Impairment of Long-lived Assets - Narrative (Details) (USD $)	0 Months Ended	12 Months Ended						0 Months Ended	3 Months Ended
	Feb. 26, 2014	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		May 31, 2014	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2000	Oct. 31, 2000
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Impairment charges		$ 24,000,000		$ 14,000,000
Interest costs incurred		1,144,000,000	[1],[2],[3]	1,423,000,000	[1],[2],[3]	1,003,000,000	[1],[2],[3]
Estimated residual value of leased assets		685,000,000		1,465,000,000						685,000,000		1,600,000,000
Capital lease net investment in direct financing leases prepayments received											1,200,000,000.0
Proceeds from termination of direct financing lease investment	335,000,000	335,000,000		0		0
Capital leases net investment in direct financing leases writeoff	336,000,000
Pre Tax Loss		1,000,000
Exelon Generation Co L L C [Member]
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Carrying amount of long lived assets to be written down		1,000,000,000		75,000,000				151,000,000		1,000,000,000
Fair value of impaired assets		556,000,000		32,000,000				65,000,000		556,000,000
Impairment charges		450,000,000		43,000,000				86,000,000
Interest costs incurred		419,000,000	[1],[2],[3]	411,000,000	[1],[2],[3]	368,000,000	[1],[2],[3]
Baltimore Gas and Electric Company [Member]
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Regulatory assets transfer changes						272,000,000			8,000,000
Interest costs incurred		118,000,000	[1],[3]	129,000,000	[1],[3]	149,000,000	[1],[3]
Constellation Energy Group LLC [Member]
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Carrying amount of long lived assets to be written down		163,000,000								163,000,000
Fair value of impaired assets		39,000,000								39,000,000
Impairment charges										124,000,000
Operating And Maintenance Expense [Member] | Exelon Generation Co L L C [Member]
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Utilities operating expense, impairments				111,000,000
Interest Expense [Member] | Exelon Generation Co L L C [Member]
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Interest costs incurred		$ 8,000,000

[1] Exelon activity for the year ended December 31, 2012 includes the results of Constellation and BGE for March 12, 2012—December 31, 2012. Generation activity for the year ended December 31, 2012 includes the results of Constellation for March 12, 2012—December 31, 2012. BGE activity represents the activity for the year ended December 31, 2012.

[2] On April 1, 2014, Generation assumed operational control of CENG’s nuclear fleet. As a result, the 2014 financial results include CENG’s financial position and results of operations beginning April 1, 2014.

[3] Includes interest expense to affiliates.