Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Fair value of commodity derivatives
Derivative Assets	$ 314	$ 239
Derivative Liabilities	(383)	(347)
Current Assets
Fair value of commodity derivatives
Derivative Assets	247	166
Noncurrent Assets
Fair value of commodity derivatives
Derivative Assets	67	73
Current Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(159)	(111)
Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(224)	(236)
Power Contracts | Current Assets
Fair value of commodity derivatives
Derivative Assets	240	162
Power Contracts | Noncurrent Assets
Fair value of commodity derivatives
Derivative Assets	64	53
Power Contracts | Current Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(154)	(102)
Power Contracts | Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(33)	(11)
FTRs | Current Assets
Fair value of commodity derivatives
Derivative Assets	7	4
FTRs | Current Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(5)	(9)
FTRs | Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(3)	(3)
NUGs | Noncurrent Assets
Fair value of commodity derivatives
Derivative Assets	3	20
NUGs | Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	$ (188)	$ (222)