Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) $ / MMBTU $ / MWh	Dec. 31, 2016 USD ($) $ / MMBTU $ / MWh
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 439	$ 549
Derivative Liabilities	(409)	(656)
Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	113	94
Derivative Liabilities	(106)	(182)
Natural Gas
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	230	348
Derivative Liabilities	(204)	(461)
Natural Gas | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	62	53
Derivative Liabilities	$ (64)	$ (149)
Natural Gas | Minimum | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(1.92)	(1.00)
Natural Gas | Maximum | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	6.36	7.90
Natural Gas | Weighted Average | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(0.03)	(0.05)
Electricity
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 202	$ 193
Derivative Liabilities	(199)	(189)
Electricity | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	47	39
Derivative Liabilities	$ (40)	$ (30)
Electricity | Minimum | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	(10)	(6)
Electricity | Maximum | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	7	12
Electricity | Weighted Average | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	1	1