Changes in Accumulated Other Comprehensive Income - Components of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2016	Mar. 23, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension and non-pension postretirement benefit plans:
Prior service benefit reclassified to periodic benefit cost	$ 7		$ 8	$ 22	$ 22
Actuarial loss reclassified to periodic cost	(29)		(35)	(88)	(105)
Pension and non-pension postretirement benefit plans valuation adjustment	1		0	1	17
Change in unrealized gain/(loss) on cash flow hedges	(1)		3	3	(3)
Change in unrealized loss on equity investments	0		0	3	0
Change in unrealized gain on marketable securities	(1)		0	(1)	0
Total	(23)		(24)	(60)	(69)
PEPCO Holdings Inc [Member]
Pension and non-pension postretirement benefit plans:
Actuarial loss reclassified to periodic cost	2	$ 0			3
Exelon Generation Co L L C [Member]
Pension and non-pension postretirement benefit plans:
Change in unrealized gain/(loss) on cash flow hedges	(2)		3	1	4
Change in unrealized loss on equity investments	0		0	3	0
Change in unrealized gain on marketable securities	0		0	0	0
Total	$ (2)		$ 3	$ 4	$ 4