Regulatory Matters Regulatory Matters - Estimated Commitments related to PJM Agreements (Details) - USD ($) $ in Millions		Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Mar. 31, 2017
Other Commitments [Line Items]
Other Receivables		$ 1,256		$ 1,132
Total		127
Accounts Payable, Current		3,800		3,532
Regulatory Assets		9,459		9,288
Regulatory Liabilities		10,203		10,388
Exelon Generation Co L L C [Member]
Other Commitments [Line Items]
Other Receivables		562		321
Accounts Payable, Current		1,847		1,773
Commonwealth Edison Co [Member]
Other Commitments [Line Items]
Other Receivables		320		266
Accounts Payable, Current		607		568
Regulatory Assets				1,279
Regulatory Liabilities		6,343		6,577
PECO Energy Co [Member]
Other Commitments [Line Items]
Other Receivables		151		105
Accounts Payable, Current		370		370
Regulatory Assets				410
Regulatory Liabilities		596		690
Baltimore Gas and Electric Company [Member]
Other Commitments [Line Items]
Other Receivables		90		94
Accounts Payable, Current		295		265
Regulatory Assets				571
Regulatory Liabilities		1,269		1,163
Potomac Electric Power Company [Member]
Other Commitments [Line Items]
Other Receivables		81		87
Accounts Payable, Current		214		139
Regulatory Assets				891
Regulatory Liabilities		829		832
Delmarva Power & Light Company [Member]
Other Commitments [Line Items]
Other Receivables		46		38
Accounts Payable, Current		111		82
Regulatory Assets				314
Regulatory Liabilities		665		635
Atlantic City Electric Company [Member]
Other Commitments [Line Items]
Other Receivables		55		56
Accounts Payable, Current		154		118
Regulatory Assets				430
Regulatory Liabilities		420		422
Pepco Holdings LLC [Member]
Other Commitments [Line Items]
Other Receivables		177		206
Total					$ 444
Accounts Payable, Current		496		348
Regulatory Assets				3,047
Regulatory Liabilities		1,948		$ 1,928
PJM Transmission Rate Design Settlement [Member]
Other Commitments [Line Items]
Other Receivables			$ 220
Accounts Payable, Current			176
PJM Transmission Rate Design Settlement [Member] | Exelon Generation Co L L C [Member]
Other Commitments [Line Items]
Other Receivables	[1]		0
Accounts Payable, Current	[1]		41
Accounts Receivable, Gross, Current		$ 16
PJM Transmission Rate Design Settlement [Member] | Commonwealth Edison Co [Member]
Other Commitments [Line Items]
Other Receivables			122
Accounts Payable, Current			0
PJM Transmission Rate Design Settlement [Member] | PECO Energy Co [Member]
Other Commitments [Line Items]
Other Receivables			85
Accounts Payable, Current			0
PJM Transmission Rate Design Settlement [Member] | Baltimore Gas and Electric Company [Member]
Other Commitments [Line Items]
Other Receivables			0
Accounts Payable, Current			51
PJM Transmission Rate Design Settlement [Member] | Potomac Electric Power Company [Member]
Other Commitments [Line Items]
Other Receivables			0
Accounts Payable, Current			84
PJM Transmission Rate Design Settlement [Member] | Delmarva Power & Light Company [Member]
Other Commitments [Line Items]
Other Receivables			10
Accounts Payable, Current			0
PJM Transmission Rate Design Settlement [Member] | Atlantic City Electric Company [Member]
Other Commitments [Line Items]
Other Receivables			3
Accounts Payable, Current			0
PJM Transmission Rate Design Settlement [Member] | Pepco Holdings LLC [Member]
Other Commitments [Line Items]
Other Receivables			13
Accounts Payable, Current			84
PJM Transmission Rate Design Settlement [Member]
Other Commitments [Line Items]
Regulatory Assets			136
PJM Transmission Rate Design Settlement [Member] | Exelon Generation Co L L C [Member]
Other Commitments [Line Items]
Regulatory Assets	[1]		0
PJM Transmission Rate Design Settlement [Member] | Commonwealth Edison Co [Member]
Other Commitments [Line Items]
Regulatory Assets			0
PJM Transmission Rate Design Settlement [Member] | PECO Energy Co [Member]
Other Commitments [Line Items]
Regulatory Assets			0
PJM Transmission Rate Design Settlement [Member] | Baltimore Gas and Electric Company [Member]
Other Commitments [Line Items]
Regulatory Assets			51
PJM Transmission Rate Design Settlement [Member] | Potomac Electric Power Company [Member]
Other Commitments [Line Items]
Regulatory Assets			84
PJM Transmission Rate Design Settlement [Member] | Delmarva Power & Light Company [Member]
Other Commitments [Line Items]
Regulatory Assets			0
PJM Transmission Rate Design Settlement [Member] | Atlantic City Electric Company [Member]
Other Commitments [Line Items]
Regulatory Assets			1
PJM Transmission Rate Design Settlement [Member] | Pepco Holdings LLC [Member]
Other Commitments [Line Items]
Regulatory Assets			85
PJM Transmission Rate Design Settlement [Member]
Other Commitments [Line Items]
Regulatory Liabilities			221
PJM Transmission Rate Design Settlement [Member] | Exelon Generation Co L L C [Member]
Other Commitments [Line Items]
Regulatory Liabilities	[1]		0
PJM Transmission Rate Design Settlement [Member] | Commonwealth Edison Co [Member]
Other Commitments [Line Items]
Regulatory Liabilities			122
PJM Transmission Rate Design Settlement [Member] | PECO Energy Co [Member]
Other Commitments [Line Items]
Regulatory Liabilities			85
PJM Transmission Rate Design Settlement [Member] | Baltimore Gas and Electric Company [Member]
Other Commitments [Line Items]
Regulatory Liabilities			0
PJM Transmission Rate Design Settlement [Member] | Potomac Electric Power Company [Member]
Other Commitments [Line Items]
Regulatory Liabilities			0
PJM Transmission Rate Design Settlement [Member] | Delmarva Power & Light Company [Member]
Other Commitments [Line Items]
Regulatory Liabilities			10
PJM Transmission Rate Design Settlement [Member] | Atlantic City Electric Company [Member]
Other Commitments [Line Items]
Regulatory Liabilities			4
PJM Transmission Rate Design Settlement [Member] | Pepco Holdings LLC [Member]
Other Commitments [Line Items]
Regulatory Liabilities			$ 14

[1] Does not include an offsetting receivable from New Jersey Utilities of $16 million as of December 31, 2018.