Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2010
Significant Accounting Policies [Line Items]
Unbilled revenue	$ 172	$ 177
Taxes included in gross revenues	321	346	356
Restricted stock awards vesting period	1 year	1 year
Unbilled receivables billing period	1 month
Estimated fair value of an asset	Greater than 50%
AFUDC for borrowed funds	7	7	7
AFUDC for equity component	13	11	14
Number of subsidiaries, allocated	3
Retained earnings	565	595
Asset Removal Costs [Member]
Significant Accounting Policies [Line Items]
Regulatory liabilities	250	275
Maximum [Member]
Significant Accounting Policies [Line Items]
Original maturities of cash and cash equivalents	3 months
Time-Based Restricted Stock [Member]
Significant Accounting Policies [Line Items]
Restricted stock awards vesting period	3 years
Performance Based Restricted Stock Units [Member]
Significant Accounting Policies [Line Items]
Restricted stock awards vesting period	3 years
Potomac Electric Power Co [Member]
Significant Accounting Policies [Line Items]
Unbilled revenue	77	80
Taxes included in gross revenues	304	318	324
Unbilled receivables billing period	1 month
Awards received from the U.S. DOE				149
AFUDC for borrowed funds	5	5	4
AFUDC for equity component	10	9	8
Preferred stock, shares outstanding	0
Retained earnings	1,077	992
Potomac Electric Power Co [Member] | Asset Removal Costs [Member]
Significant Accounting Policies [Line Items]
Regulatory liabilities	84	102
Potomac Electric Power Co [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Original maturities of cash and cash equivalents	3 months
Potomac Electric Power Co [Member] | Transmission and Distribution [Member]
Significant Accounting Policies [Line Items]
Annual Depreciation Rate	2.30%	2.20%	2.50%
Atlantic City Electric Co [Member]
Significant Accounting Policies [Line Items]
Unbilled revenue	32	36
Taxes included in gross revenues	1	11	15
Unbilled receivables billing period	1 month
Awards received from the U.S. DOE				19
AFUDC for borrowed funds	1		2
AFUDC for equity component	1		3
Retained earnings	209	190
Securitization debt capitalization rate	30.00%
Prior period item reclassification adjustment in Net cash from operating activities		246	133
Prior period item reclassification adjustment change in Net cash from operating activities		5	3
Net Cash From Operating Activities		241	136
Prior period item reclassification adjustment in cash from financing activities		7	37
Prior period item reclassification adjustment Change in cash from financing activities		5	3
Net Cash From Financing Activities		12	34
Atlantic City Electric Co [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Original maturities of cash and cash equivalents	3 months
AFUDC for borrowed funds		1
AFUDC for equity component		1
Atlantic City Electric Co [Member] | Transmission and Distribution [Member]
Significant Accounting Policies [Line Items]
Annual Depreciation Rate	2.60%	2.80%	3.00%
Delmarva Power & Light Co/De [Member]
Significant Accounting Policies [Line Items]
Unbilled revenue	63	61
Taxes included in gross revenues	16	17	15
Unbilled receivables billing period	1 month
Estimated fair value of an asset	Greater than 50%
AFUDC for borrowed funds	1	2	2
AFUDC for equity component	2	2	3
Preferred stock, shares outstanding	0
Retained earnings	641	637
Delmarva Power & Light Co/De [Member] | Asset Removal Costs [Member]
Significant Accounting Policies [Line Items]
Regulatory liabilities	$ 166	$ 173
Delmarva Power & Light Co/De [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Original maturities of cash and cash equivalents	3 months
Delmarva Power & Light Co/De [Member] | Transmission and Distribution [Member]
Significant Accounting Policies [Line Items]
Annual Depreciation Rate	2.60%	2.60%	2.70%