Derivative Financial Instruments - Summary of AOCI related to Cash Flow Hedges (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Derivative [Line Items]
Beginning Balance		$ (2,660)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1]	(144)		$ (150)
Ending Balance		(2,487)		(2,660)
Income Tax Expense (Benefit)		125		(761)		$ (1,073)
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Beginning Balance		(54)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1]	(4)		(13)
Ending Balance		(37)		(54)
Income Tax Expense (Benefit)		1,375		(290)		(502)
Cash Flow Hedging [Member]
Derivative [Line Items]
Beginning Balance		(17)		(19)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net		(1)		(6)
Ending Balance		(14)		(17)		(19)
Cash Flow Hedging [Member] | Interest Expense [Member]
Derivative [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		4	[2]	8	[3]
Cash Flow Hedging [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Beginning Balance		(19)		(21)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net		(1)		(6)
Ending Balance		(16)		(19)		$ (21)
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		4	[2]	8	[3]
Reclassification out of Accumulated Other Comprehensive Income [Member]
Derivative [Line Items]
Income Tax Expense (Benefit)	[1]			0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Income Tax Expense (Benefit)	[1]			0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Income Tax Expense (Benefit)		$ 1		$ 5

[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.

[3] Amount is net of related income tax expense of $5 million for the year ended December 31, 2016.