Derivative Instruments (Details 3) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Summary of Derivative Instruments by Risk Exposure [Abstract]
Power contracts included in operating revenues	$ 1,646		$ 1,965
Natural gas contracts included in fuel and purchased energy expense	1,010		1,357
Interest expense	154		166
Gain (Loss) on Derivative Instruments, Net, Pretax	130		(111)
Gain (Loss) on Sale of Derivatives	59	[1]	(39)	[1]
Mark-to-market gain (loss)	71	[2]	(72)	[2]
Power [Member]
Summary of Derivative Instruments by Risk Exposure [Abstract]
Power contracts included in operating revenues	119		(237)
Interest Rate Swap [Member]
Summary of Derivative Instruments by Risk Exposure [Abstract]
Interest expense	1		1
Mark-to-market gain (loss)	1	[2]	1	[2]
Energy Related Derivative [Member]
Summary of Derivative Instruments by Risk Exposure [Abstract]
Gain (Loss) on Sale of Derivatives	59	[1]	(39)	[1]
Mark-to-market gain (loss)	70	[2]	(73)	[2]
Natural Gas [Member]
Summary of Derivative Instruments by Risk Exposure [Abstract]
Natural gas contracts included in fuel and purchased energy expense	$ 10		$ 125

[1] Does not include the realized value associated with derivative instruments that settle through physical delivery.

[2] In addition to changes in market value on derivatives not designated as hedges, changes in mark-to-market gain (loss) also includes de-designation of interest rate swap cash flow hedges and related reclassification from AOCI into earnings, hedge ineffectiveness and adjustments to reflect changes in credit default risk exposure.