Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) $ / MWh $ / MMBTU	Dec. 31, 2015 USD ($) $ / MWh $ / MMBTU
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	$ 549	$ 711
Derivative Liabilities	(656)	(705)
Natural Gas
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	348	387
Derivative Liabilities	(461)	(383)
Electricity
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	193	307
Derivative Liabilities	$ (189)	$ (305)
Level 3 | Discounted Cash Flow Valuation Technique | Natural Gas | Minimum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(1.00)	(1.50)
Level 3 | Discounted Cash Flow Valuation Technique | Natural Gas | Maximum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	7.90	2.77
Level 3 | Discounted Cash Flow Valuation Technique | Natural Gas | Weighted Average
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(0.05)	(0.19)
Level 3 | Discounted Cash Flow Valuation Technique | Electricity | Minimum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MWh	(6)	(11)
Level 3 | Discounted Cash Flow Valuation Technique | Electricity | Maximum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MWh	12	14
Level 3 | Discounted Cash Flow Valuation Technique | Electricity | Weighted Average
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MWh	1	2
Recurring | Level 3
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	$ 94	$ 174
Derivative Liabilities	(182)	(178)
Recurring | Level 3 | Natural Gas
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	53	103
Derivative Liabilities	(149)	(108)
Recurring | Level 3 | Electricity
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	39	68
Derivative Liabilities	$ (30)	$ (62)