Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013 MMBTU	Dec. 31, 2012 MMBTU
Fair Value, Inputs, Level 3 [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Weighted Average Forward Basis Price	(0.16)	0.03
Fair Value, Inputs, Level 3 [Member] | Electricity Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Weighted Average Forward Basis Price	3	3
Fair Value, Measurements, Recurring [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 126	$ 147
Derivative Liabilities	(238)	(151)
Fair Value, Measurements, Recurring [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	14	40
Derivative Liabilities	(108)	(56)
Fair Value, Measurements, Recurring [Member] | Electricity Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	109	102
Derivative Liabilities	(129)	(93)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	176	160
Derivative Liabilities	(212)	(173)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	34	24
Derivative Liabilities	(86)	(62)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Electricity Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	139	134
Derivative Liabilities	$ (126)	$ (111)
Minimum [Member] | Fair Value, Inputs, Level 3 [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Weighted Average Range for Unobservable Inputs	(0.88)	(0.63)
Minimum [Member] | Fair Value, Inputs, Level 3 [Member] | Electricity Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Weighted Average Range for Unobservable Inputs	(7)	(2)
Maximum [Member] | Fair Value, Inputs, Level 3 [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Weighted Average Range for Unobservable Inputs	5.07	1.95
Maximum [Member] | Fair Value, Inputs, Level 3 [Member] | Electricity Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Weighted Average Range for Unobservable Inputs	15	16