Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) $ / MMBTU $ / MWh	Dec. 31, 2017 USD ($) $ / MMBTU $ / MWh
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 397	$ 652
Derivative Liabilities	(397)	(657)
Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	92	148
Derivative Liabilities	(111)	(157)
Natural gas
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	207	357
Derivative Liabilities	(196)	(378)
Natural gas | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	66	97
Derivative Liabilities	$ (76)	$ (126)
Natural gas | Minimum | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(1.37)	(1.10)
Natural gas | Maximum | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	4.10	9.75
Natural gas | Weighted Average | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(0.08)	(0.03)
Electricity
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 183	$ 285
Derivative Liabilities	(198)	(275)
Electricity | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	21	42
Derivative Liabilities	$ (34)	$ (30)
Electricity | Minimum | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	(6)	(5)
Electricity | Maximum | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	8	15
Electricity | Weighted Average | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	0	2