Derivative Instruments (Details) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Fair value of commodity derivatives
Economic Hedges Derivative assets	$ 404	$ 268
Economic Hedges Derivative liabilities	896	714
Power Contracts | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	88	38
Power Contracts | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	102	40
Power Contracts | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	210	96
Power Contracts | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	274	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	13	0
FTRs | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	7	0
NUGs | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	205	238
NUGs | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	71	119
NUGs | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	4	3
NUGs | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	317	229
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	4	0
Interest Rate Swap | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	5	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