Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / MMBTU $ / MWh	Dec. 31, 2016 USD ($) $ / MMBTU $ / MWh
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	$ 652	$ 549
Derivative Liabilities	(657)	(656)
Natural Gas
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	357	348
Derivative Liabilities	(378)	(461)
Electricity
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	285	193
Derivative Liabilities	$ (275)	$ (189)
Level 3 | Discounted Cash Flow | Natural Gas | Minimum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(1.10)	(1.00)
Level 3 | Discounted Cash Flow | Natural Gas | Maximum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	9.75	7.90
Level 3 | Discounted Cash Flow | Natural Gas | Weighted Average
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(0.03)	(0.05)
Level 3 | Discounted Cash Flow | Electricity | Minimum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MWh	(5)	(6)
Level 3 | Discounted Cash Flow | Electricity | Maximum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MWh	15	12
Level 3 | Discounted Cash Flow | Electricity | Weighted Average
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MWh	2	1
Recurring | Level 3
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	$ 148	$ 94
Derivative Liabilities	(157)	(182)
Recurring | Level 3 | Natural Gas
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	97	53
Derivative Liabilities	(126)	(149)
Recurring | Level 3 | Electricity
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	42	39
Derivative Liabilities	$ (30)	$ (30)