Income Taxes - Remeasurement of Deferred Income Tax Balances - State (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Income Tax Expense (Benefit)	$ 774	$ 118	$ (126)
State and Local Jurisdiction [Member]
Income Taxes [Line Items]
Increase (Decrease) in Deferred Income Taxes	23	(50)	0
ILLINOIS | State and Local Jurisdiction [Member]
Income Taxes [Line Items]
Increase (Decrease) in Deferred Income Taxes			250
Increase (Decrease) in Other Regulatory Assets			270
Income Tax Expense (Benefit)			(20)
Exelon Generation Co L L C [Member]
Income Taxes [Line Items]
Income Tax Expense (Benefit)	516	(108)	(1,376)
Exelon Generation Co L L C [Member] | State and Local Jurisdiction [Member]
Income Taxes [Line Items]
Increase (Decrease) in Deferred Income Taxes	9	(53)	0
Exelon Generation Co L L C [Member] | ILLINOIS
Income Taxes [Line Items]
Increase (Decrease) in Deferred Income Taxes			20
Increase (Decrease) in Other Regulatory Assets			0
Income Tax Expense (Benefit)			20
Commonwealth Edison Co [Member]
Income Taxes [Line Items]
Income Tax Expense (Benefit)	$ 163	$ 168	417
Commonwealth Edison Co [Member] | State and Local Jurisdiction [Member]
Income Taxes [Line Items]
Increase (Decrease) in Deferred Income Taxes			0
Commonwealth Edison Co [Member] | ILLINOIS
Income Taxes [Line Items]
Increase (Decrease) in Deferred Income Taxes			270
Increase (Decrease) in Other Regulatory Assets			270
Income Tax Expense (Benefit)			$ 0