Mergers, Acquisitions and Dispositions - Narrative (Details) - USD ($) $ / shares in Units, shares in Millions			2 Months Ended	3 Months Ended										6 Months Ended	12 Months Ended			20 Months Ended
		Nov. 01, 2014	Dec. 31, 2014	Dec. 31, 2015		Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014		Jun. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	May. 31, 2014	May. 30, 2012
Business Acquisition [Line Items]
Junior Subordinated Notes, Noncurrent							$ 1,150,000,000							$ 1,150,000,000
Forward Contract Indexed to Issuer's Equity, Indexed Shares							23							23
Other noncurrent assets			$ 923,000,000	$ 1,445,000,000					$ 923,000,000						$ 1,445,000,000	$ 923,000,000		$ 1,445,000,000
Total acquisition costs															40,000,000	386,000,000	$ 0
Business Acquisition, Transaction Costs				259,000,000											259,000,000			259,000,000
Business Combination, Integration Related Costs															80,000,000	179,000,000		121,000,000
Bridge loan							$ 3,200,000,000.0							$ 3,200,000,000.0					$ 7,200,000,000.0
Revenues				6,702,000,000		$ 7,401,000,000	6,514,000,000	$ 8,830,000,000	7,255,000,000	$ 6,912,000,000	$ 6,024,000,000	$ 7,237,000,000			29,447,000,000	27,429,000,000	24,888,000,000
Comprehensive Income (Loss), Net of Tax, Attributable to Parent															2,310,000,000	1,176,000,000	2,456,000,000
Net income				309,000,000	[1]	629,000,000	638,000,000	693,000,000	18,000,000	993,000,000	522,000,000	90,000,000	[2]		2,250,000,000	1,820,000,000	1,729,000,000
Derivative, Loss on Derivative														$ 64,000,000
Merger and integration related costs																44,000,000
Change in capital expenditures not paid															96,000,000	220,000,000	(38,000,000)
Net present value of settlement amount															430,000,000
Cost of Reimbursable Expense															40,000,000
Payments of Merger Related Costs, Financing Activities															138,000,000
Payments of Debt Restructuring Costs	[3]														22,000,000
New Jersey Board Of Public Utilities [Member]
Business Acquisition [Line Items]
Business Acquisition, Preacquisition Contingency, Amount of Settlement				62,000,000											62,000,000			62,000,000
Delaware Public Service Commission [Member]
Business Acquisition [Line Items]
Business Acquisition, Preacquisition Contingency, Amount of Settlement				40,000,000											40,000,000			40,000,000
Maryland Public Service Commission [Member]
Business Acquisition [Line Items]
Business Acquisition, Preacquisition Contingency, Amount of Settlement				66,000,000											66,000,000			66,000,000
District of Columbia Public Service Commission [Member]
Business Acquisition [Line Items]
Business Acquisition, Preacquisition Contingency, Amount of Settlement				78,000,000											78,000,000			78,000,000
Energy Efficiency Program [Member] | New Jersey Board Of Public Utilities [Member]
Business Acquisition [Line Items]
Business Acquisition, Preacquisition Contingency, Amount of Settlement				15,000,000											15,000,000			15,000,000
Energy Efficiency Program [Member] | Delaware Public Service Commission [Member]
Business Acquisition [Line Items]
Business Acquisition, Preacquisition Contingency, Amount of Settlement				2,000,000											2,000,000			2,000,000
Energy Efficiency Program [Member] | Maryland Public Service Commission [Member]
Business Acquisition [Line Items]
Business Acquisition, Preacquisition Contingency, Amount of Settlement				43,000,000											43,000,000			43,000,000
Workforce Development [Member] | Delaware Public Service Commission [Member]
Business Acquisition [Line Items]
Business Acquisition, Preacquisition Contingency, Amount of Settlement				2,000,000											2,000,000			2,000,000
Workforce Development [Member] | District of Columbia Public Service Commission [Member]
Business Acquisition [Line Items]
Business Acquisition, Preacquisition Contingency, Amount of Settlement				5,000,000											5,000,000			5,000,000
Green Sustainability Fund [Member] | Maryland Public Service Commission [Member]
Business Acquisition [Line Items]
Business Acquisition, Preacquisition Contingency, Amount of Settlement				14,000,000											14,000,000			14,000,000
Rate Bill Credits [Member] | District of Columbia Public Service Commission [Member]
Business Acquisition [Line Items]
Business Acquisition, Preacquisition Contingency, Amount of Settlement				26,000,000											26,000,000			26,000,000
OneTimeBillCredit [Member] | District of Columbia Public Service Commission [Member]
Business Acquisition [Line Items]
Business Acquisition, Preacquisition Contingency, Amount of Settlement				14,000,000											14,000,000			14,000,000
LowIncomeAssistance [Member] | District of Columbia Public Service Commission [Member]
Business Acquisition [Line Items]
Business Acquisition, Preacquisition Contingency, Amount of Settlement				16,000,000											16,000,000			16,000,000
Charitable Contributions [Member]
Business Acquisition [Line Items]
Business Acquisition, Preacquisition Contingency, Amount of Settlement				53,000,000											53,000,000			53,000,000
Charitable Contributions [Member] | District of Columbia Public Service Commission [Member]
