Derivative Financial Instruments - Summary of AOCI related to Cash Flow Hedges (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017		Sep. 30, 2018	Sep. 30, 2017
Derivative [Line Items]
Beginning Balance					$ (3,026)
Ending Balance		$ (2,869)			(2,869)
Income taxes		137	$ 451		262	$ 601
Amounts reclassified from AOCI	[1]	(45)	(34)		(137)	(108)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Derivative [Line Items]
Amounts reclassified from AOCI	[2],[3]				(1)	(3)
Cash Flow Hedging [Member]
Derivative [Line Items]
Beginning Balance		(2)	(12)		(14)	(17)
Ending Balance		(2)	(12)		(2)	(12)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net			1		11	2
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Beginning Balance					(37)
Ending Balance		(31)			(31)
Income taxes		78	239		110	215
Amounts reclassified from AOCI	[1]	0	1		(1)	(3)
Exelon Generation Co L L C [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Derivative [Line Items]
Amounts reclassified from AOCI	[2],[3]				(1)	(3)
Exelon Generation Co L L C [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Beginning Balance		(4)	(14)		(16)	(19)
Ending Balance		(4)	(14)		(4)	(14)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net			1		11	2
Interest Expense [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Amounts reclassified from AOCI		0	(1)	[4]	1	3	[5]
Interest Expense [Member] | Exelon Generation Co L L C [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Amounts reclassified from AOCI		0	(1)	[4]	1	3	[5]
Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Income taxes			1			2
Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedging [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Derivative [Line Items]
Income taxes	[1]	0	(1)		0	2
Reclassification out of Accumulated Other Comprehensive Income [Member] | Exelon Generation Co L L C [Member] | Cash Flow Hedging [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Derivative [Line Items]
Income taxes	[1]	$ 0	$ (1)		$ 0	$ 2

[1] Amounts in parenthesis represent a decrease in AOCI.

[2] All amounts are net of tax and noncontrolling interests. Amounts in parenthesis represent a decrease in AOCI.

[3] See next tables for details about these reclassifications.

[4] Amount is net of related income tax benefit of $1 million for the three months ended September 30, 2017.

[5] Amount is net of related income tax expense of $2 million for the nine months ended September 30, 2017.