Mergers, Acquisitions and Dispositions Purchase Price Allocation Table - FitzPatrick (Details) - USD ($) $ in Millions		12 Months Ended
	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross		$ 154	$ 208	$ 6,923
Business Combination, Bargain Purchase, Gain Recognized, Amount		0	233	0
Exelon Generation Co L L C [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross		154	208	293
Business Combination, Bargain Purchase, Gain Recognized, Amount		$ 0	233	$ 0
Exelon Generation Co L L C [Member] | Fitzpatrick [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross			110
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets			110
Business Combination, Consideration Transferred, Other	$ 179		54
Business Combination, Consideration Transferred	289		289
Current assets			60
Property, plant and equipment			298
Unamortized energy contracts			807
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Assets			114
Total assets			1,279
Current liabilities			6
Unamortized energy contract liabilities			(444)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Noncurrent			149
BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRegulatedLiabilities			110
Other liabilities			(15)
Total liabilities			757
Total net assets			522
_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRetirmentObligations			33
Business Combination, Bargain Purchase, Gain Recognized, Amount			233
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