Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair value of commodity derivatives
Derivative Assets	$ 305	$ 296
Derivative Liabilities	(573)	(597)
Current Assets
Fair value of commodity derivatives
Derivative Assets	183	160
Noncurrent Assets
Fair value of commodity derivatives
Derivative Assets	122	136
Current Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(126)	(125)
Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(447)	(472)
Power Contracts | Current Assets
Fair value of commodity derivatives
Derivative Assets	175	153
Power Contracts | Noncurrent Assets
Fair value of commodity derivatives
Derivative Assets	87	99
Power Contracts | Current Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(122)	(115)
Power Contracts | Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(53)	(36)
FTRs | Current Assets
Fair value of commodity derivatives
Derivative Assets	1	7
FTRs | Noncurrent Assets
Fair value of commodity derivatives
Derivative Assets	1	1
FTRs | Current Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(4)	(7)
FTRs | Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(1)	(2)
NUGs | Noncurrent Assets
Fair value of commodity derivatives
Derivative Assets	34	36
NUGs | Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(247)	(290)
LCAPP | Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(146)	(144)
Other | Current Assets
Fair value of commodity derivatives
Derivative Assets	7	0
Other | Current Liabilities
Fair value of commodity derivatives
Derivative Liabilities	$ 0	$ (3)