Derivative Financial Instruments - Summary of Derivative Fair Value Balances (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Mark-to-market derivative assets, current	$ 679	$ 804
Mark-to-market derivative assets, noncurrent	508	452
Mark-to-market derivative liabilities, current	(247)	(475)
Mark-to-market derivative liabilities, noncurrent	(393)	(479)
Total mark-to-market derivative net assets (liabilities)	2	1
Margin Deposit Assets	511	(94)
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	675	804
Mark-to-market derivative assets, noncurrent	508	452
Mark-to-market derivative liabilities, current	(215)	(449)
Mark-to-market derivative liabilities, noncurrent	(123)	(252)
Margin Deposit Assets	511	94
Commonwealth Edison Co [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities, current	(32)	(26)
Mark-to-market derivative liabilities, noncurrent	(269)	(223)
Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	675	801
Mark-to-market derivative assets, noncurrent	508	445
Total mark-to-market derivative assets	1,183	1,246
Mark-to-market derivative liabilities, current	(236)	(473)
Mark-to-market derivative liabilities, noncurrent	(380)	(474)
Total mark-to-market derivative liabilities	(616)	(947)
Total mark-to-market derivative net assets (liabilities)	567	299
Commodity Contract [Member] \| Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	675	801
Mark-to-market derivative assets, noncurrent	508	445
Total mark-to-market derivative assets	1,183	1,246
Mark-to-market derivative liabilities, current	(204)	(447)
Mark-to-market derivative liabilities, noncurrent	(111)	(251)
Total mark-to-market derivative liabilities	(315)	(698)
Total mark-to-market derivative net assets (liabilities)	868	548
Commodity Contract [Member] \| Collateral [Member] \| Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	287	121
Mark-to-market derivative assets, noncurrent	122	51
Total mark-to-market derivative assets	409	172
Mark-to-market derivative liabilities, current	357	125
Mark-to-market derivative liabilities, noncurrent	163	60
Total mark-to-market derivative liabilities	520	185
Total mark-to-market derivative net assets (liabilities)	929	357
Commodity Contract [Member] \| Netting [Member] \| Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	(3,190)	(2,930)
Mark-to-market derivative assets, noncurrent	(877)	(913)
Total mark-to-market derivative assets	(4,067)	(3,843)
Mark-to-market derivative liabilities, current	3,190	2,931
Mark-to-market derivative liabilities, noncurrent	877	912
Total mark-to-market derivative liabilities	4,067	3,843
Total mark-to-market derivative net assets (liabilities)	0	0
Commodity Contract [Member] \| Not Designated as Hedging Instrument, Economic Hedge [Member] \| Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	3,506	3,505
Mark-to-market derivative assets, noncurrent	1,238	1,266
Total mark-to-market derivative assets	4,744	4,771
Mark-to-market derivative liabilities, current	(3,713)	(3,429)
Mark-to-market derivative liabilities, noncurrent	(1,140)	(1,203)
Total mark-to-market derivative liabilities	(4,853)	(4,632)
Total mark-to-market derivative net assets (liabilities)	(109)	139
Commodity Contract [Member] \| Not Designated as Hedging Instrument, Economic Hedge [Member] \| Commonwealth Edison Co [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	0	0
Mark-to-market derivative assets, noncurrent	0	0
Total mark-to-market derivative assets	0	0
Mark-to-market derivative liabilities, current	(32)	(26)
Mark-to-market derivative liabilities, noncurrent	(269)	(223)
Total mark-to-market derivative liabilities	(301)	(249)
Total mark-to-market derivative net assets (liabilities)	(301)	(249)
Commodity Contract [Member] \| Not Designated as Hedging Instrument, Trading [Member] \| Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	72	105
Mark-to-market derivative assets, noncurrent	25	41
Total mark-to-market derivative assets	97	146
Mark-to-market derivative liabilities, current	(38)	(74)
Mark-to-market derivative liabilities, noncurrent	(11)	(20)
Total mark-to-market derivative liabilities	(49)	(94)
Total mark-to-market derivative net assets (liabilities)	$ 48	$ 52

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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