Derivative Financial Instruments - Summary of AOCI related to Cash Flow Hedges (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Derivative [Line Items]
Beginning Balance			$ (3,026)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1]		(182)	$ (144)
Ending Balance		$ (2,995)	(2,995)	(3,026)
Income Tax Expense (Benefit)		(26)	(120)	126		$ (753)
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Beginning Balance			(37)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1]		(1)	(4)
Ending Balance		(38)	(38)	(37)
Income Tax Expense (Benefit)			108	1,376		(282)
Cash Flow Hedging [Member]
Derivative [Line Items]
Beginning Balance			(14)	(17)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net			11	(1)
Ending Balance		(2)	(2)	(14)		(17)
Cash Flow Hedging [Member] | Interest Expense [Member]
Derivative [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax			1	4	[2]
Cash Flow Hedging [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Beginning Balance			(16)	(19)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net			11	(1)
Ending Balance		$ (4)	(4)	(16)		$ (19)
Cash Flow Hedging [Member] | Exelon Generation Co L L C [Member] | Interest Expense [Member]
Derivative [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax			$ 1	4	[2]
Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Income Tax Expense (Benefit)				$ 1

[1] Amounts in parenthesis represent a decrease in net income.

[2] Amount is net of related income tax expense of $1 million for the year ended December 31, 2017.