3,000,000)

[1] Megawatt-hours

[2] One million British thermal units