Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 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	(0.11)	(0.16)
Fair Value, Inputs, Level 3 [Member] | Electricity Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Weighted Average Forward Basis Price	4	3
Minimum [Member] | Fair Value, Inputs, Level 3 [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	(0.80)	(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	6.98	5.07
Maximum [Member] | Fair Value, Inputs, Level 3 [Member] | Electricity Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price	16	15
Fair Value, Measurements, Recurring [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 124	$ 126
Derivative Liabilities	(177)	(238)
Fair Value, Measurements, Recurring [Member] | Natural Gas Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	12	14
Derivative Liabilities	(54)	(108)
Fair Value, Measurements, Recurring [Member] | Electricity Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	106	109
Derivative Liabilities	(122)	(129)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	154	176
Derivative Liabilities	(194)	(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	52	34
Derivative Liabilities	(72)	(86)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Electricity Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	95	139
Derivative Liabilities	$ (122)	$ (126)