Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($) $ / MMBTU $ / MWh	Dec. 31, 2015 USD ($) $ / MMBTU $ / MWh
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 489	$ 711
Derivative Liabilities	(601)	(705)
Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	85	174
Derivative Liabilities	(192)	(178)
Natural Gas
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	264	387
Derivative Liabilities	(361)	(383)
Natural Gas | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	44	103
Derivative Liabilities	$ (144)	$ (108)
Natural Gas | Minimum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(2.08)	(1.50)
Natural Gas | Maximum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	5.75	2.77
Natural Gas | Weighted Average | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(0.07)	(0.19)
Electricity
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 213	$ 307
Derivative Liabilities	(226)	(305)
Electricity | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	38	68
Derivative Liabilities	$ (44)	$ (62)
Electricity | Minimum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	(10)	(11)
Electricity | Maximum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	14	14
Electricity | Weighted Average | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	1	2