Derivative Financial Instruments - Summary of Derivative Fair Value Balances (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Mark-to-market derivatives assets	$ 601	$ 804
Derivative Asset, Noncurrent	487	452
Derivative Liability, Current	(239)	(475)
Derivative Liability, Noncurrent	(416)	(479)
Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	602	801
Derivative Asset, Noncurrent	483	445
Total mark-to-market derivative assets	1,085	1,246
Derivative Liability, Current	(224)	(473)
Derivative Liability, Noncurrent	(394)	(474)
Total mark-to-market derivative liabilities	(618)	(947)
Total mark-to-market derivative net assets (liabilities)	467	299
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	602	804
Derivative Asset, Noncurrent	483	452
Derivative Liability, Current	(212)	(449)
Derivative Liability, Noncurrent	(163)	(252)
Exelon Generation Co L L C [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	602	801
Derivative Asset, Noncurrent	483	445
Total mark-to-market derivative assets	1,085	1,246
Derivative Liability, Current	(197)	(447)
Derivative Liability, Noncurrent	(141)	(251)
Total mark-to-market derivative liabilities	(338)	(698)
Total mark-to-market derivative net assets (liabilities)	747	548
Commonwealth Edison Co [Member]
Derivative [Line Items]
Derivative Liability, Current	(27)	(26)
Derivative Liability, Noncurrent	(253)	(223)
Collateral [Member] | Exelon Generation Co L L C [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	212	121
Derivative Asset, Noncurrent	104	51
Total mark-to-market derivative assets	316	172
Derivative Liability, Current	249	125
Derivative Liability, Noncurrent	156	60
Total mark-to-market derivative liabilities	405	185
Total mark-to-market derivative net assets (liabilities)	721	357
Netting [Member] | Exelon Generation Co L L C [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	(2,205)	(2,930)
Derivative Asset, Noncurrent	(1,074)	(913)
Total mark-to-market derivative assets	(3,279)	(3,843)
Derivative Liability, Current	2,205	2,931
Derivative Liability, Noncurrent	1,074	912
Total mark-to-market derivative liabilities	3,279	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	2,452	3,505
Derivative Asset, Noncurrent	1,386	1,266
Total mark-to-market derivative assets	3,838	4,771
Derivative Liability, Current	(2,550)	(3,429)
Derivative Liability, Noncurrent	(1,324)	(1,203)
Total mark-to-market derivative liabilities	(3,874)	(4,632)
Total mark-to-market derivative net assets (liabilities)	(36)	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	(27)	(26)
Derivative Liability, Noncurrent	(253)	(223)
Total mark-to-market derivative liabilities	(280)	(249)
Total mark-to-market derivative net assets (liabilities)	(280)	(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	143	105
Derivative Asset, Noncurrent	67	41
Total mark-to-market derivative assets	210	146
Derivative Liability, Current	(101)	(74)
Derivative Liability, Noncurrent	(47)	(20)
Total mark-to-market derivative liabilities	(148)	(94)
Total mark-to-market derivative net assets (liabilities)	62	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	$ 306	$ (94)