Derivative Financial Instruments Derivative Financial Instruments - Summary of Derivative Fair Value Balances (Details) - USD ($) $ in Millions		Mar. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative Asset, Current		$ 847	$ 917
Derivative Asset, Noncurrent		539	492
Derivative, Collateral, Right to Reclaim Cash		1	29
Margin Deposit Assets		14	158
Derivative Liability, Current		(228)	(282)
Derivative Liability, Noncurrent		(420)	(392)
Derivative [Member]
Derivative [Line Items]
Derivative Asset, Current		842	909
Derivative Asset, Noncurrent		524	472
Total mark-to-market derivative net assets (liabilities)		729	719
Total mark-to-market derivative assets		1,366	1,381
Derivative Liability, Current		(221)	(274)
Derivative Liability, Noncurrent		(416)	(388)
Total mark-to-market derivative liabilities		(637)	(662)
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Derivative Asset, Current		847	917
Derivative Asset, Noncurrent		527	476
Derivative Liability, Current		(209)	(263)
Derivative Liability, Noncurrent		(157)	(153)
Exelon Generation Co L L C [Member] | Economic Hedges [Member]
Derivative [Line Items]
Derivative Asset, Current		3,398	3,623
Derivative Asset, Noncurrent		1,975	1,467
Total mark-to-market derivative net assets (liabilities)		564	651
Total mark-to-market derivative assets		5,373	5,090
Derivative Liability, Current		(3,029)	(3,165)
Derivative Liability, Noncurrent		(1,780)	(1,274)
Total mark-to-market derivative liabilities		(4,809)	(4,439)
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Current assets collateral offset			100
Noncurrent assets collateral offset			72
Derivative Liability, Collateral, Right to Reclaim Cash, Offset		444
Current liabilities collateral offset			95
Noncurrent liabilities collateral offset			62
Total cash collateral received net of cash collateral posted			329
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivative [Line Items]
Derivative, Collateral, Right to Reclaim Cash		128
Margin Deposit Assets		14	158
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative, Collateral, Obligation to Return Cash		136
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member] | Other Noncurrent Assets [Member]
Derivative [Line Items]
Derivative, Collateral, Right to Reclaim Cash		77
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member] | Other Noncurrent Liabilities [Member]
Derivative [Line Items]
Derivative, Collateral, Obligation to Return Cash		103
Exelon Generation Co L L C [Member] | Derivative [Member]
Derivative [Line Items]
Derivative Asset, Current	[1]	842	909
Derivative Asset, Noncurrent	[1]	524	472
Total mark-to-market derivative net assets (liabilities)	[1]	1,011	977
Total mark-to-market derivative assets	[1]	1,366	1,381
Derivative Liability, Current	[1]	(202)	(255)
Derivative Liability, Noncurrent	[1]	(153)	(149)
Total mark-to-market derivative liabilities	[1]	(355)	(404)
Exelon Generation Co L L C [Member] | Proprietary Trading [Member]
Derivative [Line Items]
Derivative Asset, Current		56	55
Derivative Asset, Noncurrent		24	21
Total mark-to-market derivative net assets (liabilities)		3	(3)
Total mark-to-market derivative assets		80	76
Derivative Liability, Current		(49)	(54)
Derivative Liability, Noncurrent		(28)	(25)
Total mark-to-market derivative liabilities		(77)	(79)
Exelon Generation Co L L C [Member] | Collateral And Netting [Member]
Derivative [Line Items]
Derivative Asset, Current		(2,612)	(2,769)
Derivative Asset, Noncurrent		(1,475)	(1,016)
Total mark-to-market derivative net assets (liabilities)	[2]	444	329
Total mark-to-market derivative assets	[2]	(4,087)	(3,785)
Derivative Liability, Current		2,876	2,964
Derivative Liability, Noncurrent		1,655	1,150
Total mark-to-market derivative liabilities	[2]	4,531	4,114
Commonwealth Edison Co [Member]
Derivative [Line Items]
Derivative Liability, Current		(19)	(19)
Derivative Liability, Noncurrent		(263)	(239)
Commonwealth Edison Co [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Current		0	0
Derivative Asset, Noncurrent		0	0
Total mark-to-market derivative net assets (liabilities)	[3]	(282)	(258)
Total mark-to-market derivative assets	[3]	0	0
Derivative Liability, Current		(19)	(19)
Derivative Liability, Noncurrent		(263)	(239)
Total mark-to-market derivative liabilities	[3]	(282)	(258)
Delmarva Power and Light Company [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Current		0	2
Derivative Asset, Noncurrent		0	0
Total mark-to-market derivative net assets (liabilities)		0	2
Total mark-to-market derivative assets		0	2
Derivative Liability, Current		0	0
Derivative Liability, Noncurrent		0	0
Total mark-to-market derivative liabilities		0	0
Delmarva Power and Light Company [Member] | Derivative [Member]
Derivative [Line Items]
Derivative Asset, Current	[4]	0	0
Derivative Asset, Noncurrent	[4]	0	0
Total mark-to-market derivative net assets (liabilities)		0	0
Total mark-to-market derivative assets	[4]	0	0
Derivative Liability, Current	[4]	0	0
Derivative Liability, Noncurrent	[4]	0	0
Total mark-to-market derivative liabilities	[4]	0	0
Delmarva Power and Light Company [Member] | Collateral And Netting [Member]
Derivative [Line Items]
Derivative Asset, Current		0	(2)
Derivative Asset, Noncurrent		0	0
Total mark-to-market derivative net assets (liabilities)		0	(2)
Total mark-to-market derivative assets		0	(2)
Derivative Liability, Current		0	0
Derivative Liability, Noncurrent		0	0
Total mark-to-market derivative liabilities		0	0
Pepco Holdings LLC [Member] | Successor [Member]
Derivative [Line Items]
Derivative Asset, Current		0
Derivative Asset, Noncurrent		0
Total mark-to-market derivative net assets (liabilities)		0
Total mark-to-market derivative assets		0
Derivative Liability, Current		0
Derivative Liability, Noncurrent		0
Total mark-to-market derivative liabilities		$ 0
Pepco Holdings LLC [Member] | Predecessor [Member]
Derivative [Line Items]
Derivative Asset, Current			0
Derivative Asset, Noncurrent			0
Total mark-to-market derivative net assets (liabilities)			0
Total mark-to-market derivative assets			0
Derivative Liability, Current			0
Derivative Liability, Noncurrent			0
Total mark-to-market derivative liabilities			$ 0

[1] .

[2] .

[3] .

[4] Current and noncurrent assets are shown net of collateral of $100 million and $72 million, respectively, and current and noncurrent liabilities are shown net of collateral of $95 million and $62 million, respectively. The total cash collateral posted, net of cash collateral received and offset against mark-to-market assets and liabilities was $329 million at December 31, 2016.