Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Fair value of commodity derivatives
Economic Hedges Derivative assets	$ 321	$ 268
Economic Hedges Derivative liabilities	(619)	(714)
Power Contracts | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(51)	(38)
Power Contracts | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	79	40
Power Contracts | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	185	96
Power Contracts | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(196)	(209)
FTRs | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(1)	0
FTRs | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	0	0
FTRs | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	1	0
FTRs | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(22)	0
NUGs
Fair value of commodity derivatives
Economic Hedges Derivative assets	56	122
Economic Hedges Derivative liabilities	(349)	(467)
Interest Rate Swap | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	0	0
Interest Rate Swap | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	0	0
Interest Rate Swap | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	0	0
Interest Rate Swap | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	0	0
Other | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	0	0
Other | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	0	0
Other | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	0	10
Other | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	$ 0	$ 0