Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2015 USD ($) $ / MMBTU $ / MWh	Dec. 31, 2014 USD ($) $ / MMBTU $ / MWh
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	$ 607	$ 1,178
Derivative liability, gross	(601)	(1,110)
Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	91	142
Derivative liability, gross	(95)	(118)
Natural Gas
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	251	787
Derivative liability, gross	$ (256)	$ (718)
Natural Gas | Level 3 | Discounted cash flow valuation technique | Minimum
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(1.70)	(2.28)
Natural Gas | Level 3 | Discounted cash flow valuation technique | Maximum
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	6.63	7.83
Natural Gas | Level 3 | Discounted cash flow valuation technique | Weighted Average
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(0.17)	(0.22)
Natural Gas | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	$ 42	$ 92
Derivative liability, gross	(50)	(62)
Electricity
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	312	342
Derivative liability, gross	$ (305)	$ (342)
Electricity | Level 3 | Discounted cash flow valuation technique | Minimum
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	(6)	(14)
Electricity | Level 3 | Discounted cash flow valuation technique | Maximum
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	19	15
Electricity | Level 3 | Discounted cash flow valuation technique | Weighted Average
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | $ / MWh	2	4
Electricity | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	$ 44	$ 47
Derivative liability, gross	$ (41)	$ (52)