Retirement Benefits - Reconciliation of Level 3 Assets and Liabilities measured at Fair Value for Pension and Other Postretirement Benefit Plans (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
Purchases		0	454
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
Purchases		0	0
Fair value of net plan assets at end of year		2,293	2,430
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		3,251	2,470
Relating to assets still held at the reporting date		64	279
Relating to assets sold during the period		13	3
Purchases		731	847
Sales		(123)	(60)
Settlements	[1]	(423)	(301)
Transfers into (out of) Level 3	[2]		13
Fair value of net plan assets at end of year		3,513	3,251
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		226	115
Relating to assets still held at the reporting date		9	14
Purchases		200	111
Sales			(13)
Settlements	[1]	(62)	(1)
Fair value of net plan assets at end of year		373	226
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		785	706
Relating to assets still held at the reporting date		(39)	59
Relating to assets sold during the period		4	2
Purchases		104	74
Sales		(57)	(25)
Settlements	[1]	(2)	(1)
Transfers into (out of) Level 3	[2]		(30)
Fair value of net plan assets at end of year		795	785
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		0	0
Relating to assets still held at the reporting date		1	0
Purchases		138	0
Sales			0
Settlements	[1]	0	0
Fair value of net plan assets at end of year		139	0
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		900	806
Relating to assets still held at the reporting date		60	112
Relating to assets sold during the period		0	0
Purchases		186	169
Sales		0	0
Settlements	[1]	(222)	(203)
Transfers into (out of) Level 3	[2]		16
Fair value of net plan assets at end of year		924	900
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		0	2
Relating to assets still held at the reporting date		0	0
Purchases		0	1
Sales			(2)
Settlements	[1]	0	(1)
Fair value of net plan assets at end of year		0	0
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		685	544
Relating to assets still held at the reporting date		76	81
Relating to assets sold during the period		9	0
Purchases		116	112
Sales		(54)	(19)
Settlements	[1]	(107)	(60)
Transfers into (out of) Level 3	[2]		27
Fair value of net plan assets at end of year		725	685
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		116	109
Relating to assets still held at the reporting date		15	13
Purchases		62	1
Sales			(7)
Settlements	[1]	(62)	0
Fair value of net plan assets at end of year		131	116
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		272	41
Relating to assets still held at the reporting date		(14)	7
Relating to assets sold during the period		0	0
Purchases		125	227
Sales		(7)	(3)
Settlements	[1]	(8)	0
Transfers into (out of) Level 3	[2]		0
Fair value of net plan assets at end of year		368	272
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
Settlements	[1]	0	0
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	2
Relating to assets still held at the reporting date		0	0
Relating to assets sold during the period		0	0
Purchases		0	0
Sales		0	0
Settlements	[1]	0	0
Transfers into (out of) Level 3	[2]		0
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
Settlements	[1]	0	0
Fair value of net plan assets at end of year		0	0
Private Placement [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		607	371
Relating to assets still held at the reporting date		(19)	20
Relating to assets sold during the period		0	1
Purchases		200	265
Sales		(5)	(13)
Settlements	[1]	(84)	(37)
Transfers into (out of) Level 3	[2],[3]		0
Fair value of net plan assets at end of year		699	607
Private Placement [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		110	4
Relating to assets still held at the reporting date		(7)	1
Purchases		0	109
Sales			(4)
Settlements	[1]	0	0
Fair value of net plan assets at end of year		$ 103	110
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

[1] Represents cash settlements only.

[2] As of January 1, 2015 and January 1, 2014, 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, 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 in 2014.

[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.