Regulatory Matters Regulatory Matters - Purchase of Receivables Programs (Details) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Purchase Of Receivables [Line Items]
Payments to Acquire Other Receivables		$ 379	$ 298
Allowance for Doubtful Accounts Receivable	[1]	(37)	(31)
Payments to Acquire Other Receivables, Net		342	267
Commonwealth Edison Co [Member]
Purchase Of Receivables [Line Items]
Payments to Acquire Other Receivables		120	87
Allowance for Doubtful Accounts Receivable	[1]	(19)	(14)
Payments to Acquire Other Receivables, Net		101	73
PECO Energy Co [Member]
Purchase Of Receivables [Line Items]
Payments to Acquire Other Receivables		91	70
Allowance for Doubtful Accounts Receivable	[1]	(5)	(5)
Payments to Acquire Other Receivables, Net		86	65
Baltimore Gas and Electric Company [Member]
Purchase Of Receivables [Line Items]
Payments to Acquire Other Receivables		60	58
Allowance for Doubtful Accounts Receivable	[1]	(3)	(3)
Payments to Acquire Other Receivables, Net		57	55
Pepco Holdings LLC [Member]
Purchase Of Receivables [Line Items]
Payments to Acquire Other Receivables		108	83
Allowance for Doubtful Accounts Receivable	[1]	(10)	(9)
Payments to Acquire Other Receivables, Net		98	74
Potomac Electric Power Company [Member]
Purchase Of Receivables [Line Items]
Payments to Acquire Other Receivables		69	56
Allowance for Doubtful Accounts Receivable	[1]	(5)	(5)
Payments to Acquire Other Receivables, Net		64	51
Delmarva Power and Light Company [Member]
Purchase Of Receivables [Line Items]
Payments to Acquire Other Receivables		11	9
Allowance for Doubtful Accounts Receivable	[1]	(1)	(1)
Payments to Acquire Other Receivables, Net		10	8
Atlantic City Electric Company [Member]
Purchase Of Receivables [Line Items]
Payments to Acquire Other Receivables		28	18
Allowance for Doubtful Accounts Receivable	[1]	(4)	(3)
Payments to Acquire Other Receivables, Net		$ 24	$ 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.