Derivative Financial Instruments - Summary of Derivative Fair Value Balances (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
Mark-to-market derivative assets, current	$ 472	$ 679
Derivative Asset, Noncurrent	383	508
Mark-to-market derivative liabilities, current	(177)	(247)
Mark-to-market derivative liabilities, noncurrent	(396)	(393)
Margin Deposit Assets	34	511
Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	471	675
Derivative Asset, Noncurrent	383	508
Total mark-to-market derivative assets	854	1,183
Mark-to-market derivative liabilities, current	(168)	(236)
Mark-to-market derivative liabilities, noncurrent	(378)	(380)
Total mark-to-market derivative liabilities	(546)	(616)
Total mark-to-market derivative net assets (liabilities)	308	567
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	471	675
Derivative Asset, Noncurrent	383	508
Mark-to-market derivative liabilities, current	(147)	(215)
Mark-to-market derivative liabilities, noncurrent	(121)	(123)
Margin Deposit Assets	34	511
Exelon Generation Co L L C [Member] \| Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	471	675
Derivative Asset, Noncurrent	383	508
Total mark-to-market derivative assets	854	1,183
Mark-to-market derivative liabilities, current	(138)	(204)
Mark-to-market derivative liabilities, noncurrent	(104)	(111)
Total mark-to-market derivative liabilities	(242)	(315)
Total mark-to-market derivative net assets (liabilities)	612	868
Commonwealth Edison Co [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities, current	(30)	(32)
Mark-to-market derivative liabilities, noncurrent	(274)	(269)
Collateral [Member] \| Exelon Generation Co L L C [Member] \| Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	79	287
Derivative Asset, Noncurrent	39	122
Total mark-to-market derivative assets	118	409
Mark-to-market derivative liabilities, current	84	357
Mark-to-market derivative liabilities, noncurrent	49	163
Total mark-to-market derivative liabilities	133	520
Total mark-to-market derivative net assets (liabilities)	251	929
Netting [Member] \| Exelon Generation Co L L C [Member] \| Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	(2,232)	(3,190)
Derivative Asset, Noncurrent	(1,172)	(877)
Total mark-to-market derivative assets	(3,404)	(4,067)
Mark-to-market derivative liabilities, current	2,232	3,190
Mark-to-market derivative liabilities, noncurrent	1,172	877
Total mark-to-market derivative liabilities	3,404	4,067
Total mark-to-market derivative net assets (liabilities)	0	0
Not Designated as Hedging Instrument, Economic Hedge [Member] \| Exelon Generation Co L L C [Member] \| Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	2,566	3,506
Derivative Asset, Noncurrent	1,500	1,238
Total mark-to-market derivative assets	4,066	4,744
Mark-to-market derivative liabilities, current	(2,414)	(3,713)
Mark-to-market derivative liabilities, noncurrent	(1,313)	(1,140)
Total mark-to-market derivative liabilities	(3,727)	(4,853)
Total mark-to-market derivative net assets (liabilities)	339	(109)
Not Designated as Hedging Instrument, Economic Hedge [Member] \| Commonwealth Edison Co [Member] \| Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	0	0
Derivative Asset, Noncurrent	0	0
Total mark-to-market derivative assets	0	0
Mark-to-market derivative liabilities, current	(30)	(32)
Mark-to-market derivative liabilities, noncurrent	(274)	(269)
Total mark-to-market derivative liabilities	(304)	(301)
Total mark-to-market derivative net assets (liabilities)	(304)	(301)
Not Designated as Hedging Instrument [Member] \| Exelon Generation Co L L C [Member] \| Commodity Contract [Member]
Derivative [Line Items]
Mark-to-market derivative assets, current	58	72
Derivative Asset, Noncurrent	16	25
Total mark-to-market derivative assets	74	97
Mark-to-market derivative liabilities, current	(40)	(38)
Mark-to-market derivative liabilities, noncurrent	(12)	(11)
Total mark-to-market derivative liabilities	(52)	(49)
Total mark-to-market derivative net assets (liabilities)	$ 22	$ 48

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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