Income Taxes - Tax Effects of Temporary Differences and Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Plant basis differences	$ (12,533)	$ (12,490)
Deferred Tax Liabilities, Other Finite-Lived Assets	117	150
Derivatives and other financial instruments	89	(85)
Deferred pension and postretirement obligation	1,435	1,463
Deferred Tax Liabilities, Investments	(351)	(553)
Deferred debt refinancing costs	234	217
Deferred Tax Liabilities, Regulatory Assets and Liabilities	(749)	(688)
Tax loss carryforward	237	344
Tax credit carryforward	811	861
Investment in partnerships	(797)	(434)
Other, net	934	746
Deferred income tax liabilities (net)	(10,573)	(10,469)
Unamortized investment tax credits	(724)	(732)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(11,297)	(11,201)
Exelon Generation Co L L C [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(2,495)	(2,819)
Deferred Tax Liabilities, Other Finite-Lived Assets	(44)	(66)
Derivatives and other financial instruments	35	(66)
Deferred pension and postretirement obligation	(188)	(205)
Deferred Tax Liabilities, Investments	(351)	(553)
Deferred debt refinancing costs	23	26
Deferred Tax Liabilities, Regulatory Assets and Liabilities	0	0
Tax loss carryforward	78	76
Tax credit carryforward	816	868
Investment in partnerships	(775)	(416)
Other, net	239	78
Deferred income tax liabilities (net)	(2,662)	(3,077)
Unamortized investment tax credits	(700)	(705)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(3,362)	(3,782)
Commonwealth Edison Co [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(4,059)	(3,825)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	69	(2)
Deferred pension and postretirement obligation	(255)	(285)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(7)	(8)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	300	489
Tax loss carryforward	0	33
Tax credit carryforward	0	1
Investment in partnerships	0	0
Other, net	151	141
Deferred income tax liabilities (net)	(3,801)	(3,456)
Unamortized investment tax credits	(12)	(13)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(3,813)	(3,469)
PECO Energy Co [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(1,862)	(1,762)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(26)	(15)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	0	(1)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	(129)	(90)
Tax loss carryforward	18	9
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	67	71
Deferred income tax liabilities (net)	(1,932)	(1,788)
Unamortized investment tax credits	(1)	(1)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(1,933)	(1,789)
Baltimore Gas and Electric Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(1,399)	(1,368)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(26)	(29)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(3)	(3)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	172	136
Tax loss carryforward	25	11
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	12	13
Deferred income tax liabilities (net)	(1,219)	(1,240)
Unamortized investment tax credits	(3)	(4)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(1,222)	(1,244)
Pepco Holdings LLC [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(2,577)	(2,521)
Deferred Tax Liabilities, Other Finite-Lived Assets	161	216
Derivatives and other financial instruments	3	3
Deferred pension and postretirement obligation	(102)	(130)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	187	203
Deferred Tax Liabilities, Regulatory Assets and Liabilities	(90)	(184)
Tax loss carryforward	96	156
Tax credit carryforward	0	6
Investment in partnerships	0	0
Other, net	196	193
Deferred income tax liabilities (net)	(2,126)	(2,058)
Unamortized investment tax credits	(8)	(8)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(2,134)	(2,066)
Atlantic City Electric Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(645)	(607)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(14)	(18)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(1)	(1)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	83	86
Tax loss carryforward	26	35
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	19	16
Deferred income tax liabilities (net)	(532)	(489)
Unamortized investment tax credits	(3)	(4)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(535)	(493)
Delmarva Power & Light Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(743)	(717)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(46)	(51)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(2)	(2)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	96	88
Tax loss carryforward	52	68
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	17	14
Deferred income tax liabilities (net)	(626)	(600)
Unamortized investment tax credits	(2)	(3)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(628)	(603)
Potomac Electric Power Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(1,148)	(1,152)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(78)	(78)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(4)	(4)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	58	39
Tax loss carryforward	12	40
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	98	94
Deferred income tax liabilities (net)	(1,062)	(1,061)
Unamortized investment tax credits	(2)	(2)
Total deferred income tax liabilities (net) and unamortized investment tax credits	$ (1,064)	$ (1,063)