Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($) $ / MMBTU $ / MWh	Dec. 31, 2016 USD ($) $ / MMBTU $ / MWh
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 503	$ 549
Derivative Liabilities	(489)	(656)
Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	124	94
Derivative Liabilities	(130)	(182)
Natural Gas
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	226	348
Derivative Liabilities	(216)	(461)
Natural Gas | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	56	53
Derivative Liabilities	$ (73)	$ (149)
Natural Gas | Minimum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(1.30)	(1.00)
Natural Gas | Maximum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	6.35	7.90
Natural Gas | Weighted average | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(0.08)	(0.05)
Electricity
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 266	$ 193
Derivative Liabilities	(268)	(189)
Electricity | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	60	39
Derivative Liabilities	$ (54)	$ (30)
Electricity | Minimum | Discounted cash flow valuation technique | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	(4)	(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