Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Fair value of commodity derivatives
Economic Hedges Derivative assets	$ 459	$ 321
Economic Hedges Derivative liabilities	(707)	(619)
Power Contracts | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	300	185
Power Contracts | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(282)	(196)
Power Contracts | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	115	79
Power Contracts | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(66)	(51)
FTRs | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	1	1
FTRs | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(15)	(22)
FTRs | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	0	0
FTRs | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	0	(1)
NUGs
Fair value of commodity derivatives
Economic Hedges Derivative assets	42	56
Economic Hedges Derivative liabilities	(342)	(349)
Other | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	1	0
Other | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(2)	0
Other | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	0	0
Other | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	$ 0	$ 0