Income Taxes - Tax Effects of Temporary Differences and Carryforwards (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Plant basis differences	$ (13,413)	$ (12,533)
Deferred Tax Liabilities, Other Finite-Lived Assets	61	117
Derivatives and other financial instruments	165	89
Deferred pension and postretirement obligation	1,504	1,435
Deferred Tax Liabilities, Investments	(503)	(351)
Deferred debt refinancing costs	183	234
Deferred Tax Liabilities, Regulatory Assets and Liabilities	(884)	(740)
Tax loss carryforward	240	237
Tax credit carryforward	892	811
Investment in partnerships	(830)	(797)
Other, net	926	934
Deferred income tax liabilities (net)	(11,659)	(10,564)
Unamortized investment tax credits	(668)	(724)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(12,327)	(11,288)
Income Tax Expense (Benefit)	774	118	$ (126)
Exelon Generation Co L L C [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(2,814)	(2,495)
Deferred Tax Liabilities, Other Finite-Lived Assets	(43)	(44)
Derivatives and other financial instruments	88	35
Deferred pension and postretirement obligation	(220)	(188)
Deferred Tax Liabilities, Investments	(503)	(351)
Deferred debt refinancing costs	20	23
Deferred Tax Liabilities, Regulatory Assets and Liabilities	0	0
Tax loss carryforward	55	78
Tax credit carryforward	897	816
Investment in partnerships	(808)	(775)
Other, net	236	239
Deferred income tax liabilities (net)	(3,092)	(2,662)
Unamortized investment tax credits	(648)	(700)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(3,740)	(3,362)
Income Tax Expense (Benefit)	516	(108)	(1,376)
Commonwealth Edison Co [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(4,197)	(4,059)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	84	69
Deferred pension and postretirement obligation	(270)	(255)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(7)	(7)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	183	300
Tax loss carryforward	0	0
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	196	151
Deferred income tax liabilities (net)	(4,011)	(3,801)
Unamortized investment tax credits	(10)	(12)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(4,021)	(3,813)
Income Tax Expense (Benefit)	163	168	417
PECO Energy Co [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(1,978)	(1,862)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(28)	(26)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	0	0
Deferred Tax Liabilities, Regulatory Assets and Liabilities	(169)	(129)
Tax loss carryforward	25	18
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	70	67
Deferred income tax liabilities (net)	(2,080)	(1,932)
Unamortized investment tax credits	(1)	(1)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(2,081)	(1,933)
Income Tax Expense (Benefit)	65	6	104
Baltimore Gas and Electric Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(1,578)	(1,399)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(28)	(26)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(3)	(3)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	157	172
Tax loss carryforward	49	25
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	10	12
Deferred income tax liabilities (net)	(1,393)	(1,219)
Unamortized investment tax credits	(3)	(3)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(1,396)	(1,222)
Income Tax Expense (Benefit)	79	74	218
Pepco Holdings LLC [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(2,681)	(2,577)
Deferred Tax Liabilities, Other Finite-Lived Assets	104	161
Derivatives and other financial instruments	2	3
Deferred pension and postretirement obligation	(89)	(102)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	142	187
Deferred Tax Liabilities, Regulatory Assets and Liabilities	(10)	(81)
Tax loss carryforward	93	96
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	181	196
Deferred income tax liabilities (net)	(2,258)	(2,117)
Unamortized investment tax credits	(7)	(8)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(2,265)	(2,125)
Income Tax Expense (Benefit)	38	33	217
Potomac Electric Power Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(1,204)	(1,148)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(75)	(78)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(3)	(4)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	55	67
Tax loss carryforward	13	12
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	85	98
Deferred income tax liabilities (net)	(1,129)	(1,053)
Unamortized investment tax credits	(2)	(2)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(1,131)	(1,055)
Income Tax Expense (Benefit)	16	11	105
Delmarva Power & Light Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(753)	(743)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(42)	(46)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(2)	(2)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	88	96
Tax loss carryforward	44	52
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	12	17
Deferred income tax liabilities (net)	(653)	(626)
Unamortized investment tax credits	(2)	(2)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(655)	(628)
Income Tax Expense (Benefit)	22	22	71
Atlantic City Electric Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(687)	(645)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(10)	(14)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(1)	(1)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	77	83
Tax loss carryforward	31	26
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	16	19
Deferred income tax liabilities (net)	(574)	(532)
Unamortized investment tax credits	(3)	(3)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(577)	(535)
Income Tax Expense (Benefit)	$ 0	12	$ 26
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member]
Operating Loss Carryforwards [Line Items]
Increase (Decrease) in Deferred Income Taxes		8,624
Increase (Decrease) in Regulatory Liabilities		7,315
Income Tax Expense (Benefit)		1,309
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member] | Exelon Generation Co L L C [Member]
Operating Loss Carryforwards [Line Items]
Increase (Decrease) in Deferred Income Taxes		1,895
Income Tax Expense (Benefit)		1,895
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member] | Commonwealth Edison Co [Member]
Operating Loss Carryforwards [Line Items]
Increase (Decrease) in Deferred Income Taxes		2,819
Increase (Decrease) in Regulatory Liabilities		2,818
Income Tax Expense (Benefit)		1
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member] | PECO Energy Co [Member]
Operating Loss Carryforwards [Line Items]
Increase (Decrease) in Deferred Income Taxes		1,407
Increase (Decrease) in Regulatory Liabilities		1,394
Income Tax Expense (Benefit)		13
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member] | Baltimore Gas and Electric Company [Member]
Operating Loss Carryforwards [Line Items]
Increase (Decrease) in Deferred Income Taxes		1,120
Increase (Decrease) in Regulatory Liabilities		1,124
Income Tax Expense (Benefit)		(4)
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member] | Pepco Holdings LLC [Member]
Operating Loss Carryforwards [Line Items]
Increase (Decrease) in Deferred Income Taxes		1,944
Increase (Decrease) in Regulatory Liabilities		1,979
Income Tax Expense (Benefit)		(35)
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member] | Potomac Electric Power Company [Member]
Operating Loss Carryforwards [Line Items]
Increase (Decrease) in Deferred Income Taxes		968
Increase (Decrease) in Regulatory Liabilities		976
Income Tax Expense (Benefit)		(8)
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member] | Delmarva Power & Light Company [Member]
Operating Loss Carryforwards [Line Items]
Increase (Decrease) in Deferred Income Taxes		540
Increase (Decrease) in Regulatory Liabilities		545
Income Tax Expense (Benefit)		(5)
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member] | Atlantic City Electric Company [Member]
Operating Loss Carryforwards [Line Items]
Increase (Decrease) in Deferred Income Taxes		456
Increase (Decrease) in Regulatory Liabilities		458
Income Tax Expense (Benefit)		$ (2)