Mergers, Acquisitions, and Dispositions Purchase Price Allocation Table - FitzPatrick (Details) - USD ($) $ in Millions			3 Months Ended				6 Months Ended		12 Months Ended
		Mar. 31, 2017	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Business Acquisition [Line Items]
Bargain Purchase Gain			$ 0		$ 0		$ 0	$ 226
Fitzpatrick [Member]
Business Acquisition [Line Items]
Bargain Purchase Gain						$ 226
Exelon Generation Co L L C [Member]
Business Acquisition [Line Items]
Bargain Purchase Gain			$ 0		$ 0		$ 0	$ 226
Exelon Generation Co L L C [Member] | Fitzpatrick [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred		$ 289							$ 289
Business Combination, Consideration Transferred, Other		179
Current assets									60
Property, plant and equipment									298
Nuclear decommissioning trust funds									807
Other assets	[1]								114
Total assets									1,279
Current liabilities									6
Nuclear decommissioning ARO									444
Pension and OPEB obligations									33
Deferred income taxes									149
Spent nuclear fuel obligation									110
Other liabilities									15
Total liabilities									757
Total net identifiable assets, at fair value									522
Bargain Purchase Gain				$ 7					233
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets									110
Exelon Generation Co L L C [Member] | Fitzpatrick [Member] | Cash and Cash Equivalents [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred		$ 110							110
Business Combination, Consideration Transferred, Other									125
Exelon Generation Co L L C [Member] | Fitzpatrick [Member] | Nuclear Fuel [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred, Other									$ 54

[1] 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 of the Exelon 2017 Form 10-K for additional information regarding SNF obligations to the DOE.