Regulatory Matters - Schedule of Regulatory Assets and Regulatory Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Public Utilities, General Disclosures [Line Items]
Regulatory assets	$ 2,409	$ 2,087
Regulatory Liabilities	343	399
Pension and Other Postretirement Benefit Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	946	667
Securitized Stranded Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	278	350
Recoverable Income Taxes [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	274	225
Demand-Side Management Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	264	125
Smart Grid Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	261	251
Deferred Energy Supply Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	73	136
Incremental Storm Restoration Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	51	72
Deferred Debt Extinguishment Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	42	47
MAPP Abandonment Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	33	68
Recoverable Workers' Compensation and Long-Term Disability Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	30	26
Deferred Losses on Gas Derivatives [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	4
Other Regulatory Assets [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	153	120
Asset Removal Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	250	275
Deferred Income Taxes Due to Customers [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	44	45
Deferred Energy Supply Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	3	46
Deferred Gains on Gas Derivatives [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities		1
Other Regulatory Liabilities [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	46	32
Potomac Electric Power Co [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	697	563
Regulatory Liabilities	104	113
Potomac Electric Power Co [Member] | Recoverable Income Taxes [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	148	107
Potomac Electric Power Co [Member] | Demand-Side Management Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	197	98
Potomac Electric Power Co [Member] | Smart Grid Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	175	168
Potomac Electric Power Co [Member] | Deferred Energy Supply Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	3	6
Potomac Electric Power Co [Member] | Incremental Storm Restoration Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	29	37
Potomac Electric Power Co [Member] | Deferred Debt Extinguishment Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	22	25
Potomac Electric Power Co [Member] | MAPP Abandonment Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	19	37
Potomac Electric Power Co [Member] | Recoverable Workers' Compensation and Long-Term Disability Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	30	26
Potomac Electric Power Co [Member] | Other Regulatory Assets [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	74	59
Potomac Electric Power Co [Member] | Asset Removal Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	84	102
Potomac Electric Power Co [Member] | Other Regulatory Liabilities [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	20	11
Delmarva Power & Light Co/De [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	356	311
Regulatory Liabilities	225	229
Delmarva Power & Light Co/De [Member] | Recoverable Income Taxes [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	84	76
Delmarva Power & Light Co/De [Member] | Demand-Side Management Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	67	27
Delmarva Power & Light Co/De [Member] | Smart Grid Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	86	83
Delmarva Power & Light Co/De [Member] | Deferred Energy Supply Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	12	13
Delmarva Power & Light Co/De [Member] | Incremental Storm Restoration Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	7	9
Delmarva Power & Light Co/De [Member] | Deferred Debt Extinguishment Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	12	13
Delmarva Power & Light Co/De [Member] | MAPP Abandonment Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	14	31
Delmarva Power & Light Co/De [Member] | Deferred Losses on Gas Derivatives [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	4
Delmarva Power & Light Co/De [Member] | Other Regulatory Assets [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	52	37
Delmarva Power & Light Co/De [Member] | COPCO Acquisition Adjustment [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	18	22
Delmarva Power & Light Co/De [Member] | Asset Removal Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	166	173
Delmarva Power & Light Co/De [Member] | Deferred Income Taxes Due to Customers [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	37	37
Delmarva Power & Light Co/De [Member] | Deferred Energy Supply Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities		3
Delmarva Power & Light Co/De [Member] | Deferred Gains on Gas Derivatives [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities		1
Delmarva Power & Light Co/De [Member] | Other Regulatory Liabilities [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	22	15
Atlantic City Electric Co [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	427	569
Regulatory Liabilities	14	57
Atlantic City Electric Co [Member] | Securitized Stranded Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	278	350
Atlantic City Electric Co [Member] | Recoverable Income Taxes [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	42	42
Atlantic City Electric Co [Member] | Deferred Energy Supply Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	58	117
Atlantic City Electric Co [Member] | Incremental Storm Restoration Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	15	26
Atlantic City Electric Co [Member] | Other Regulatory Assets [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory assets	34	34
Atlantic City Electric Co [Member] | Deferred Energy Supply Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities		38
Atlantic City Electric Co [Member] | Other Regulatory Liabilities [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	6	6
Atlantic City Electric Co [Member] | Federal and State Tax Benefits, Related to Securitized Stranded Costs [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 8	$ 13