Business Acquisition [Line Items]
Business Acquisition, Preacquisition Contingency, Amount of Settlement				19,000,000											19,000,000			19,000,000
Exelon Generation Co L L C [Member]
Business Acquisition [Line Items]
Other noncurrent assets			578,000,000	627,000,000					578,000,000						627,000,000	578,000,000		627,000,000
Total acquisition costs															40,000,000	386,000,000	0
Business Combination, Integration Related Costs															25,000,000	11,000,000
Revenues			386,000,000	4,294,000,000		4,768,000,000	4,232,000,000	5,840,000,000	4,802,000,000	4,412,000,000	3,789,000,000	4,390,000,000			19,135,000,000	17,393,000,000	15,630,000,000
Comprehensive Income (Loss), Net of Tax, Attributable to Parent															1,313,000,000	769,000,000	761,000,000
Net income			(42,000,000)	154,000,000		377,000,000	398,000,000	443,000,000	(91,000,000)	771,000,000	340,000,000	(185,000,000)			1,340,000,000	1,019,000,000	1,060,000,000
Merger and integration related costs																44,000,000
Expected pre-tax proceeds																1,800,000,000
Expected after-tax proceeds																1,400,000,000
Change in capital expenditures not paid															82,000,000	(61,000,000)	(107,000,000)
Development costs expensed															22,000,000	13,000,000	10,000,000
Baltimore Gas and Electric Company [Member]
Business Acquisition [Line Items]
Other noncurrent assets			9,000,000	8,000,000					9,000,000						8,000,000	9,000,000		8,000,000
Business Combination, Integration Related Costs															5,000,000	2,000,000
Revenues				746,000,000		725,000,000	628,000,000	1,036,000,000	761,000,000	697,000,000	653,000,000	1,054,000,000			3,135,000,000	3,165,000,000	3,065,000,000
Comprehensive Income (Loss), Net of Tax, Attributable to Parent															288,000,000	211,000,000	210,000,000
Net income				74,000,000		51,000,000	44,000,000	106,000,000	52,000,000	46,000,000	16,000,000	85,000,000			288,000,000	211,000,000	210,000,000
Merger and integration related costs																0
Maximum loss contingency																				$ 1,700,000.0
Change in capital expenditures not paid															(9,000,000)	25,000,000	48,000,000
Commonwealth Edison Co [Member]
Business Acquisition [Line Items]
Other noncurrent assets			237,000,000	324,000,000					237,000,000						324,000,000	237,000,000		324,000,000
Business Combination, Integration Related Costs															10,000,000	4,000,000
Revenues				1,196,000,000		1,376,000,000	1,148,000,000	1,185,000,000	1,079,000,000	1,222,000,000	1,128,000,000	1,134,000,000			4,905,000,000	4,564,000,000	4,464,000,000
Comprehensive Income (Loss), Net of Tax, Attributable to Parent															426,000,000	408,000,000	249,000,000
Net income				87,000,000		149,000,000	99,000,000	90,000,000	73,000,000	126,000,000	111,000,000	98,000,000			426,000,000	408,000,000	249,000,000
Merger and integration related costs																0
Change in capital expenditures not paid															34,000,000	78,000,000	(8,000,000)
PECO Energy Co [Member]
Business Acquisition [Line Items]
Other noncurrent assets			20,000,000	21,000,000					20,000,000						21,000,000	20,000,000		$ 21,000,000
Business Combination, Integration Related Costs															5,000,000	2,000,000
Revenues				645,000,000		740,000,000	661,000,000	985,000,000	750,000,000	693,000,000	656,000,000	993,000,000			3,032,000,000	3,094,000,000	3,100,000,000
Comprehensive Income (Loss), Net of Tax, Attributable to Parent															378,000,000	352,000,000	395,000,000
Net income				$ 79,000,000		$ 90,000,000	$ 70,000,000	$ 139,000,000	$ 98,000,000	$ 81,000,000	$ 84,000,000	$ 89,000,000			378,000,000	352,000,000	395,000,000
Merger and integration related costs																0
Change in capital expenditures not paid															$ (13,000,000)	$ 0	$ 13,000,000
Pepco Holdings [Member]
Business Acquisition [Line Items]
Share price				$ 27.25											$ 27.25			$ 27.25
Payments to acquire businesses															$ 6,900,000,000
Pepco Holdings [Member] | Minimum [Member]
Business Acquisition [Line Items]
Business exit costs															259,000,000
Pepco Holdings [Member] | Maximum [Member]
Business Acquisition [Line Items]
Other long-term investments				$ 180,000,000											180,000,000			$ 180,000,000
Business exit costs															293,000,000
Intergrys Energy Group Inc [Member] | Exelon Generation Co L L C [Member]
Business Acquisition [Line Items]
Total consideration transferred		$ 332,000,000
Bargain purchase gain (after-tax)		28,000,000
Derivative, Loss on Derivative		$ 108,000,000
Merger and integration related costs			$ 7,000,000												$ 5,000,000

[1] Includes charges to earnings related to the impairments of certain generating assets which were held for sale and certain Upstream exploration assets. See Note 8—Impairment of Long-Lived Assets of the Combined Notes to Consolidated Financial Statements for additional information.

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[3] (e)See Note 14—Debt and Credit Agreements and 4—Mergers, Acquisitions, and Dispositions for additional information on the PHI merger related debt exchange.