Derivative Financial Instruments - Summary of Interest Rate and Foreign Currency Hedges (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)
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)
InterestRateAndForeignExchangeContract [Member]
Derivative [Line Items]
Mark-to-market derivatives assets		2	3
Derivative Asset, Noncurrent		12	6
Total mark-to-market derivative assets		14	9
Derivative Liability, Current		0	(2)
Derivative Liability, Noncurrent		(5)	(2)
Total mark-to-market derivative liabilities		(5)	(4)
Total mark-to-market derivative net assets (liabilities)		9	5
InterestRateAndForeignExchangeContract [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivatives assets		2	3
Derivative Asset, Noncurrent		12	3
Total mark-to-market derivative assets		14	6
Derivative Liability, Current		0	(2)
Derivative Liability, Noncurrent		(1)	(2)
Total mark-to-market derivative liabilities		(1)	(4)
Total mark-to-market derivative net assets (liabilities)		13	2
Collateral And Netting [Member] | InterestRateAndForeignExchangeContract [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivatives assets	[1]	(3)	(7)
Derivative Asset, Noncurrent	[1]	(1)	0
Total mark-to-market derivative assets	[1]	(4)	(7)
Derivative Liability, Current	[1]	3	7
Derivative Liability, Noncurrent	[1]	1	0
Total mark-to-market derivative liabilities	[1]	4	7
Total mark-to-market derivative net assets (liabilities)	[1]	0	0
Designated as Hedging Instrument [Member] | InterestRateAndForeignExchangeContract [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivatives assets		0	0
Derivative Asset, Noncurrent		12	3
Total mark-to-market derivative assets		12	3
Derivative Liability, Current		0	(2)
Derivative Liability, Noncurrent		0	0
Total mark-to-market derivative liabilities		0	(2)
Total mark-to-market derivative net assets (liabilities)		12	1
Designated as Hedging Instrument [Member] | InterestRateAndForeignExchangeContract [Member] | Corporate, Non-Segment [Member]
Derivative [Line Items]
Mark-to-market derivatives assets		0	0
Derivative Asset, Noncurrent		0	3
Total mark-to-market derivative assets		0	3
Derivative Liability, Current		0	0
Derivative Liability, Noncurrent		(4)	0
Total mark-to-market derivative liabilities		(4)	0
Total mark-to-market derivative net assets (liabilities)		(4)	3
Not Designated as Hedging Instrument, Economic Hedge [Member] | InterestRateAndForeignExchangeContract [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivatives assets		5	10
Derivative Asset, Noncurrent		1	0
Total mark-to-market derivative assets		6	10
Derivative Liability, Current		(3)	(7)
Derivative Liability, Noncurrent		(2)	(2)
Total mark-to-market derivative liabilities		(5)	(9)
Total mark-to-market derivative net assets (liabilities)		$ 1	$ 1

[1] Exelon and Generation 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 counterparty exposures subject to a master netting or similar agreement, such as accrued interest, transactions that do not qualify as derivatives, letters of credit and other forms of non-cash collateral, which are not reflected in the table above.