Changes in Accumulated Other Comprehensive Income - Components of Other Comprehensive Income (Loss) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Sep. 30, 2013	Dec. 31, 2014		Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, net	$ (2,673)	[1]	$ (2,036)	[1]		$ (2,684)	[1]	$ (2,040)	[1]
Pension and non-pension postretirement benefit plans:
Prior service benefit reclassified to periodic benefit cost	8				(1)
Actuarial gain (loss) reclassified to periodic cost	(35)				(23)
Pension and non-pension postretirement benefit plans valuation adjustment	17				7
Change in unrealized gain (loss) on cash flow hedges	(2)				18
Change in unrealized income on equity investments	0				(7)
Total	(12)		(6)
Exelon Generation Co L L C [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, net	(53)	[1]	193	[1]		(36)	[1]	214	[1]
Pension and non-pension postretirement benefit plans:
Change in unrealized gain (loss) on cash flow hedges	5				19
Change in unrealized income on equity investments	0				(7)
Change in marketable securities	0				(2)
Total	$ 5		$ 10

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