Fair Value Disclosures - Reconciliations of Fair Value Measurements Using Significant Unobservable Inputs (Level 3) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Preferred Stock [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 0
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	2	3
Included in accumulated other comprehensive loss
Included in regulatory liabilities and regulatory assets
Purchases
Issuances	(1)	(1)
Settlements		(1)
Transfers in (out) of level 3
Ending balance	20	19
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
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)
Potomac Electric Power Co [Member] | Life Insurance Contracts [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	18	18
Included in income	2	3
Included in accumulated other comprehensive loss
Purchases
Issuances	(1)	(1)
Settlements		(1)
Transfers in (out) of level 3
Ending balance	19	19
Delmarva Power & Light Co/De [Member] | Life Insurance Contracts [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
Delmarva Power & Light Co/De [Member] | 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
Atlantic City Electric Co [Member] | 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)