Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair value of commodity derivatives
Economic Hedges Derivative assets	$ 285	$ 321
Economic Hedges Derivative liabilities	(630)	(619)
Power Contracts | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	178	185
Power Contracts | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(146)	(196)
Power Contracts | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	79	79
Power Contracts | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(31)	(51)
FTRs | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	7	1
FTRs | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(9)	(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	(2)	(1)
NUGs
Fair value of commodity derivatives
Economic Hedges Derivative assets	18	56
Economic Hedges Derivative liabilities	(300)	(349)
LCAPP
Fair value of commodity derivatives
Economic Hedges Derivative assets	0	0
Economic Hedges Derivative liabilities	(142)	0
Other | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	3	0
Other | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	$ 0	$ 0