Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) $ / MMBTU $ / MWh	Dec. 31, 2014 USD ($) $ / MMBTU $ / MWh
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	$ 586	$ 1,178
Derivative liability, gross	(551)	(1,110)
Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	106	142
Derivative liability, gross	(83)	(118)
Natural Gas
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	303	787
Derivative liability, gross	(277)	(718)
Natural Gas | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	54	92
Derivative liability, gross	$ (33)	$ (62)
Natural Gas | Minimum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(1.73)	(2.28)
Natural Gas | Maximum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	6.57	7.83
Natural Gas | Weighted Average | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(0.11)	(0.22)
Electricity
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	$ 251	$ 342
Derivative liability, gross	(244)	(342)
Electricity | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	47	47
Derivative liability, gross	$ (44)	$ (52)
Electricity | Minimum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	(8)	(14)
Electricity | Maximum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	15	15
Electricity | Weighted Average | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	1	4