Stock-Based Compensation Plans - Schedule of Stock-based Compensation Expense (Details) - Share-based Payment Arrangement [Member] - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Expense	$ 64	$ 77	$ 208
Share-based Payment Arrangement, Expense, Tax Benefit	(16)	(20)	(54)
Share-based Payment Arrangement, Expense, after Tax	48	57	154
Exelon Generation Co L L C [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Expense	27	37	77
Share-based Payment Arrangement, Expense, Tax Benefit	(7)	(10)	(20)
Share-based Payment Arrangement, Expense, after Tax	$ 20	$ 27	$ 57

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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