Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($) $ / MWh $ / MMBTU	Dec. 31, 2015 USD ($) $ / MWh $ / MMBTU
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets, gross	$ 685	$ 711
Derivative Liabilities	(763)	(705)
Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets, gross	149	174
Derivative Liabilities	(206)	(178)
Natural Gas
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets, gross	298	387
Derivative Liabilities	(350)	(383)
Natural Gas | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets, gross	76	103
Derivative Liabilities	$ (110)	$ (108)
Natural Gas | Minimum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(1.21)	(1.50)
Natural Gas | Maximum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	3.62	2.77
Natural Gas | Weighted Average | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(0.12)	(0.19)
Electricity
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets, gross	$ 377	$ 307
Derivative Liabilities	(396)	(305)
Electricity | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets, gross	72	68
Derivative Liabilities	$ (88)	$ (62)
Electricity | Minimum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	(14)	(11)
Electricity | Maximum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	13	14
Electricity | Weighted Average | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	1	2