Income Taxes - Tax Effects of Temporary Differences and Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Plant basis differences	$ (12,490)	$ (17,966)
Deferred Tax Liabilities, Other Finite-Lived Assets	150	434
Derivatives and other financial instruments	(85)	(179)
Deferred pension and postretirement obligation	1,463	2,287
Deferred Tax Liabilities, Investments	(553)	(509)
Deferred debt refinancing costs	217	325
Deferred Tax Liabilities, Regulatory Assets and Liabilities	(688)	(3,319)
Tax loss carryforward	344	189
Tax credit carryforward	861	446
Investment in partnerships	(434)	(650)
Other, net	746	1,485
Deferred income tax liabilities (net)	(10,469)	(17,457)
Unamortized investment tax credits	(732)	(658)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(11,201)	(18,115)
Exelon Generation Co L L C [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(2,819)	(4,192)
Deferred Tax Liabilities, Other Finite-Lived Assets	(66)	(115)
Derivatives and other financial instruments	(66)	(162)
Deferred pension and postretirement obligation	(205)	(316)
Deferred Tax Liabilities, Investments	(553)	(509)
Deferred debt refinancing costs	26	44
Deferred Tax Liabilities, Regulatory Assets and Liabilities	0	0
Tax loss carryforward	76	61
Tax credit carryforward	868	493
Investment in partnerships	(416)	(650)
Other, net	78	403
Deferred income tax liabilities (net)	(3,077)	(4,943)
Unamortized investment tax credits	(705)	(626)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(3,782)	(5,569)
Commonwealth Edison Co [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(3,825)	(5,034)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	(2)	(3)
Deferred pension and postretirement obligation	(285)	(453)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(8)	(13)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	489	(226)
Tax loss carryforward	33	29
Tax credit carryforward	1	0
Investment in partnerships	0	0
Other, net	141	351
Deferred income tax liabilities (net)	(3,456)	(5,349)
Unamortized investment tax credits	(13)	(15)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(3,469)	(5,364)
PECO Energy Co [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(1,762)	(3,095)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(15)	(18)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(1)	(1)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	(90)	10
Tax loss carryforward	9	0
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	71	99
Deferred income tax liabilities (net)	(1,788)	(3,005)
Unamortized investment tax credits	(1)	(1)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(1,789)	(3,006)
Baltimore Gas and Electric Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(1,368)	(1,977)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(29)	(43)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(3)	(3)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	136	(240)
Tax loss carryforward	11	22
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	13	27
Deferred income tax liabilities (net)	(1,240)	(2,214)
Unamortized investment tax credits	(4)	(5)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(1,244)	(2,219)
Pepco Holdings LLC [Member] | Successor [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(2,521)	(3,586)
Deferred Tax Liabilities, Other Finite-Lived Assets	216	548
Derivatives and other financial instruments	3	(1)
Deferred pension and postretirement obligation	(130)	(111)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	203	293
Deferred Tax Liabilities, Regulatory Assets and Liabilities	(184)	(1,205)
Tax loss carryforward	156	77
Tax credit carryforward	6	0
Investment in partnerships	0	0
Other, net	193	225
Deferred income tax liabilities (net)	(2,058)	(3,760)
Unamortized investment tax credits	(8)	(9)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(2,066)	(3,769)
Atlantic City Electric Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(607)	(869)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(18)	(21)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(1)	(2)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	86	(69)
Tax loss carryforward	35	14
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	16	34
Deferred income tax liabilities (net)	(489)	(913)
Unamortized investment tax credits	(4)	(4)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(493)	(917)
Delmarva Power and Light Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(717)	(973)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(51)	(74)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(2)	(4)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	88	(75)
Tax loss carryforward	68	39
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	14	34
Deferred income tax liabilities (net)	(600)	(1,053)
Unamortized investment tax credits	(3)	(3)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(603)	(1,056)
Potomac Electric Power Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(1,152)	(1,678)
Deferred Tax Liabilities, Other Finite-Lived Assets	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(78)	(122)
Deferred Tax Liabilities, Investments	0	0
Deferred debt refinancing costs	(4)	(7)
Deferred Tax Liabilities, Regulatory Assets and Liabilities	39	(194)
Tax loss carryforward	40	27
Tax credit carryforward	0	0
Investment in partnerships	0	0
Other, net	94	66
Deferred income tax liabilities (net)	(1,061)	(1,908)
Unamortized investment tax credits	(2)	(2)
Total deferred income tax liabilities (net) and unamortized investment tax credits	$ (1,063)	$ (1,910)