Derivative Financial Instruments - Summary of AOCI related to Cash Flow Hedges (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Derivative [Line Items]
Beginning Balance		$ (3,026)
Ending Balance		(2,965)
Income taxes		59	$ 211
Amounts reclassified from AOCI		(44)	(40)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Derivative [Line Items]
Amounts reclassified from AOCI	[1],[2]	0	(4)
Cash Flow Hedging [Member]
Derivative [Line Items]
Beginning Balance		(14)	(17)
Ending Balance		(6)	(11)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net		8	2
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Beginning Balance		(37)
Ending Balance		(34)
Income taxes		9	123
Amounts reclassified from AOCI			(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	[1],[2]	0	(4)
Exelon Generation Co L L C [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Beginning Balance		(16)	(19)
Ending Balance		(9)	(13)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net		$ 7	2
Interest Expense [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Amounts reclassified from AOCI	[3]		4
Interest Expense [Member] | Exelon Generation Co L L C [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Amounts reclassified from AOCI	[3]		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	[4]		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	[4]		$ 3

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

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

[3] Amount is net of related income tax expense of $3 million for the three months ended March 31, 2017.

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