Retirement Benefits - Summary of Changes in Plan Assets (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of net plan assets at beginning of year		$ 14,874	$ 13,571
Actual return on plan assets		(32)	1,443
Employer contributions		462	332
Plan participants’ contributions		0	0
Gross benefits paid		(923)	(896)
Acquisitions/divestitures(a)		0	454
Settlements	[1]	(34)	(30)
Fair value of net plan assets at end of year		14,347	14,874
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of net plan assets at beginning of year		2,430	2,238
Actual return on plan assets		4	90
Employer contributions		40	291
Plan participants’ contributions		42	42
Gross benefits paid		(223)	(231)
Acquisitions/divestitures(a)		0	0
Settlements	[1]	0	0
Fair value of net plan assets at end of year		$ 2,293	$ 2,430

[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.