Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair value of commodity derivatives
Economic Hedges Derivative assets	$ 296	$ 321
Economic Hedges Derivative liabilities	(597)	(619)
Power Contracts | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	153	185
Power Contracts | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	99	79
Power Contracts | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(115)	(196)
Power Contracts | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(36)	(51)
FTRs | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	7	1
FTRs | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	1	0
FTRs | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(7)	(22)
FTRs | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(2)	(1)
NUGs | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	36	56
NUGs | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(290)	(349)
LCAPP | Noncurrent Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	0	0
LCAPP | Noncurrent Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	(144)	0
Other | Current Assets
Fair value of commodity derivatives
Economic Hedges Derivative assets	0	0
Other | Current Liabilities
Fair value of commodity derivatives
Economic Hedges Derivative liabilities	$ (3)	$ 0