Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair value of commodity derivatives
Economic Hedges Derivative assets	$ 366	$ 321
Economic Hedges Derivative liabilities	(742)	(619)
Power Contracts | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	232	185
Power Contracts | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(213)	(196)
Power Contracts | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	106	79
Power Contracts | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(49)	(51)
FTRs | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	12	1
FTRs | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(8)	(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	(1)	(1)
NUGs
Fair value of commodity derivatives
Economic Hedges Derivative assets	9	56
Economic Hedges Derivative liabilities	(302)	(349)
LCAPP
Fair value of commodity derivatives
Economic Hedges Derivative assets	0	0
Economic Hedges Derivative liabilities	(145)	0
Interest rate swaps | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	3	0
Interest rate swaps | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(23)	0
Other | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	4	0
Other | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	$ (1)	$ 0