Income Taxes - Narrative (Details) - USD ($) $ in Millions		3 Months Ended				9 Months Ended			12 Months Ended
		Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	[1]	Sep. 30, 2018	Sep. 30, 2017	[1]	Dec. 31, 2017	Jul. 27, 2018	Dec. 31, 2009
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%		35.00%		21.00%	35.00%		35.00%
Allowable Expensing For Cost Of Qualified Property						30.00%
Limitation on Deductible Interest Expense						100.00%
Net Regulatory Liabilities		$ 10,445				$ 10,445			$ 10,388
Unrecognized tax benefits that if recognized would affect the effective tax rate		473				473				$ 20
Decrease in Unrecognized Tax Benefits is Reasonably Possible		515				515
Regulatory Assets		9,342				9,342			9,288
Like kind exchange [Member]
Income Tax Additional Narrative Information [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible		$ 33				33
Deferred Gain on Sale of Property											$ 1,200
IRS asserted penalties for understatement of tax						$ 90
Exelon Generation Co L L C [Member]
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%		35.00%		21.00%	35.00%
Increase in Unrecognized Tax Benefits is Reasonably Possible										71
Unrecognized tax benefits that if recognized would affect the effective tax rate		$ 473				$ 473				$ 20
Decrease in Unrecognized Tax Benefits is Reasonably Possible		$ 501				$ 501
Commonwealth Edison Co [Member]
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%		35.00%		21.00%	35.00%
Net Regulatory Liabilities		$ 6,638				$ 6,638			6,577
Regulatory Assets		1,485				1,485			1,279
Commonwealth Edison Co [Member] | Like kind exchange [Member]
Income Tax Additional Narrative Information [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible		$ 2				$ 2
PECO Energy Co [Member]
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%		35.00%		21.00%	35.00%
Net Regulatory Liabilities		$ 740				$ 740			690
Regulatory Assets		$ 532				$ 532			410
Baltimore Gas and Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%		35.00%		21.00%	35.00%
Net Regulatory Liabilities		$ 1,165				$ 1,165			1,163
Regulatory Assets		$ 597				$ 597			571
Pepco Holdings LLC [Member]
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%		35.00%		21.00%	35.00%
Net Regulatory Liabilities		$ 1,882				$ 1,882			1,928
Decrease in Unrecognized Tax Benefits is Reasonably Possible		14				14
Regulatory Assets		$ 2,895				$ 2,895			3,047
Potomac Electric Power Company [Member]
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%		35.00%		21.00%	35.00%
Net Regulatory Liabilities		$ 764				$ 764			832
Regulatory Assets		$ 946				$ 946			891
Delmarva Power and Light Company [Member]
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%		35.00%		21.00%	35.00%
Net Regulatory Liabilities		$ 654				$ 654			635
Regulatory Assets		$ 301				$ 301			314
Atlantic City Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%		35.00%		21.00%	35.00%
Net Regulatory Liabilities		$ 428				$ 428			422
Decrease in Unrecognized Tax Benefits is Reasonably Possible		14				14
Regulatory Assets		427				427			430
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers Subject To IRS Normalization Rules
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									3,040
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers Subject To IRS Normalization Rules | Commonwealth Edison Co [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									1,400
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers Subject To IRS Normalization Rules | PECO Energy Co [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities	[2]								533
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers Subject To IRS Normalization Rules | Baltimore Gas and Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									459
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers Subject To IRS Normalization Rules | Pepco Holdings LLC [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									648
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers Subject To IRS Normalization Rules | Potomac Electric Power Company [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									299
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers Subject To IRS Normalization Rules | Delmarva Power and Light Company [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									195
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers Subject To IRS Normalization Rules | Atlantic City Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									153
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers Subject To Rate Regulator Determinations
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									1,694
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers Subject To Rate Regulator Determinations | Commonwealth Edison Co [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									573
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers Subject To Rate Regulator Determinations | PECO Energy Co [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities	[2]								43
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers Subject To Rate Regulator Determinations | Baltimore Gas and Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									324
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers Subject To Rate Regulator Determinations | Pepco Holdings LLC [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									754
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers Subject To Rate Regulator Determinations | Potomac Electric Power Company [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									391
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers Subject To Rate Regulator Determinations | Delmarva Power and Light Company [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									194
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers Subject To Rate Regulator Determinations | Atlantic City Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									170
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									4,734
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers | Commonwealth Edison Co [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									1,973
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers | PECO Energy Co [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities	[2]								576
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers | Baltimore Gas and Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									783
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers | Pepco Holdings LLC [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									1,402
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers | Potomac Electric Power Company [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									690
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers | Delmarva Power and Light Company [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									389
Net Regulatory Liabilities Associated With Ultimate Settlement To Customers | Atlantic City Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities									$ 323
State and Local Jurisdiction [Member]
Income Tax Additional Narrative Information [Line Items]
Unrecognized tax benefits that if recognized would affect the effective tax rate		26				26
Decrease in Unrecognized Tax Benefits is Reasonably Possible		241				241
Increase (Decrease) in Deferred Income Taxes		(50)	$ 16
Increase (Decrease) in Income Tax Expense		(50)
State and Local Jurisdiction [Member] | Exelon Generation Co L L C [Member]
Income Tax Additional Narrative Information [Line Items]
Unrecognized tax benefits that if recognized would affect the effective tax rate		26				26
Decrease in Unrecognized Tax Benefits is Reasonably Possible		33				33
Increase (Decrease) in Deferred Income Taxes		(53)	5
Increase (Decrease) in Income Tax Expense		(53)
State and Local Jurisdiction [Member] | Baltimore Gas and Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible		120				120
State and Local Jurisdiction [Member] | Pepco Holdings LLC [Member]
Income Tax Additional Narrative Information [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible		88				88
Increase (Decrease) in Deferred Income Taxes		(4)	17
Increase (Decrease) in Income Tax Expense		(3)
State and Local Jurisdiction [Member] | Potomac Electric Power Company [Member]
Income Tax Additional Narrative Information [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible		67				67
Increase (Decrease) in Deferred Income Taxes		1	16
State and Local Jurisdiction [Member] | Delmarva Power and Light Company [Member]
Income Tax Additional Narrative Information [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible		21				21
Increase (Decrease) in Deferred Income Taxes		(2)	$ 1
State and Local Jurisdiction [Member] | Net Regulatory Liabilities Associated With Marginal State Income Tax Rates [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities		1				1
State and Local Jurisdiction [Member] | Net Regulatory Liabilities Associated With Marginal State Income Tax Rates [Member] | Pepco Holdings LLC [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities		1				1
State and Local Jurisdiction [Member] | Net Regulatory Liabilities Associated With Marginal State Income Tax Rates [Member] | Delmarva Power and Light Company [Member]
Income Tax Additional Narrative Information [Line Items]
Net Regulatory Liabilities		2				2
State and Local Jurisdiction [Member] | Net Regulatory Assets Associated With Marginal State Income Tax Rates [Member] | Potomac Electric Power Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Assets		$ 1				$ 1

[1] Exelon retrospectively adopted the new standard Revenue from Contracts with Customers. The standard was adopted as of January 1, 2018. The effective income tax rates are recast to reflect the impact of the new standard.

[2] Given the regulatory treatment of income tax benefits related to electric and gas distribution repairs, PECO remains in an overall net regulatory asset position as of December 31, 2017 after recording the impacts related to the TCJA. As a result, the amount of customer benefits resulting from the TCJA subject to the discretion of PECO's rate regulators are lower relative to the other Utility Registrants. See Note 6 - Regulatory Matters for additional information.