Mergers, Acquisitions, and Dispositions Purchase Price Allocation Table - FitzPatrick (Details) - USD ($) $ in Millions			3 Months Ended			9 Months Ended		12 Months Ended
		Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross						$ 57	$ 208
Bargain Purchase Gain			$ 0	$ 7		0	233
Exelon Generation Co L L C [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross						57	208
Bargain Purchase Gain			$ 0	7		$ 0	$ 233
Exelon Generation Co L L C [Member] | Fitzpatrick [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross								$ 110
Business Combination, Consideration Transferred		$ 289						289
Business Combination, Consideration Transferred, Other		179						54
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]
Payments to Acquire Businesses, Gross								$ 125
Business Combination, Consideration Transferred		$ 110

[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.