Derivative Instruments and Hedging Activities (Gains and Losses) (Details) - Energy Commodities, net - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014		Sep. 30, 2015	Sep. 30, 2014
Cash Flow Hedges
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing)	$ 6	$ 7		$ 23	$ 29
Cash Flow Hedges | Wholesale energy
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing)	(3)	(2)		(3)	(1)
Cash Flow Hedges | Energy purchases
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing)	9	8		25	23
Cash Flow Hedges | Depreciation
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing)	0	0		1	1
Cash Flow Hedges | Discontinued operations
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing)		1			6
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on derivative, net	124	124		138	(614)
Not Designated as Hedging Instrument | Wholesale energy
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on derivative, net	298	617	[1]	328	(2,520)	[1]
Not Designated as Hedging Instrument | Retail Energy
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on derivative, net	15	18		(16)	(34)
Not Designated as Hedging Instrument | Fuel
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on derivative, net	(6)	(8)		(5)	(1)
Not Designated as Hedging Instrument | Energy purchases
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on derivative, net	$ (183)	(505)	[2]	$ (169)	1,937	[2]
Not Designated as Hedging Instrument | Discontinued operations
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on derivative, net		$ 2			$ 4

[1] The nine month period ended September 30, 2014 includes significant realized and unrealized losses on physical and financial commodity sales contracts due to unusual market and weather volatility.

[2] The nine month period ended September 30, 2014 includes significant realized and unrealized gains on physical and financial commodity purchase contracts due to unusual market and weather volatility.