Mergers, Acquisitions, and Dispositions Purchase Price Allocation Table - FitzPatrick (Details) - USD ($) $ in Millions			3 Months Ended			12 Months Ended
		Mar. 31, 2017	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017	Dec. 31, 2017
Business Acquisition [Line Items]
Bargain Purchase Gain			$ 0		$ 226
Fitzpatrick [Member]
Business Acquisition [Line Items]
Bargain Purchase Gain				$ 7	226	$ 233
Exelon Generation Co L L C [Member]
Business Acquisition [Line Items]
Bargain Purchase Gain			$ 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	$ 226	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 background regarding SNF obligations to the DOE.