Derivative Financial Instruments Derivative Financial Instruments - Summary of Derivative Fair Value Balances (Details) - USD ($) $ in Millions		Sep. 30, 2017		Dec. 31, 2016
Derivative [Line Items]
Mark-to-market derivative assets		$ 699		$ 917
Derivative Asset, Noncurrent		426		492
Derivative Liability, Current		(178)		(282)
Derivative Liability, Noncurrent		(410)		(392)
Derivative, Collateral, Right to Reclaim Cash		75		29
Margin Deposit Assets		(27)		158
Derivative [Member]
Derivative [Line Items]
Mark-to-market derivative assets		694		909
Derivative Asset, Noncurrent		416		472
Total mark-to-market derivative assets		1,110		1,381
Derivative Liability, Current		(170)		(274)
Derivative Liability, Noncurrent		(409)		(388)
Total mark-to-market derivative liabilities		(579)		(662)
Total mark-to-market derivative net assets (liabilities)		531		719
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets		699		917
Derivative Asset, Noncurrent		416		476
Derivative Liability, Current		(158)		(263)
Derivative Liability, Noncurrent		(153)		(153)
Derivative Liability, Fair Value, Amount Offset Against Collateral	[1]	(638)		(627)
Exelon Generation Co L L C [Member] | Derivative [Member]
Derivative [Line Items]
Mark-to-market derivative assets		694	[2]	909	[3]
Derivative Asset, Noncurrent		416	[2]	472	[3]
Total mark-to-market derivative assets		1,110	[2]	1,381	[3]
Derivative Liability, Current		(150)	[2]	(255)	[3]
Derivative Liability, Noncurrent		(152)	[2]	(149)	[3]
Total mark-to-market derivative liabilities		(302)	[2]	(404)	[3]
Total mark-to-market derivative net assets (liabilities)		808	[2]	977	[3]
Commonwealth Edison Co [Member]
Derivative [Line Items]
Derivative Liability, Current		(20)		(19)
Derivative Liability, Noncurrent		(257)		(239)
Delmarva Power and Light Company [Member] | Derivative [Member]
Derivative [Line Items]
Mark-to-market derivative 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
Pepco Holdings LLC [Member] | Successor [Member]
Derivative [Line Items]
Mark-to-market derivative 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]
Derivative [Line Items]
Mark-to-market derivative assets	[4]	(1,969)	[5]	(2,769)	[6]
Derivative Asset, Noncurrent	[4]	(1,197)	[5]	(1,016)	[6]
Total mark-to-market derivative assets	[4]	(3,166)	[5]	(3,785)	[6]
Derivative Liability, Current	[4]	2,230	[5]	2,964	[6]
Derivative Liability, Noncurrent	[4]	1,351	[5]	1,150	[6]
Total mark-to-market derivative liabilities	[4]	3,581	[5]	4,114	[6]
Total mark-to-market derivative net assets (liabilities)	[4]	415	[5]	329	[6]
Collateral And Netting [Member] | Delmarva Power and Light Company [Member]
Derivative [Line Items]
Mark-to-market derivative assets	[4]	0		(2)
Derivative Asset, Noncurrent	[4]	0		0
Total mark-to-market derivative assets	[4]	0		(2)
Derivative Liability, Current	[4]	0		0
Derivative Liability, Noncurrent	[4]	0		0
Total mark-to-market derivative liabilities	[4]	0		0
Total mark-to-market derivative net assets (liabilities)	[4]	0		(2)
Not Designated as Hedging Instrument, Economic Hedge [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets		2,608		3,623
Derivative Asset, Noncurrent		1,583		1,467
Total mark-to-market derivative assets		4,191		5,090
Derivative Liability, Current		(2,334)		(3,165)
Derivative Liability, Noncurrent		(1,476)		(1,274)
Total mark-to-market derivative liabilities		(3,810)		(4,439)
Total mark-to-market derivative net assets (liabilities)		381		651
Not Designated as Hedging Instrument, Economic Hedge [Member] | Commonwealth Edison Co [Member]
Derivative [Line Items]
Mark-to-market derivative assets	[7]	0		0
Derivative Asset, Noncurrent	[7]	0		0
Total mark-to-market derivative assets	[7]	0		0
Derivative Liability, Current	[7]	(20)		(19)
Derivative Liability, Noncurrent	[7]	(257)		(239)
Total mark-to-market derivative liabilities	[7]	(277)		(258)
Total mark-to-market derivative net assets (liabilities)	[7]	(277)		(258)
Not Designated as Hedging Instrument, Economic Hedge [Member] | Delmarva Power and Light Company [Member]
Derivative [Line Items]
Mark-to-market derivative assets	[8]	0		2
Derivative Asset, Noncurrent	[8]	0		0
Total mark-to-market derivative assets	[8]	0		2
Derivative Liability, Current	[8]	0		0
Derivative Liability, Noncurrent	[8]	0		0
Total mark-to-market derivative liabilities	[8]	0		0
Total mark-to-market derivative net assets (liabilities)	[8]	0		2
Not Designated as Hedging Instrument [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets		55		55
Derivative Asset, Noncurrent		30		21
Total mark-to-market derivative assets		85		76
Derivative Liability, Current		(46)		(54)
Derivative Liability, Noncurrent		(27)		(25)
Total mark-to-market derivative liabilities		(73)		(79)
Total mark-to-market derivative net assets (liabilities)		12		(3)
Designated as Hedging Instrument [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Derivative Liability, Collateral, Right to Reclaim Cash, Offset		(415)		(329)
Current liabilities collateral offset		138		95
Noncurrent liabilities collateral offset		93		62
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		123		100
Margin Deposit Assets		27
Derivative Liability, Fair Value, Amount Offset Against Collateral				(158)
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		$ 61		$ 72

[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 $123 million and $61 million, respectively, and current and noncurrent liabilities are shown net of collateral of $138 million and $93 million, respectively. The total cash collateral posted, net of cash collateral received and offset against mark-to-market assets and liabilities was $415 million at September 30, 2017.

[3] Current and noncurrent assets are shown net of collateral of $100 million and $72 million, respectively, and current and noncurrent liabilities are shown net of collateral of $95 million and $62 million, respectively. The total cash collateral posted, net of cash collateral received and offset against mark-to-market assets and liabilities was $329 million at December 31, 2016.

[4] Exelon, Generation, PHI and DPL net all available amounts allowed under the derivative accounting 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), $27 million represents variation margin on the exchanges.

[6] Of the collateral posted/(received), $(158) 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.