Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	Dec. 31, 2015 USD ($) $ / MWh $ / MMBTU	Dec. 31, 2014 USD ($) $ / MWh $ / MMBTU
Unobservable Input Valuation Techniques [Line Items]
Derivative asset, gross	$ 711	$ 1,178
Derivative Liabilities	(705)	(1,110)
Natural Gas
Unobservable Input Valuation Techniques [Line Items]
Derivative asset, gross	387	787
Derivative Liabilities	(383)	(718)
Electricity
Unobservable Input Valuation Techniques [Line Items]
Derivative asset, gross	307	342
Derivative Liabilities	$ (305)	$ (342)
Level 3 | Discounted Cash Flow Valuation Technique | Natural Gas | Minimum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(1.50)	(2.28)
Level 3 | Discounted Cash Flow Valuation Technique | Natural Gas | Maximum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	2.77	7.83
Level 3 | Discounted Cash Flow Valuation Technique | Natural Gas | Weighted Average
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(0.19)	(0.22)
Level 3 | Discounted Cash Flow Valuation Technique | Electricity | Minimum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MWh	(11)	(14)
Level 3 | Discounted Cash Flow Valuation Technique | Electricity | Maximum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MWh	14	15
Level 3 | Discounted Cash Flow Valuation Technique | Electricity | Weighted Average
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MWh	2	4
Recurring | Level 3
Unobservable Input Valuation Techniques [Line Items]
Derivative asset, gross	$ 174	$ 142
Derivative Liabilities	(178)	(118)
Recurring | Level 3 | Natural Gas
Unobservable Input Valuation Techniques [Line Items]
Derivative asset, gross	103	92
Derivative Liabilities	(108)	(62)
Recurring | Level 3 | Electricity
Unobservable Input Valuation Techniques [Line Items]
Derivative asset, gross	68	47
Derivative Liabilities	$ (62)	$ (52)