Assets and Liabilities with Recurring Fair Value Measurements (Textuals) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value Measurement [Domain]
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]
Cash and Cash Equivalents, at Carrying Value		$ 880	$ 679
Cash Equivalents Included In Restricted Cash, Fair Value Disclosure		203	217
Balance, beginning of period		85	14	$ 16
Included in operating revenues	[1]	218	70	5
Included in fuel and purchased energy expense	[2]	(7)	5	0
Purchases		(70)	6	6
Issuances		0	0	(2)
Settlements		(29)	(10)	(11)
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount		0	0	0
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount		0	0	0
Transfers into level 3	[3],[4]	0	0	0
Transfers out of level 3	[4],[5]	(243)	0	0
Balance, end of period		(46)	85	14
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		211	75	5
Cash and Cash Equivalents, at Carrying Value		906	717	$ 941	$ 1,284
Cash Equivalents Included In Restricted Cash, Fair Value Disclosure		228	$ 244
Transfer to Level 2 [Member]
Purchases, issuances and settlements:
Transfers out of level 3	[4],[5]	(4)
Transfer to Other Assets [Member]
Purchases, issuances and settlements:
Transfers out of level 3	[4],[5]	$ (239)

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

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

[3] There were no transfers out of level 2 into level 3 for the years ended December 31, 2015, 2014 and 2013.

[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 during the years ended December 31, 2015, 2014 and 2013.

[5] We had $4 million in gains transferred out of level 3 into level 2 during the year ended December 31, 2015 due to changes in market liquidity in various power markets and $239 million in gains transferred out of level 3 during the year ended December 31, 2015 to other assets following the election of the normal purchase normal sales exemption and the discontinuance of derivative accounting treatment as of the date of this election. There were no transfers out of level 3 for the years ended December 31, 2014 and 2013.