Fair Value Disclosures - Reconciliations of Fair Value Measurements Using Significant Unobservable Inputs (Level 3) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Preferred Stock [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance
Included in income
Included in accumulated other comprehensive loss
Included in regulatory liabilities and regulatory assets
Purchases
Issuances	3
Settlements
Transfers in (out) of level 3
Ending balance	3
Life Insurance Contracts [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	19	18
Included in income	3	3
Included in accumulated other comprehensive loss
Included in regulatory liabilities and regulatory assets
Purchases
Issuances	(3)	(3)
Settlements
Transfers in (out) of level 3
Ending balance	19	18
Life Insurance Contracts [Member] | Potomac Electric Power Co [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	18	18
Included in income	3	3
Included in accumulated other comprehensive loss
Purchases
Issuances	(3)	(3)
Settlements		(1)
Transfers in (out) of level 3
Ending balance	18	17
Life Insurance Contracts [Member] | Delmarva Power & Light Co/De [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	1	1
Included in income
Included in accumulated other comprehensive loss
Included in regulatory liabilities and regulatory assets
Purchases
Issuances
Settlements
Transfers in (out) of level 3
Ending balance	1	1
Natural Gas [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		(4)
Included in income
Included in accumulated other comprehensive loss
Included in regulatory liabilities and regulatory assets
Purchases
Issuances
Settlements		4
Transfers in (out) of level 3
Ending balance
Natural Gas [Member] | Delmarva Power & Light Co/De [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		(4)
Included in income
Included in accumulated other comprehensive loss
Included in regulatory liabilities and regulatory assets
Purchases
Issuances
Settlements		4
Transfers in (out) of level 3
Ending balance
Capacity [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		(3)
Included in income
Included in accumulated other comprehensive loss
Included in regulatory liabilities and regulatory assets		(7)
Purchases
Issuances
Settlements
Transfers in (out) of level 3
Ending balance		(10)
Capacity [Member] | Atlantic City Electric Co [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		(3)
Included in income
Included in accumulated other comprehensive loss
Included in regulatory liabilities and regulatory assets		(7)
Purchases
Issuances
Settlements
Transfers in (out) of level 3
Ending balance		$ (10)