Supplemental Financial Information - Operations (Detail) - USD ($) $ in Millions		3 Months Ended				6 Months Ended	9 Months Ended
		Sep. 30, 2016	Mar. 23, 2016	Sep. 30, 2015		Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2015
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		$ 255		$ 151			$ 624	$ 430
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units		57		39	[1]		181	203	[1]
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units		35		18	[1]		95	122	[1]
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		155		(301)			286	(385)
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		116		(218)			216	(274)
Net unrealized income (losses) on pledged assets		(5)					(2)	9
Regulatory offset to decommissioning trust fund-related activities		(168)		207	[2]		(380)	129	[2]
Total decommissioning-related activities		190		(255)			396	(196)
Investment income		2		4			14	6
Long-term lease income				4			4	12
Unrecognized Tax Benefits Interest Income		8					13	0
Income Tax Examination, Penalties and Interest Expense		106					106
AFUDC - equity		19		6			43	16
Gain (Loss) on Disposition of Assets	[3]							(26)
Unrecognized Tax Benefits, Income Tax Penalties Expense		(90)
Gains (Losses) on Extinguishment of Debt							(3)
Other Income		7		(3)			16	9
Other, net		120		(244)			377	(179)
Exelon Generation Co L L C [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		35		28			90	79
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units		57		39	[1]		181	203	[1]
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units		35		18	[1]		95	122	[1]
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units		155		(301)			286	(385)
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement		116		(218)			216	(274)
Net unrealized income (losses) on pledged assets		(5)					(2)	9
Regulatory offset to decommissioning trust fund-related activities		(168)		207	[2]		(380)	129	[2]
Total decommissioning-related activities		190		(255)			396	(196)
Investment income		1		1			6	1
Unrecognized Tax Benefits Interest Income		0					0	1
Gain (Loss) on Disposition of Assets								0
Gains (Losses) on Extinguishment of Debt							(2)
Other Income		(6)		(3)			(5)	1
Other, net		185		(257)			395	(193)
Commonwealth Edison Co [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		67		63			186	180
Decommissioning-Related Activities [Abstract]
Investment income		0
Unrecognized Tax Benefits Interest Income							0	0
Income Tax Examination, Penalties and Interest Expense							86
AFUDC - equity		5		1			8	2
Other Income		1		3			6	12
Other, net		(80)		4			(72)	14
PECO Energy Co [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		40		37			106	104
Decommissioning-Related Activities [Abstract]
Investment income		(1)		(1)			(1)	(1)
Unrecognized Tax Benefits Interest Income		0					0	0
AFUDC - equity		2		1			6	4
Other Income		1		1			1	0
Other, net		2		1			6	3
Baltimore Gas and Electric Company [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		21		23			66	67
Decommissioning-Related Activities [Abstract]
Investment income		0		1	[4]		2	3	[4]
Unrecognized Tax Benefits Interest Income		0					0
AFUDC - equity		5		4			14	10
Other Income		0		(1)			0	0
Other, net		5		4			16	13
Potomac Electric Power Company [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		87		82			240	239
Decommissioning-Related Activities [Abstract]
Investment income		0		0			0	0
Long-term lease income				0			0	0
Unrecognized Tax Benefits Interest Income		0					1	0
AFUDC - equity		5		3			14	9
Gain (Loss) on Disposition of Assets		0					8
Other Income		7		5			13	12
Other, net		12		8			28	21
Pepco Holdings LLC [Member] | Successor [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		92				$ 176
Decommissioning-Related Activities [Abstract]
Investment income						1
Long-term lease income						0
Unrecognized Tax Benefits Interest Income						0
AFUDC - equity		7				15
Other Income		12				15
Other, net		19				$ 31
Pepco Holdings LLC [Member] | Predecessor [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes			$ 78	86				253
Decommissioning-Related Activities [Abstract]
Investment income			0					0
Long-term lease income			0					0
Unrecognized Tax Benefits Interest Income			0					0
AFUDC - equity			7	2				11
Other Income			(11)	25				37
Other, net			$ (4)	27				48
Delmarva Power and Light Company [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		5		4			14	14
Decommissioning-Related Activities [Abstract]
Investment income		0		0			0	0
Long-term lease income				0			0	0
Unrecognized Tax Benefits Interest Income		0					0	0
AFUDC - equity		1		0			3	1
Gain (Loss) on Disposition of Assets		4		0			4	0
Other Income		2		4			6	7
Other, net		3		4			9	8
Atlantic City Electric Company [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes		0		0			0	0
Decommissioning-Related Activities [Abstract]
Investment income		0		0			0	0
Long-term lease income				0			0	0
Unrecognized Tax Benefits Interest Income		0					1	1
AFUDC - equity		1		0			5	1
Gain (Loss) on Disposition of Assets		0					1
Other Income		1		1			2	2
Other, net		$ 2		$ 1			$ 8	$ 4

[1] Includes investment income and realized gains and losses on sales of investments of the trust funds.

[2] Includes the elimination of NDT fund activity for the Regulatory Agreement Units, including the elimination of net income taxes related to all NDT fund activity for those units. See Note 16 — Asset Retirement Obligations of the Exelon 2015 Form 10-K for additional information regarding the accounting for nuclear decommissioning.

[3] In January 2015, in connection with Generation's $750 million issuance of five-year Senior Unsecured Notes, Exelon terminated certain floating-to-fixed interest rate swaps. As the original forecasted transactions were a series of future interest payments over a ten year period, a portion of the anticipated interest payments were probable not to occur. As a result, $26 million of anticipated payments were reclassified from AOCI to Other, net in Exelon's Consolidated Statement of Operations and Comprehensive Income.

[4] Relates to the cash return on BGE’s rate stabilization deferral. See Note 3 — Regulatory Matters of the Exelon 2015 Form 10-K for additional information regarding the rate stabilization deferral.