Changes in Accumulated Other Comprehensive Income - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Prior service costs	$ 30	$ 19	$ 0
Actuarial loss reclassified to periodic cost, taxes	(140)	(93)	(133)
Pension and non-pension postretirement benefit plan valuation adjustment, taxes	62	317	(430)
Change in unrealized gain (loss) on cash flow hedges, taxes	(6)	96	166
Change in unrealized gain (loss) on equity investments taxes	1	73	(71)
Other comprehensive income, income taxes	(53)	412	(468)
Exelon Generation Co L L C [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in unrealized gain (loss) on cash flow hedges, taxes	2	84	262
Change in unrealized gain (loss) on equity investments taxes	1	73	(72)
Other comprehensive income, income taxes	$ 3	$ 157	$ 190