Income Taxes - Components of Consolidated Deferred Tax Liabilities (Assets) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Deferred Income Tax Assets And Liabilities [Line Items]
Depreciation and other basis differences related to plant and equipment	$ 2,962	$ 2,628
Deferred electric service and electric restructuring liabilities	67	91
Cross-border energy lease investments		(6)
Federal and state net operating losses	(400)	(350)
Valuation allowances on state net operating losses	61	21
Pension and other postretirement benefits	116	135
Deferred taxes on amounts to be collected through future rates	94	75
Other	325	285
Total Deferred Tax Liabilities, net	3,225	2,879
Deferred tax assets included in Current Assets	50	51
Deferred tax liabilities included in Other Current Liabilities	(9)	(2)
Total Consolidated Deferred Tax Liabilities, net non-current	3,266	2,928
Potomac Electric Power Co [Member]
Schedule Of Deferred Income Tax Assets And Liabilities [Line Items]
Depreciation and other basis differences related to plant and equipment	1,423	1,240
Federal and state net operating losses	(186)	(169)
Pension and other postretirement benefits	103	105
Deferred taxes on amounts to be collected through future rates	59	43
Other	180	145
Total Deferred Tax Liabilities, net	1,579	1,364
Deferred tax assets included in Current Assets	14	48
Deferred tax liabilities included in Other Current Liabilities	(9)
Total Consolidated Deferred Tax Liabilities, net non-current	1,584	1,412
Delmarva Power & Light Co/De [Member]
Schedule Of Deferred Income Tax Assets And Liabilities [Line Items]
Depreciation and other basis differences related to plant and equipment	797	712
Deferred electric service and electric restructuring liabilities	(4)	(5)
Federal and state net operating losses	(115)	(125)
Pension and other postretirement benefits	80	80
Deferred taxes on amounts to be collected through future rates	19	16
Other	101	80
Total Deferred Tax Liabilities, net	878	758
Deferred tax assets included in Current Assets	16	59
Deferred tax liabilities included in Other Current Liabilities	(1)	(1)
Total Consolidated Deferred Tax Liabilities, net non-current	893	816
Atlantic City Electric Co [Member]
Schedule Of Deferred Income Tax Assets And Liabilities [Line Items]
Depreciation and other basis differences related to plant and equipment	691	627
Deferred electric service and electric restructuring liabilities	71	96
Federal and state net operating losses	(26)	(49)
Pension and other postretirement benefits	25	29
Deferred taxes on amounts to be collected through future rates	16	16
Payment for termination of purchased power contracts with NUGs	38	43
Purchased energy	1	2
Other	39	55
Total Deferred Tax Liabilities, net	855	819
Deferred tax assets included in Current Assets	10	15
Deferred tax liabilities included in Other Current Liabilities		(1)
Total Consolidated Deferred Tax Liabilities, net non-current	$ 865	$ 833