Discontinued Operations - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended			9 Months Ended	12 Months Ended
	Jan. 31, 2011	Dec. 31, 2015	Jun. 30, 2013	Mar. 31, 2013	Sep. 30, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
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
Penalties on tax settlement	$ 1,000,000			0
Pre-tax loss reclassified into Income from Discontinued operations, Before Taxes						$ (1,000,000)	$ (1,000,000)	$ (1,000,000)
Cross-Border Energy Lease Investments [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income tax benefit, income from operations of discontinued operations		$ (9,000,000)				9,000,000		44,000,000
Non-cash pre-tax charge								373,000,000
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
Pepco Energy Services [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Derivatives		0				0	$ 0	$ 0
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
After-tax non-cash charges			$ 6,000,000	323,000,000
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				0
Penalties on tax settlement						$ 0
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			$ (6,000,000)	(4,000,000)
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