Assets and Liabilities with Recurring Fair Value Measurements (Textuals) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	$ 85		$ 14
Included in net income:
Included in operating revenues	131	[1]	(10)	[1]
Fair Value, Assets Measured with Unobservable Inputs on Recurring Basis, Gain (Loss) Included In Fuel And Purchased Energy Expense	3	[2]	5	[2]
Purchases, issuances and settlements:
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	2		1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(10)		(3)
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount	0		0
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount	0		0
Transfers into level 3	(1)	[3],[4]	0	[3],[4]
Transfers out of Level 3	(7)	[3],[5]	1	[3],[5]
Balance, end of period	203		8
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	134		(5)
Assets and Liabilities with Recurring Fair Value Measurements (Textuals) [Abstract]
Cash and Cash Equivalents, at Carrying Value	796		515		717	941
Restricted Cash and Cash Equivalents	209				244
Fair Value Measurement [Domain]
Assets and Liabilities with Recurring Fair Value Measurements (Textuals) [Abstract]
Cash and Cash Equivalents, at Carrying Value	757				679
Restricted Cash and Cash Equivalents	$ 208				$ 217

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

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

[3] 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 months ended March 31, 2015 and 2014.

[4] We had $(1) million in losses transferred out of level 2 into level 3 for the three months ended March 31, 2015. There were no transfers out of level 2 into level 3 for the three months ended March 31, 2014.

[5] We had $(7) million in gains and $1 million in losses transferred out of level 3 into level 2 for the three months ended March 31, 2015 and 2014, respectively, due to changes in market liquidity in various power markets.