Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair value of commodity derivatives
Derivative Assets	$ 284	$ 296
Derivative Liabilities	(591)	(598)
Current Assets
Fair value of commodity derivatives
Derivative Assets	173	160
Noncurrent Assets
Fair value of commodity derivatives
Derivative Assets	111	136
Current Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(142)	(126)
Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(449)	(472)
Power Contracts | Current Assets
Fair value of commodity derivatives
Derivative Assets	166	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	(133)	(119)
Power Contracts | Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(35)	(36)
FTRs | Current Assets
Fair value of commodity derivatives
Derivative Assets	7	7
FTRs | Noncurrent Assets
Fair value of commodity derivatives
Derivative Assets	0	1
FTRs | Current Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(9)	(7)
FTRs | Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	0	(2)
NUGs | Noncurrent Assets
Fair value of commodity derivatives
Derivative Assets	24	36
NUGs | Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(256)	(290)
LCAPP | Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	$ (158)	$ (144)