Mergers, Acquisitions and Dispositions Purchase Price Allocation Table - FitzPatrick (Details) - USD ($) $ in Millions			3 Months Ended	12 Months Ended
		Mar. 31, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2017
Business Acquisition [Line Items]
Business Combination, Bargain Purchase, Gain Recognized, Amount				$ 233	$ 0	$ 0
Fitzpatrick [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets							$ 110
Business Combination, Consideration Transferred		$ 289
Current assets		60	$ 60
Property, plant and equipment		298	298
Unamortized energy contracts		807	807
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Assets	[1]	114	114
Total assets		1,279	1,279
Current liabilities		6	6
Asset retirement obligation		444	444
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Noncurrent		149	149
BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedSpentNuclearFuel			110
Other liabilities		(15)	(15)
Total liabilities		757	757
Total net assets		522	522
_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRetirmentObligations		33	$ 33
Business Combination, Bargain Purchase, Gain Recognized, Amount				$ 233
Fitzpatrick [Member] | Cash and Cash Equivalents [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred, Other		125
Business Combination, Consideration Transferred		110
Fitzpatrick [Member] | Nuclear Fuel [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred, Other		$ 54

[1] (a)Includes a $110 million asset associated with a contractual right to reimbursement from the New York Power Authority (NYPA), a prior owner of FitzPatrick, associated with the DOE one-time fee obligation. See Note 23-Commitments and Contingencies for additional background regarding SNF obligations to the DOE.