Discontinued Operations - Additional Information (Detail) (USD $)	3 Months Ended			12 Months Ended		9 Months Ended
	Mar. 31, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2014	Dec. 31, 2012	Sep. 30, 2013	Dec. 31, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
After-tax non-cash charges	$ 377,000,000
After-tax effect of revised lease rerun	313,000,000
Pepco Energy Services [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Derivatives				0			0
Cash collateral posted				2,000,000			3,000,000
Letters of credit posted				0
Pepco Energy Services [Member] | Less Than [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Letters of credit posted							1,000,000
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
After-tax non-cash charges	323,000,000		6,000,000
Non-cash pre-tax charge	373,000,000
After-tax effect of revised lease rerun	313,000,000
After-tax non-cash charge	16,000,000
Penalties associated with re-assessment of tax positions				0
Maximum percentage of penalty on the amount of additional taxes due				20.00%
Amount of net pre-tax loss arising during the period included in Accumulated Other Comprehensive Loss					(10,000,000)
Amount of net tax loss arising during the period included in Accumulated Other Comprehensive Loss					(6,000,000)
Pre-tax loss reclassified into Income from Discontinued operations, Before Taxes	(4,000,000)	(6,000,000)
Net of tax loss reclassified into Income from Discontinued operations, Net of Income Taxes	(2,000,000)
Discontinued Operations [Member] | PHI [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net pre-tax gain / loss						(3,000,000)
Loss on early termination of finance leases held in trust, after-tax						(2,000,000)
After-tax non-cash charge	70,000,000
Discontinued Operations [Member] | PHI [Member] | Proceeds From Early Termination [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Aggregate of net cash proceeds (payments) of lease investments						873,000,000
Discontinued Operations [Member] | PHI [Member] | Payment Paid From Early Termination [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Aggregate of net cash proceeds (payments) of lease investments						2,000,000,000
Discontinued Operations [Member] | Power Delivery [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Interest benefit on uncertain tax positions	12,000,000
Discontinued Operations [Member] | Other Non-Regulated [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Interest expense on uncertain tax positions	16,000,000
Discontinued Operations [Member] | Corporate and Other [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Interest expense on uncertain tax positions	$ 66,000,000