Regulatory Matters Regulatory Matters - Purchase of Receivables Programs (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Purchase Of Receivables [Line Items]
Payments to Acquire Other Receivables		$ 317	$ 298
Allowance for Doubtful Accounts Receivable	[1]	(35)	(31)
Payments to Acquire Other Receivables, Net		282	267
Commonwealth Edison Co [Member]
Purchase Of Receivables [Line Items]
Payments to Acquire Other Receivables		88	87
Allowance for Doubtful Accounts Receivable	[1]	(16)	(14)
Payments to Acquire Other Receivables, Net		72	73
PECO Energy Co [Member]
Purchase Of Receivables [Line Items]
Payments to Acquire Other Receivables		73	70
Allowance for Doubtful Accounts Receivable	[1]	(6)	(5)
Payments to Acquire Other Receivables, Net		67	65
Baltimore Gas and Electric Company [Member]
Purchase Of Receivables [Line Items]
Payments to Acquire Other Receivables		64	58
Allowance for Doubtful Accounts Receivable	[1]	(4)	(3)
Payments to Acquire Other Receivables, Net		60	55
Pepco Holdings LLC [Member]
Purchase Of Receivables [Line Items]
Payments to Acquire Other Receivables		92	83
Allowance for Doubtful Accounts Receivable	[1]	(9)	(9)
Payments to Acquire Other Receivables, Net		83	74
Potomac Electric Power Company [Member]
Purchase Of Receivables [Line Items]
Payments to Acquire Other Receivables		55	56
Allowance for Doubtful Accounts Receivable	[1]	(5)	(5)
Payments to Acquire Other Receivables, Net		50	51
Delmarva Power and Light Company [Member]
Purchase Of Receivables [Line Items]
Payments to Acquire Other Receivables		17	9
Allowance for Doubtful Accounts Receivable	[1]	(1)	(1)
Payments to Acquire Other Receivables, Net		16	8
Atlantic City Electric Company [Member]
Purchase Of Receivables [Line Items]
Payments to Acquire Other Receivables		20	18
Allowance for Doubtful Accounts Receivable	[1]	(3)	(3)
Payments to Acquire Other Receivables, Net		$ 17	$ 15

[1] For ComEd, BGE, Pepco and DPL, reflects the incremental allowance for uncollectible accounts recorded, which is in addition to the purchase discount. For ComEd, the incremental uncollectible accounts expense is recovered through its Purchase of Receivables with Consolidated Billing tariff.