Fair Value Disclosures - Reconciliations of Fair Value Measurements Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Preferred Stock [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 3
Included in income	0	$ 0
Included in accumulated other comprehensive loss	0	0
Included in regulatory liabilities and regulatory assets	0	0
Purchases	0	0
Issuances		3
Settlements	0	0
Transfers in (out) of Level 3	0	0
Ending balance	3	3
Life Insurance Contracts [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	19	19
Included in income	3	2
Included in accumulated other comprehensive loss	0	0
Included in regulatory liabilities and regulatory assets	0	0
Purchases	0	0
Issuances	(1)	(1)
Settlements	(1)
Transfers in (out) of Level 3	0	0
Ending balance	20	20
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	2
Included in accumulated other comprehensive loss	0	0
Purchases	0	0
Issuances	(1)	(1)
Settlements	0	0
Transfers in (out) of Level 3	0	0
Ending balance	20	19
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	0	0
Included in accumulated other comprehensive loss	0	0
Included in regulatory liabilities and regulatory assets	0	0
Purchases	0	0
Issuances	0	0
Settlements	(1)
Transfers in (out) of Level 3	$ 0	0
Ending balance		$ 1