Retirement Benefits - Reconciliation of Level 3 Assets and Liabilities measured at Fair Value for Pension and Other Postretirement Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
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	$ 13,571		$ 13,357
Purchases	454		0
Fair value of net plan assets at end of year	14,874		13,571
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	2,238		2,135
Purchases	0		0
Fair value of net plan assets at end of year	2,430		2,238
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Transfers into (out of) Level 3	56
Fair Value, Inputs, Level 3 [Member] | 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	2,470		2,415
Relating to assets still held at the reporting date	279		292
Relating to assets sold during the period	3		(1)
Purchases	847		752
Sales	(60)		(167)
Settlements	(301)	[1]	(198)	[1]
Transfers into (out of) Level 3	13	[2],[3]	(623)	[2]
Fair value of net plan assets at end of year	3,251		2,470
Fair Value, Inputs, Level 3 [Member] | Pension Plan, Defined Benefit [Member] | Constellation Energy Group Acquisition [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Transfers into (out of) Level 3			825
Fair Value, Inputs, Level 3 [Member] | 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	115		108
Relating to assets still held at the reporting date	14		12
Purchases	111		4
Sales	(13)		(1)
Settlements	(1)	[1]	(4)	[1]
Transfers into (out of) Level 3			(4)	[2]
Fair value of net plan assets at end of year	226		115
Hedge Funds [Member] | Fair Value, Inputs, Level 3 [Member] | 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	1,039		1,235
Relating to assets still held at the reporting date	77		143
Relating to assets sold during the period	3		3
Purchases	311		360
Sales	(38)		(76)
Settlements	(33)	[1]	(3)	[1]
Transfers into (out of) Level 3	(30)	[2],[3]	(623)	[2]
Fair value of net plan assets at end of year	1,329		1,039
Hedge Funds [Member] | Fair Value, Inputs, Level 3 [Member] | 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	4		12
Relating to assets still held at the reporting date	1		1
Purchases	109		0
Sales	(4)		(1)
Settlements	0	[1]	(4)	[1]
Transfers into (out of) Level 3			(4)	[2]
Fair value of net plan assets at end of year	110		4
Private Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member] | 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	806		754
Relating to assets still held at the reporting date	112		86
Relating to assets sold during the period	0		0
Purchases	173		123
Sales	0		0
Settlements	(203)	[1]	(157)	[1]
Transfers into (out of) Level 3	16	[2],[3]	0	[2]
Fair value of net plan assets at end of year	904		806
Private Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member] | 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		1
Relating to assets still held at the reporting date	0		0
Purchases	1		1
Sales	(2)		0
Settlements	(1)	[1]	0	[1]
Transfers into (out of) Level 3			0	[2]
Fair value of net plan assets at end of year	0		2
Real Estate Funds [Member] | Fair Value, Inputs, Level 3 [Member] | 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	582		426
Relating to assets still held at the reporting date	83		63
Relating to assets sold during the period	0		(4)
Purchases	136		226
Sales	(19)		(91)
Settlements	(65)	[1]	(38)	[1]
Transfers into (out of) Level 3	27	[2],[3]	0	[2]
Fair value of net plan assets at end of year	744		582
Real Estate Funds [Member] | Fair Value, Inputs, Level 3 [Member] | 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	109		95
Relating to assets still held at the reporting date	13		11
Purchases	1		3
Sales	(7)		0
Settlements	0	[1]	0	[1]
Transfers into (out of) Level 3			0	[2]
Fair value of net plan assets at end of year	116		109
Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | 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	41		0
Relating to assets still held at the reporting date	7		0
Relating to assets sold during the period	0		0
Purchases	227		41
Sales	(3)		0
Settlements	0	[1]	0	[1]
Transfers into (out of) Level 3	0	[2],[3]	0	[2]
Fair value of net plan assets at end of year	272		41
Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | 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	0		0
Relating to assets still held at the reporting date	0		0
Purchases	0		0
Sales	0		0
Settlements	0	[1]	0	[1]
Transfers into (out of) Level 3			0	[2]
Fair value of net plan assets at end of year	0		0
Equity Security Individually Held [Member] | Fair Value, Inputs, Level 3 [Member] | 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	2		0
Relating to assets still held at the reporting date	0		0
Relating to assets sold during the period	0		0
Purchases	0		2
Sales	0		0
Settlements	0	[1]	0	[1]
Transfers into (out of) Level 3	0	[2],[3]	0	[2]
Fair value of net plan assets at end of year	2		2
Equity Security Individually Held [Member] | Fair Value, Inputs, Level 3 [Member] | 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	0		0
Relating to assets still held at the reporting date	0		0
Purchases	0		0
Sales	0		0
Settlements	0	[1]	0	[1]
Transfers into (out of) Level 3			0	[2]
Fair value of net plan assets at end of year	0		0
Hedge Fund Investments [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Transfers into (out of) Level 3	$ 43		$ 627

[1] Represents cash settlements only.

[2] As of January 1, 2014 and January 1, 2013, hedge fund investments that contained redemption restrictions limiting Exelon’s ability to redeem the investments within a reasonable period of time were classified as Level 3 investments. As of December 31, 2014 and December 31, 2013, restrictions for certain investments no longer applied, therefore allowing redemption within a reasonable period of time from the measurement date at NAV. As such, these hedge fund investments are reflected as transfers out of Level 3 to Level 2 of $43 million and $627 million in 2014 and 2013 respectively.

[3] In connection with the Employee Matters Agreement between EDF and Exelon, Exelon assumed the pension plan assets of Nine Mile Point Nuclear Station, LLC and Constellation Energy Nuclear Group, LLC resulting in transfers into Level 3 of $56 million.