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Fair Value Disclosures - Reconciliations of Fair Value Measurements Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Preferred Stock [Member]    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning balance $ 3  
Included in income 15  
Included in accumulated other comprehensive loss 0 $ 0
Included in regulatory liabilities 0 0
Purchases 0 0
Issuances   3
Transfers in (out) of Level 3 0 0
Ending balance 18 3
Life Insurance Contracts [Member]    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning balance 19 19
Included in income 5 3
Included in accumulated other comprehensive loss 0 0
Included in regulatory liabilities 0 0
Purchases 0 0
Issuances (3) (3)
Settlements (2)  
Transfers in (out) of Level 3 0 0
Ending balance 19 19
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 5 3
Included in accumulated other comprehensive loss 0 0
Purchases 0 0
Issuances (3) (3)
Settlements (1)  
Transfers in (out) of Level 3 0 0
Ending balance 19 18
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 accumulated other comprehensive loss 0 0
Included in regulatory liabilities 0 0
Purchases 0 0
Settlements (1)  
Transfers in (out) of Level 3 $ 0 0
Ending balance   $ 1