Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) $ / MWh $ / MMBTU	Dec. 31, 2016 USD ($) $ / MWh $ / MMBTU
Unobservable Inputs Valuation Techniques [Line Items]
Derivative assets, gross	$ 496	$ 549
Derivative liabilities, gross	(491)	(656)
Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative assets, gross	106	94
Derivative liabilities, gross	(129)	(182)
Natural Gas
Unobservable Inputs Valuation Techniques [Line Items]
Derivative assets, gross	304	348
Derivative liabilities, gross	(289)	(461)
Natural Gas | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative assets, gross	71	53
Derivative liabilities, gross	$ (86)	$ (149)
Natural Gas | Minimum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(0.87)	(1.00)
Natural Gas | Maximum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	6.25	7.90
Natural Gas | Weighted average | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(0.14)	(0.05)
Electricity
Unobservable Inputs Valuation Techniques [Line Items]
Derivative assets, gross	$ 186	$ 193
Derivative liabilities, gross	(197)	(189)
Electricity | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative assets, gross	34	39
Derivative liabilities, gross	$ (40)	$ (30)
Electricity | Minimum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	(6)	(6)
Electricity | Maximum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	8	12
Electricity | Weighted average | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	1	1