Retirement Benefits - Summary of Changes in Benefit Obligations (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation at beginning of year		$ 18,256	$ 15,459
Service cost		326	293	$ 317
Interest cost		710	749	650
Plan participants’ contributions		0	0
Actuarial (gain) loss		(582)	2,095
Plan amendments		0	0
Acquisitions/divestitures(a)		0	594
Curtailments		0	(8)
Settlements	[1]	(34)	(30)
Gross benefits paid		(923)	(896)
Net benefit obligation at end of year		17,753	18,256	15,459
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation at beginning of year		4,197	4,451
Service cost		119	117	162
Interest cost		167	186	194
Plan participants’ contributions		42	42
Actuarial (gain) loss		(341)	502
Plan amendments		(23)	(1,012)
Acquisitions/divestitures(a)		0	142
Curtailments		0	0
Settlements	[1]	0	0
Gross benefits paid		(223)	(231)
Net benefit obligation at end of year		$ 3,938	$ 4,197	$ 4,451

[1] _______________________ (a)On April 1, 2014, Generation assumed operational control of CENG’s nuclear fleet. As a result, Exelon became a sponsor of CENG’s pension and OPEB plans effective July 14, 2014. See Note 5 - Investment in Constellation Energy Nuclear Group, LLC for further information.