Changes in Accumulated Other Comprehensive Income - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 23, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income, income taxes		$ (90)	$ 30	$ (53)
Prior service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income, income taxes		36	30	30
Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income, income taxes		(128)	(118)	(140)
Pension and Non-Pension Postretirement Benefit Plan Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income, income taxes		13	115	62
Gains and (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income, income taxes		(7)	0	(6)
Unrealized Gains and (losses) on Marketable Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in unrealized gain (loss) on equity investments taxes		(3)	3	1
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax		(1)	0	0
Exelon Generation Co L L C [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income, income taxes		(10)	1	3
Exelon Generation Co L L C [Member] | Gains and (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income, income taxes		(6)	(2)	2
Exelon Generation Co L L C [Member] | Unrealized Gains and (losses) on Marketable Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in unrealized gain (loss) on equity investments taxes		(3)	3	1
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax		$ (1)	$ 0	0
Predecessor [Member] | PEPCO Holdings Inc [Member] | Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income, income taxes	$ 0			$ 6