Leasing Activities (Narrative) (Detail) (USD $)	3 Months Ended		12 Months Ended				3 Months Ended		12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 1994	Mar. 31, 2013 Subsequent Event [Member] Minimum [Member]	Mar. 31, 2013 Subsequent Event [Member] Maximum [Member]	Dec. 31, 2012 Potomac Electric Power Co [Member]	Dec. 31, 2011 Potomac Electric Power Co [Member]	Dec. 31, 2010 Potomac Electric Power Co [Member]	Dec. 31, 2012 Delmarva Power & Light Co/De [Member]	Dec. 31, 2011 Delmarva Power & Light Co/De [Member]	Dec. 31, 2010 Delmarva Power & Light Co/De [Member]	Dec. 31, 2012 Atlantic City Electric Co [Member]	Dec. 31, 2011 Atlantic City Electric Co [Member]	Dec. 31, 2010 Atlantic City Electric Co [Member]
Leases [Line Items]
Lease investments			$ 1,200,000,000	$ 1,300,000,000
Aggregate cash proceeds			202,000,000	161,000,000
Lease termination payments			520,000,000	423,000,000
Pre-tax gain	39,000,000	39,000,000	39,000,000	39,000,000
Reversal of permanent tax benefits			16,000,000	22,000,000
After-tax gain	9,000,000	3,000,000	9,000,000	3,000,000
Income tax provision at statutory federal rate			14,000,000	14,000,000
Pre-tax charge due to tax law change				7,000,000	2,000,000
Aftertax effect of revised lease rerun							355,000,000	380,000,000
Minimum lease payment receivable, 2013			0
Minimum lease payment receivable, 2014			0
Minimum lease payment receivable, 2015			0
Minimum lease payment receivable, 2016			0
Minimum lease payment receivable, 2017			0
Minimum lease payment receivable, thereafter			1,237,000,000
Present value of future minimum lease payments						152,000,000						88,000,000
Semi-annual payments			8,000,000						8,000,000
Description of lease term			Over a 25-year period that began in December 1994						Over a 25-year period that began in December 1994
Guaranteed residual value due at end of lease term			1						1
Approximate annual commitments, 2013			15,000,000						15,000,000
Approximate annual commitments, 2014			15,000,000						15,000,000
Approximate annual commitments, 2015			15,000,000						15,000,000
Approximate annual commitments, 2016			15,000,000						15,000,000
Approximate annual commitments, 2017			15,000,000						15,000,000
Approximate annual commitments, thereafter			32,000,000						32,000,000
Rental expense for operating leases			52,000,000	46,000,000	45,000,000				6,000,000	4,000,000	4,000,000	12,000,000	11,000,000	10,000,000	11,000,000	10,000,000	9,000,000
Future minimum operating lease payments, 2013			43,000,000						6,000,000			13,000,000			5,000,000
Future minimum operating lease payments, 2014			40,000,000						6,000,000			13,000,000			5,000,000
Future minimum operating lease payments, 2015			38,000,000						6,000,000			12,000,000			5,000,000
Future minimum operating lease payments, 2016			36,000,000						5,000,000			11,000,000			4,000,000
Future minimum operating lease payments, 2017			35,000,000						4,000,000			10,000,000			4,000,000
Future minimum operating lease payments, thereafter			$ 369,000,000						$ 21,000,000			$ 125,000,000			$ 27,000,000
Ownership interest												11.90%