Derivative Financial Instruments - Summary of Derivative Fair Value Balances (Details) - USD ($) $ in Millions		Mar. 31, 2019		Dec. 31, 2018
Derivative [Line Items]
Mark-to-market derivatives assets		$ 652		$ 804
Derivative Asset, Noncurrent		454		452
Derivative Liability, Current		(345)		(475)
Derivative Liability, Noncurrent		(434)		(479)
Derivative, Collateral, Right to Reclaim Cash		43		50
Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets		653		801
Derivative Asset, Noncurrent		454		445
Total mark-to-market derivative assets		1,107		1,246
Derivative Liability, Current		(342)		(473)
Derivative Liability, Noncurrent		(428)		(474)
Total mark-to-market derivative liabilities		(770)		(947)
Total mark-to-market derivative net assets (liabilities)		337		299
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivatives assets		652		804
Derivative Asset, Noncurrent		454		452
Derivative Liability, Current		(318)		(449)
Derivative Liability, Noncurrent		(219)		(252)
Derivative, Collateral, Right to Reclaim Cash		43		50
Derivative Liability, Fair Value, Amount Offset Against Collateral		(1,177)	[1]	1,105
Exelon Generation Co L L C [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets		653	[2]	801	[3]
Derivative Asset, Noncurrent		454	[2]	445	[3]
Total mark-to-market derivative assets		1,107	[2]	1,246	[3]
Derivative Liability, Current		(315)	[2]	(447)	[3]
Derivative Liability, Noncurrent		(215)	[2]	(251)	[3]
Total mark-to-market derivative liabilities		(530)	[2]	(698)	[3]
Total mark-to-market derivative net assets (liabilities)		577	[2]	548	[3]
Commonwealth Edison Co [Member]
Derivative [Line Items]
Derivative Liability, Current		(27)		(26)
Derivative Liability, Noncurrent		(213)		(223)
Collateral And Netting [Member] | Exelon Generation Co L L C [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	[4]	(2,156)	[5]	(2,809)	[6]
Derivative Asset, Noncurrent	[4]	(794)	[5]	(862)	[6]
Total mark-to-market derivative assets	[4]	(2,950)	[5]	(3,671)	[6]
Derivative Liability, Current	[4]	2,485	[5]	3,056	[6]
Derivative Liability, Noncurrent	[4]	957	[5]	972	[6]
Total mark-to-market derivative liabilities	[4]	3,442	[5]	4,028	[6]
Total mark-to-market derivative net assets (liabilities)	[4]	492	[5]	357	[6]
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		2,691		3,505
Derivative Asset, Noncurrent		1,188		1,266
Total mark-to-market derivative assets		3,879		4,771
Derivative Liability, Current		(2,711)		(3,429)
Derivative Liability, Noncurrent		(1,142)		(1,203)
Total mark-to-market derivative liabilities		(3,853)		(4,632)
Total mark-to-market derivative net assets (liabilities)		26		139
Not Designated as Hedging Instrument, Economic Hedge [Member] | Commonwealth Edison Co [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	[7]	0		0
Derivative Asset, Noncurrent	[7]	0		0
Total mark-to-market derivative assets	[7]	0		0
Derivative Liability, Current	[7]	(27)		(26)
Derivative Liability, Noncurrent	[7]	(213)		(223)
Total mark-to-market derivative liabilities	[7]	(240)		(249)
Total mark-to-market derivative net assets (liabilities)	[7]	(240)		(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		118		105
Derivative Asset, Noncurrent		60		41
Total mark-to-market derivative assets		178		146
Derivative Liability, Current		(89)		(74)
Derivative Liability, Noncurrent		(30)		(20)
Total mark-to-market derivative liabilities		(119)		(94)
Total mark-to-market derivative net assets (liabilities)		59		52
Designated as Hedging Instrument [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Derivative Liability, Collateral, Right to Reclaim Cash, Offset		(492)		(357)
Current liabilities collateral offset		177		125
Noncurrent liabilities collateral offset		100		60
Designated as Hedging Instrument [Member] | Exelon Generation Co L L C [Member] | Other Current Assets [Member]
Derivative [Line Items]
Derivative, Collateral, Right to Reclaim Cash		152		121
Derivative Liability, Fair Value, Amount Offset Against Collateral		(33)		(94)
Designated as Hedging Instrument [Member] | Exelon Generation Co L L C [Member] | Other Noncurrent Assets [Member]
Derivative [Line Items]
Derivative, Collateral, Right to Reclaim Cash		$ 63		$ 51

[1] Amount represents the offsetting fair value of in-the-money derivative contracts under legally enforceable master netting agreements with the same counterparty, which reduces the amount of any liability for which a Registrant could potentially be required to post collateral.

[2] Current and noncurrent assets are shown net of collateral of $152 million and $63 million, respectively, and current and noncurrent liabilities are shown net of collateral of $177 million and $100 million, respectively. The total cash collateral posted, net of cash collateral received and offset against mark-to-market assets and liabilities was $492 million at March 31, 2019.

[3] Current and noncurrent assets are shown net of collateral of $121 million and $51 million, respectively, and current and noncurrent liabilities are shown net of collateral of $125 million and $60 million, respectively. The total cash collateral posted, net of cash collateral received and offset against mark-to-market assets and liabilities was $357 million at December 31, 2018.

[4] Exelon and Generation net all available amounts allowed under the derivative authoritative guidance in the balance sheet. These amounts include unrealized derivative transactions with the same counterparty under legally enforceable master netting agreements and cash collateral. In some cases Exelon and Generation may have other offsetting exposures, subject to a master netting or similar agreement, such as trade receivables and payables, transactions that do not qualify as derivatives, letters of credit and other forms of non-cash collateral. These are not reflected in the table above.

[5] Of the collateral posted/(received), $(33) million represents variation margin on the exchanges.

[6] Of the collateral posted/(received), $(94) million represents variation margin on the exchanges.

[7] Includes current and noncurrent liabilities relating to floating-to-fixed energy swap contracts with unaffiliated suppliers.