Derivative Financial Instruments - Summary of Derivative Fair Value Balances (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Mark-to-market derivatives assets	$ 526	$ 804
Derivative Asset, Noncurrent	537	452
Derivative Liability, Current	(163)	(475)
Derivative Liability, Noncurrent	(450)	(479)
Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	526	801
Derivative Asset, Noncurrent	532	445
Total mark-to-market derivative assets	1,058	1,246
Derivative Liability, Current	(157)	(473)
Derivative Liability, Noncurrent	(437)	(474)
Total mark-to-market derivative liabilities	(594)	(947)
Total mark-to-market derivative net assets (liabilities)	464	299
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	526	804
Derivative Asset, Noncurrent	533	452
Derivative Liability, Current	(134)	(449)
Derivative Liability, Noncurrent	(206)	(252)
Exelon Generation Co L L C [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	526	801
Derivative Asset, Noncurrent	532	445
Total mark-to-market derivative assets	1,058	1,246
Derivative Liability, Current	(128)	(447)
Derivative Liability, Noncurrent	(193)	(251)
Total mark-to-market derivative liabilities	(321)	(698)
Total mark-to-market derivative net assets (liabilities)	737	548
Commonwealth Edison Co [Member]
Derivative [Line Items]
Derivative Liability, Current	(29)	(26)
Derivative Liability, Noncurrent	(244)	(223)
Collateral [Member] | Exelon Generation Co L L C [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	218	121
Derivative Asset, Noncurrent	122	51
Total mark-to-market derivative assets	340	172
Derivative Liability, Current	265	125
Derivative Liability, Noncurrent	151	60
Total mark-to-market derivative liabilities	416	185
Total mark-to-market derivative net assets (liabilities)	756	357
Netting [Member] | Exelon Generation Co L L C [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	(2,724)	(2,930)
Derivative Asset, Noncurrent	(1,269)	(913)
Total mark-to-market derivative assets	(3,993)	(3,843)
Derivative Liability, Current	2,724	2,931
Derivative Liability, Noncurrent	1,269	912
Total mark-to-market derivative liabilities	3,993	3,843
Total mark-to-market derivative net assets (liabilities)	0	0
Not Designated as Hedging Instrument, Economic Hedge [Member] | Exelon Generation Co L L C [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	2,910	3,505
Derivative Asset, Noncurrent	1,618	1,266
Total mark-to-market derivative assets	4,528	4,771
Derivative Liability, Current	(3,039)	(3,429)
Derivative Liability, Noncurrent	(1,576)	(1,203)
Total mark-to-market derivative liabilities	(4,615)	(4,632)
Total mark-to-market derivative net assets (liabilities)	(87)	139
Not Designated as Hedging Instrument, Economic Hedge [Member] | Commonwealth Edison Co [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	0	0
Derivative Asset, Noncurrent	0	0
Total mark-to-market derivative assets	0	0
Derivative Liability, Current	(29)	(26)
Derivative Liability, Noncurrent	(244)	(223)
Total mark-to-market derivative liabilities	(273)	(249)
Total mark-to-market derivative net assets (liabilities)	(273)	(249)
Not Designated as Hedging Instrument [Member] | Exelon Generation Co L L C [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	122	105
Derivative Asset, Noncurrent	61	41
Total mark-to-market derivative assets	183	146
Derivative Liability, Current	(78)	(74)
Derivative Liability, Noncurrent	(37)	(20)
Total mark-to-market derivative liabilities	(115)	(94)
Total mark-to-market derivative net assets (liabilities)	68	52
Designated as Hedging Instrument [Member] | Exelon Generation Co L L C [Member] | Other Current Assets [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Offset Against Collateral	$ 358	$ (94)