Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Unobservable Input Valuation Techniques [Line Items]
Derivative asset, gross	$ 1,178	$ 833
Total fair value of derivatives	(1,110)	(936)
Natural Gas Commodity Contract [Member]
Unobservable Input Valuation Techniques [Line Items]
Derivative asset, gross	787	396
Total fair value of derivatives	(718)	(503)
Electricity Commodity Contract [Member]
Unobservable Input Valuation Techniques [Line Items]
Derivative asset, gross	342	400
Total fair value of derivatives	(342)	(398)
Level 3 [Member] | Discounted Cash Flow Valuation Technique [Member] | Natural Gas Commodity Contract [Member] | Minimum [Member]
Unobservable Input Valuation Techniques [Line Items]
Forward Basis Price	(2.28)	(0.88)
Level 3 [Member] | Discounted Cash Flow Valuation Technique [Member] | Natural Gas Commodity Contract [Member] | Maximum
Unobservable Input Valuation Techniques [Line Items]
Forward Basis Price	7.83	5.07
Level 3 [Member] | Discounted Cash Flow Valuation Technique [Member] | Natural Gas Commodity Contract [Member] | Weighted Average [Member]
Unobservable Input Valuation Techniques [Line Items]
Forward Basis Price	(0.22)	(0.16)
Level 3 [Member] | Discounted Cash Flow Valuation Technique [Member] | Electricity Commodity Contract [Member] | Minimum [Member]
Unobservable Input Valuation Techniques [Line Items]
Forward Basis Price	(14)	(7)
Level 3 [Member] | Discounted Cash Flow Valuation Technique [Member] | Electricity Commodity Contract [Member] | Maximum
Unobservable Input Valuation Techniques [Line Items]
Forward Basis Price	15	15
Level 3 [Member] | Discounted Cash Flow Valuation Technique [Member] | Electricity Commodity Contract [Member] | Weighted Average [Member]
Unobservable Input Valuation Techniques [Line Items]
Forward Basis Price	4	3
Recurring [Member] | Level 3 [Member]
Unobservable Input Valuation Techniques [Line Items]
Derivative asset, gross	142	176
Total fair value of derivatives	(118)	(212)
Recurring [Member] | Level 3 [Member] | Natural Gas Commodity Contract [Member]
Unobservable Input Valuation Techniques [Line Items]
Derivative asset, gross	92	34
Total fair value of derivatives	(62)	(86)
Recurring [Member] | Level 3 [Member] | Electricity Commodity Contract [Member]
Unobservable Input Valuation Techniques [Line Items]
Derivative asset, gross	47	139
Total fair value of derivatives	$ (52)	$ (126)