Derivative Financial Instruments - Summary of Interest Rate and Foreign Currency Hedges (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Mark-to-market derivative assets (current assets)		$ 804	$ 976
Mark-to-market derivative assets (noncurrent assets)		452	337
Derivative Liability, Current		(475)	(232)
Derivative Liability, Noncurrent		(479)	(409)
Total mark-to-market derivative net assets (liabilities)		1	6
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)		804	976
Mark-to-market derivative assets (noncurrent assets)		452	334
Derivative Liability, Current		(449)	(211)
Derivative Liability, Noncurrent		(252)	(174)
InterestRateAndForeignExchangeContract [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)		3	3
Mark-to-market derivative assets (noncurrent assets)		7	6
Total mark-to-market derivative assets		10	9
Derivative Liability, Current		(2)	(2)
Derivative Liability, Noncurrent		(5)	(2)
Total mark-to-market derivative liabilities		7	4
Total mark-to-market derivative net assets (liabilities)		3	5
InterestRateAndForeignExchangeContract [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)		3	3
Mark-to-market derivative assets (noncurrent assets)		7	3
Total mark-to-market derivative assets		10	6
Derivative Liability, Current		(2)	(2)
Derivative Liability, Noncurrent		(1)	(2)
Total mark-to-market derivative liabilities		3	4
Total mark-to-market derivative net assets (liabilities)		7	2
Designated as Hedging Instrument [Member] | InterestRateAndForeignExchangeContract [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)			0
Mark-to-market derivative assets (noncurrent assets)			3
Total mark-to-market derivative assets			3
Derivative Liability, Current			(2)
Derivative Liability, Noncurrent			0
Total mark-to-market derivative liabilities			2
Total mark-to-market derivative net assets (liabilities)			1
Not Designated as Hedging Instrument, Economic Hedge [Member] | InterestRateAndForeignExchangeContract [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)		5	10
Mark-to-market derivative assets (noncurrent assets)		8	0
Total mark-to-market derivative assets		13	10
Derivative Liability, Current		(4)	(7)
Derivative Liability, Noncurrent		(2)	(2)
Total mark-to-market derivative liabilities		6	9
Total mark-to-market derivative net assets (liabilities)		7	1
Corporate, Non-Segment [Member] | Designated as Hedging Instrument [Member] | InterestRateAndForeignExchangeContract [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)			0
Mark-to-market derivative assets (noncurrent assets)			3
Total mark-to-market derivative assets			3
Derivative Liability, Current			0
Derivative Liability, Noncurrent			0
Total mark-to-market derivative liabilities			0
Total mark-to-market derivative net assets (liabilities)			3
Corporate, Non-Segment [Member] | Not Designated as Hedging Instrument, Economic Hedge [Member] | InterestRateAndForeignExchangeContract [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)		0
Mark-to-market derivative assets (noncurrent assets)		0
Total mark-to-market derivative assets		0
Derivative Liability, Current		0
Derivative Liability, Noncurrent		(4)
Total mark-to-market derivative liabilities		4
Total mark-to-market derivative net assets (liabilities)		(4)
Collateral and Netting | InterestRateAndForeignExchangeContract [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	[1]	(2)	(7)
Mark-to-market derivative assets (noncurrent assets)	[1]	(1)	0
Total mark-to-market derivative assets	[1]	(3)	(7)
Derivative Liability, Current	[1]	2	7
Derivative Liability, Noncurrent	[1]	1	0
Total mark-to-market derivative liabilities	[1]	(3)	(7)
Total mark-to-market derivative net assets (liabilities)	[1]	$ 0	$ 0

[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.