Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	$ 764	$ 1,178
Derivative liability, gross	(770)	(1,110)
Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	78	142
Derivative liability, gross	(90)	(118)
Natural Gas
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	443	787
Derivative liability, gross	(455)	(718)
Natural Gas | Level 3 | Discounted cash flow valuation technique | Minimum
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	(2.46)	(2.28)
Natural Gas | Level 3 | Discounted cash flow valuation technique | Maximum
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	7.00	7.83
Natural Gas | Level 3 | Discounted cash flow valuation technique | Weighted Average
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	(0.25)	(0.22)
Natural Gas | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	41	92
Derivative liability, gross	(41)	(62)
Electricity
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	270	342
Derivative liability, gross	(262)	(342)
Electricity | Level 3 | Discounted cash flow valuation technique | Minimum
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	(7)	(14)
Electricity | Level 3 | Discounted cash flow valuation technique | Maximum
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	19	15
Electricity | Level 3 | Discounted cash flow valuation technique | Weighted Average
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	3	4
Electricity | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative asset, gross	36	47
Derivative liability, gross	$ (43)	$ (52)