Retirement Benefits - Components of Net Periodic Benefit Cost (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Contractual termination benefit cost					$ 1
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Contractual termination benefit cost					4
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	293		317		280
Interest cost	749		650		698
Expected return on assets	(994)		(1,015)		(988)
Transition obligation	0		0		0
Prior service cost (credit)	14		14		15
Actuarial loss	420		562		450
Curtailment benefits	0		0		0
Settlement charges	2		9		31
Contractual termination benefit cost	0	[1]	0	[1]	14	[1]
Net periodic benefit cost	484		537		500
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	117		162		156
Interest cost	186		194		205
Expected return on assets	(154)		(132)		(115)
Transition obligation	0		0		11
Prior service cost (credit)	(122)		(19)		(17)
Actuarial loss	50		83		81
Curtailment benefits	0		0		(7)
Settlement charges	0		0		0
Contractual termination benefit cost	0	[1]	0	[1]	6	[1]
Net periodic benefit cost	$ 77		$ 288		$ 320

[1] )ComEd and BGE established regulatory assets of $1 million and $4 million, respectively, for their portion of the contractual termination benefit charge in