Significant Accounting Policies - Annual Depreciation Rates (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Transmission and Distribution [Member] | Potomac Electric Power Co [Member]
Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	2.30%	2.20%	2.50%
Transmission and Distribution [Member] | Delmarva Power & Light Co/De [Member]
Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	2.60%	2.60%	2.70%
Transmission and Distribution [Member] | Atlantic City Electric Co [Member]
Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	2.60%	2.80%	3.00%
Transmission and Distribution [Member] | Pepco Energy Services [Member]
Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	0.00%
Generation [Member] | Potomac Electric Power Co [Member]
Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	0.00%
Generation [Member] | Delmarva Power & Light Co/De [Member]
Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	0.00%
Generation [Member] | Atlantic City Electric Co [Member]
Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	0.00%
Generation [Member] | Pepco Energy Services [Member]
Property, Plant and Equipment [Line Items]
Annual Depreciation Rate	1.20%	0.40%	6.40%