Assets and Liabilities with Recurring Fair Value Measurements (Textuals) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Fair Value Measurement [Domain]
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]
Cash and Cash Equivalents, at Carrying Value		$ 561		$ 561		$ 906
Restricted Cash and Cash Equivalents		225		225		228
Balance, beginning of period		(63)	$ 243	(46)	$ 85
Included in operating revenues	[1]	30	70	9	236
Fair Value, Assets Measured with Unobservable Inputs on Recurring Basis, Gain (Loss) Included In Fuel And Purchased Energy Expense	[2]	(31)	(2)	(24)	(2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		1	0	4	3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		15	(8)	(4)	(24)
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount		0	0	0	0
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount		0	0	0	0
Transfers into level 3	[3],[4]	1	0	0	0
Transfers out of Level 3	[4],[5]	75	(11)	89	(6)
Balance, end of period		28	292	28	292
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		(1)	68	(15)	234
Cash and Cash Equivalents, at Carrying Value		561	$ 659	561	$ 659	906	$ 717
Restricted Cash and Cash Equivalents		$ 225		$ 225		$ 228

[1] For power contracts and other power-related products, included on our Consolidated Condensed Statements of Operations.

[2] For natural gas and power contracts, swaps and options, included on our Consolidated Condensed Statements of Operations.

[3] We had $1 million and nil in gains transferred out of level 2 into level 3 for the three months ended September 30, 2016 and 2015, respectively. There were no transfers out of level 2 into level 3 for each of the nine months ended September 30, 2016 and 2015.

[4] We transfer amounts among levels of the fair value hierarchy as of the end of each period. There were no transfers into or out of level 1 for each of the three and nine months ended September 30, 2016 and 2015.

[5] We had $(75) million in losses and $11 million in gains transferred out of level 3 into level 2 for the three months ended September 30, 2016 and 2015, respectively, and $(89) million in losses and $6 million in gains transferred out of level 3 into level 2 for the nine months ended September 30, 2016 and 2015, respectively, due to changes in market liquidity in various power markets.