Income Taxes - Tax Effects of Temporary Differences and Carryforwards (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Loss Carryforwards [Line Items]
Plant basis differences	$ (13,868)	$ (13,413)
Deferred Tax Liabilities, Other Finite-Lived Assets	40	61
Derivatives and other financial instruments	41	165
Deferred pension and postretirement obligation	1,559	1,504
Deferred Tax Liabilities, Investments	(742)	(503)
Deferred debt refinancing costs	169	183
Deferred Tax Liabilities, Regulatory Assets and Liabilities	(1,107)	(884)
Tax loss carryforward	286	240
Tax credit carryforward	841	892
Investment in partnerships	(835)	(830)
Other, net	1,070	926
Deferred income tax liabilities (net)	(12,546)	(11,659)
Unamortized investment tax credits(a)	(464)	(668)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(13,010)	(12,327)
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Increase (Decrease) in Deferred Income Taxes	66	23	$ (50)
Exelon Generation Co L L C [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(2,592)	(2,814)
Deferred Tax Liabilities, Other Finite-Lived Assets	(37)	(43)
Derivatives and other financial instruments	(41)	88
Deferred pension and postretirement obligation	(236)	(220)
Deferred Tax Liabilities, Investments	(742)	(503)
Deferred debt refinancing costs	16	20
Deferred Tax Liabilities, Regulatory Assets and Liabilities	0	0
Tax loss carryforward	55	55
Tax credit carryforward	838	897
Investment in partnerships	(813)	(808)
Other, net	347	236
Deferred income tax liabilities (net)	(3,205)	(3,092)
Unamortized investment tax credits(a)	(445)	(648)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(3,650)	(3,740)
Exelon Generation Co L L C [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Increase (Decrease) in Deferred Income Taxes	(26)	9	$ (53)
Commonwealth Edison Co [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(4,432)	(4,197)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	84	84
Deferred pension and postretirement obligation	(288)	(270)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(6)	(7)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	87	183
Tax loss carryforward	0	0
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	223	196
Deferred income tax liabilities (net)	(4,332)	(4,011)
Unamortized investment tax credits(a)	(9)	(10)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(4,341)	(4,021)
PECO Energy Co [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(2,131)	(1,978)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(30)	(28)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	0	0
Deferred Tax Liabilities, Regulatory Assets and Liabilities	(231)	(169)
Tax loss carryforward	47	25
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	104	70
Deferred income tax liabilities (net)	(2,241)	(2,080)
Unamortized investment tax credits(a)	(1)	(1)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(2,242)	(2,081)
Baltimore Gas and Electric Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(1,711)	(1,578)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(33)	(28)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(2)	(3)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	142	157
Tax loss carryforward	57	49
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	29	10
Deferred income tax liabilities (net)	(1,518)	(1,393)
Unamortized investment tax credits(a)	(3)	(3)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(1,521)	(1,396)
Pepco Holdings LLC [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(2,822)	(2,681)
Deferred Tax Liabilities, Other Finite-Lived Assets	77	104
Derivatives and other financial instruments	2	2
Deferred pension and postretirement obligation	(80)	(89)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	131	142
Deferred Tax Liabilities, Regulatory Assets and Liabilities	(41)	(10)
Tax loss carryforward	90	93
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	220	181
Deferred income tax liabilities (net)	(2,423)	(2,258)
Unamortized investment tax credits(a)	(6)	(7)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(2,429)	(2,265)
Potomac Electric Power Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(1,259)	(1,204)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(74)	(75)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(3)	(3)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	38	55
Tax loss carryforward	4	13
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	107	85
Deferred income tax liabilities (net)	(1,187)	(1,129)
Unamortized investment tax credits(a)	(2)	(2)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(1,189)	(1,131)
Delmarva Power & Light Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(806)	(753)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(40)	(42)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(1)	(2)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	67	88
Tax loss carryforward	49	44
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	18	12
Deferred income tax liabilities (net)	(713)	(653)
Unamortized investment tax credits(a)	(2)	(2)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(715)	(655)
Atlantic City Electric Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(725)	(687)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(7)	(10)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(1)	(1)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	46	77
Tax loss carryforward	38	31
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	27	16
Deferred income tax liabilities (net)	(622)	(574)
Unamortized investment tax credits(a)	(3)	(3)
Total deferred income tax liabilities (net) and unamortized investment tax credits	$ (625)	$ (577)