Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Asset, Gross	$ 693	$ 833
Derivative Liability, Gross	(787)	(936)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Asset, Gross	116	176
Derivative Liability, Gross	(158)	(212)
Electricity Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Asset, Gross	386	400
Derivative Liability, Gross	(394)	(398)
Electricity Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Asset, Gross	74	139
Derivative Liability, Gross	(91)	(126)
Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Asset, Gross	254	396
Derivative Liability, Gross	(342)	(503)
Natural Gas Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Asset, Gross	38	34
Derivative Liability, Gross	$ (66)	$ (86)
Discounted cash flow valuation technique [Member] | Electricity Commodity Contract [Member] | Minimum [Member] | Fair Value, Inputs, Level 3 [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	(15)	(7)
Discounted cash flow valuation technique [Member] | Electricity Commodity Contract [Member] | Maximum [Member] | Fair Value, Inputs, Level 3 [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	24	15
Discounted cash flow valuation technique [Member] | Electricity Commodity Contract [Member] | Weighted Average [Member] | Fair Value, Inputs, Level 3 [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	4	3
Discounted cash flow valuation technique [Member] | Natural Gas Commodity Contract [Member] | Minimum [Member] | Fair Value, Inputs, Level 3 [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	(2.28)	(0.88)
Discounted cash flow valuation technique [Member] | Natural Gas Commodity Contract [Member] | Maximum [Member] | Fair Value, Inputs, Level 3 [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	9.97	5.07
Discounted cash flow valuation technique [Member] | Natural Gas Commodity Contract [Member] | Weighted Average [Member] | Fair Value, Inputs, Level 3 [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	(0.13)	(0.16)