Changes in Accumulated Other Comprehensive Income - Components of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, net	[1]	$ (2,596)	$ (1,917)	$ (2,596)	$ (1,917)	$ (2,684)	$ (2,040)
Pension and non-pension postretirement benefit plans:
Prior service benefit reclassified to periodic benefit cost		8	8	22	11
Actuarial loss reclassified to periodic cost		(35)	(24)	(105)	(69)
Pension and non-pension postretirement benefit plans valuation adjustment		0	5	17	(153)
Change in unrealized (gain) loss on cash flow hedges		3	15	(3)	62
Change in unrealized income on equity investments		0	3	0	73
Change in marketable securities		0	1	0	(1)
Total		(24)	8	(69)	(77)
Exelon Generation Co L L C [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, net	[1]	(60)	11	(60)	11	$ (36)	$ 214
Pension and non-pension postretirement benefit plans:
Change in unrealized (gain) loss on cash flow hedges		3	13	4	57
Change in unrealized income on equity investments		0	3	0	73
Change in marketable securities		0	1	0	(1)
Total		$ 3	$ 17	$ 4	$ 129

[1] All amounts are net of tax. Amounts in parentheses represent a decrease in accumulated other comprehensive income.