Derivative Financial Instruments Derivative Financial Instruments - Summary of Derivative Fair Value Balances (Details) - USD ($) $ in Millions		Mar. 31, 2018		Dec. 31, 2017
Derivative [Line Items]
Mark-to-market derivatives assets		$ 978		$ 976
Derivative Asset, Noncurrent		527		337
Derivative Liability, Current		(415)		(232)
Derivative Liability, Noncurrent		(468)		(409)
Derivative, Collateral, Right to Reclaim Cash		53		77
Margin Deposit Assets		(156)		117
Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets		976		973
Derivative Asset, Noncurrent		515		331
Total mark-to-market derivative assets		1,491		1,304
Derivative Liability, Current		(415)		(230)
Derivative Liability, Noncurrent		(463)		(407)
Total mark-to-market derivative liabilities		(878)		(637)
Total mark-to-market derivative net assets (liabilities)		613		667
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivatives assets		978		976
Derivative Asset, Noncurrent		527		334
Derivative Liability, Current		(391)		(211)
Derivative Liability, Noncurrent		(221)		(174)
Derivative Liability, Fair Value, Amount Offset Against Collateral	[1]	(1,562)		(577)
Exelon Generation Co L L C [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets		976	[2]	973	[3]
Derivative Asset, Noncurrent		515	[2]	331	[3]
Total mark-to-market derivative assets		1,491	[2]	1,304	[3]
Derivative Liability, Current		(391)	[2]	(209)	[3]
Derivative Liability, Noncurrent		(220)	[2]	(172)	[3]
Total mark-to-market derivative liabilities		(611)	[2]	(381)	[3]
Total mark-to-market derivative net assets (liabilities)		880	[2]	923	[3]
Commonwealth Edison Co [Member]
Derivative [Line Items]
Derivative Liability, Current		(24)		(21)
Derivative Liability, Noncurrent		(243)		(235)
Delmarva Power and Light Company [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		0		0
Derivative Liability, Noncurrent		0		0
Total mark-to-market derivative liabilities		0		0
Total mark-to-market derivative net assets (liabilities)		0		0
Collateral And Netting [Member] | Exelon Generation Co L L C [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	[4]	(2,533)	[5]	(2,144)	[6]
Derivative Asset, Noncurrent	[4]	(1,286)	[5]	(845)	[6]
Total mark-to-market derivative assets	[4]	(3,819)	[5]	(2,989)	[6]
Derivative Liability, Current	[4]	2,945	[5]	2,480	[6]
Derivative Liability, Noncurrent	[4]	1,558	[5]	975	[6]
Total mark-to-market derivative liabilities	[4]	4,503	[5]	3,455	[6]
Total mark-to-market derivative net assets (liabilities)	[4]	684	[5]	466	[6]
Collateral And Netting [Member] | Delmarva Power and Light Company [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	[4]	0		0
Derivative Asset, Noncurrent	[4]	0		0
Total mark-to-market derivative assets	[4]	0		0
Derivative Liability, Current	[4]	0		1
Derivative Liability, Noncurrent	[4]	0		0
Total mark-to-market derivative liabilities	[4]	0		1
Total mark-to-market derivative net assets (liabilities)	[4]	0		1
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		3,343		3,061
Derivative Asset, Noncurrent		1,758		1,164
Total mark-to-market derivative assets		5,101		4,225
Derivative Liability, Current		(3,185)		(2,646)
Derivative Liability, Noncurrent		(1,750)		(1,137)
Total mark-to-market derivative liabilities		(4,935)		(3,783)
Total mark-to-market derivative net assets (liabilities)		166		442
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]	(24)		(21)
Derivative Liability, Noncurrent	[7]	(243)		(235)
Total mark-to-market derivative liabilities	[7]	(267)		(256)
Total mark-to-market derivative net assets (liabilities)	[7]	(267)		(256)
Not Designated as Hedging Instrument, Economic Hedge [Member] | Delmarva Power and Light Company [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	[8]	0		0
Derivative Asset, Noncurrent	[8]	0		0
Total mark-to-market derivative assets	[8]	0		0
Derivative Liability, Current	[8]	0		(1)
Derivative Liability, Noncurrent	[8]	0		0
Total mark-to-market derivative liabilities	[8]	0		(1)
Total mark-to-market derivative net assets (liabilities)	[8]	0		(1)
Not Designated as Hedging Instrument [Member] | Exelon Generation Co L L C [Member] | Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets		166		56
Derivative Asset, Noncurrent		43		12
Total mark-to-market derivative assets		209		68
Derivative Liability, Current		(151)		(43)
Derivative Liability, Noncurrent		(28)		(10)
Total mark-to-market derivative liabilities		(179)		(53)
Total mark-to-market derivative net assets (liabilities)		30		15
Designated as Hedging Instrument [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Derivative Liability, Collateral, Right to Reclaim Cash, Offset		(684)		(466)
Current liabilities collateral offset		220		167
Noncurrent liabilities collateral offset		169		77
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		192		169
Margin Deposit Assets		156
Derivative Liability, Fair Value, Amount Offset Against Collateral				(117)
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		$ 103		$ 53

[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 $192 million and $103 million, respectively, and current and noncurrent liabilities are shown net of collateral of $220 million and $169 million, respectively. The total cash collateral posted, net of cash collateral received and offset against mark-to-market assets and liabilities was $684 million at March 31, 2018.

[3] Current and noncurrent assets are shown net of collateral of $169 million and $53 million, respectively, and current and noncurrent liabilities are shown net of collateral of $167 million and $77 million, respectively. The total cash collateral posted, net of cash collateral received and offset against mark-to-market assets and liabilities was $466 million at December 31, 2017.

[4] Exelon, Generation and DPL net all available amounts allowed under the derivative authoritative guidance on 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), $156 million represents variation margin on the exchanges.

[6] Of the collateral posted/(received), $(117) million represents variation margin on the exchanges.

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

[8] Represents natural gas futures purchased by DPL as part of a natural gas hedging program approved by the DPSC.