Derivative Financial Instruments - Summary of AOCI related to Cash Flow Hedges (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative [Line Items]
Beginning Balance				$ (3,026)
Ending Balance		$ (2,921)		(2,921)
Income taxes		66	$ (62)	125	$ 149
Amounts reclassified from AOCI	[1]	(43)	(34)	(87)	(74)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Derivative [Line Items]
Amounts reclassified from AOCI	[2],[3]			1	(4)
Cash Flow Hedging [Member]
Derivative [Line Items]
Beginning Balance		(6)	(11)	(14)	(17)
Ending Balance		(2)	(12)	(2)	(12)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net		3	(1)	11	1
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Beginning Balance				(37)
Ending Balance		(33)		(33)
Income taxes		23	(148)	32	(25)
Amounts reclassified from AOCI	[1]	1		1	(4)
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	(4)
Exelon Generation Co L L C [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Beginning Balance		(9)	(13)	(16)	(19)
Ending Balance		(4)	(14)	(4)	(14)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net		4	$ (1)	11	1
Interest Expense [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Amounts reclassified from AOCI		1		1	4	[4]
Interest Expense [Member] | Exelon Generation Co L L C [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Amounts reclassified from AOCI		1		1	4	[4]
Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Income taxes					3
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		0	3
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		$ 0	$ 3

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

[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 expense of $3 million for the six months ended June 30, 2017.