Changes in Accumulated Other Comprehensive Income - Components of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Tax	$ (27)	$ (28)
Prior service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Tax	6	10
Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Tax	(22)	(32)
Pension and Non-Pension Postretirement Benefit Plan Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Tax	(7)	0
Gains and (losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Tax	(3)	(1)
Unrealized Gains and (losses) on Marketable Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in unrealized (loss) on investments in unconsolidated affiliates	(1)	(4)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax	0	(1)
Exelon Generation Co L L C [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Tax	(4)	(4)
Exelon Generation Co L L C [Member] | Gains and (losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Tax	(3)	(1)
Exelon Generation Co L L C [Member] | Unrealized Gains and (losses) on Marketable Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in unrealized (loss) on investments in unconsolidated affiliates	$ (1)	$ (3)