Derivative Instruments (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Fair value of commodity derivatives
Economic Hedges Derivative assets	$ 288	$ 268
Economic Hedges Derivative liabilities	812	714
Power Contracts | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	67	38
Power Contracts | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	68	40
Power Contracts | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	157	96
Power Contracts | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	190	209
FTRs | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	0	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	4	0
FTRs | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	13	0
NUGs
Fair value of commodity derivatives
Economic Hedges Derivative assets	59	122
Economic Hedges Derivative liabilities	542	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