Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014 MMBTU	Dec. 31, 2013 MMBTU
Fair Value, Inputs, Level 3 [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Weighted Average Forward Basis Price	(0.17)	(0.16)
Fair Value, Inputs, Level 3 [Member] | Electricity Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Weighted Average Forward Basis Price	5	3
Minimum [Member] | Fair Value, Inputs, Level 3 [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	(1.50)	(0.88)
Minimum [Member] | Fair Value, Inputs, Level 3 [Member] | Electricity Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	(3)	(7)
Maximum [Member] | Fair Value, Inputs, Level 3 [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	1.50	5.07
Maximum [Member] | Fair Value, Inputs, Level 3 [Member] | Electricity Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	23	15
Fair Value, Measurements, Recurring [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 96	$ 126
Derivative Liabilities	(156)	(238)
Fair Value, Measurements, Recurring [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	16	14
Derivative Liabilities	(47)	(108)
Fair Value, Measurements, Recurring [Member] | Electricity Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	81	109
Derivative Liabilities	(107)	(129)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	138	176
Derivative Liabilities	(181)	(212)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	60	34
Derivative Liabilities	(70)	(86)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Electricity Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	77	139
Derivative Liabilities	$ (111)	$ (126)