Severance - Narrative (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended				3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012		Dec. 31, 2014	Dec. 31, 2013
Restructuring Cost and Reserve [Line Items]
Severance costs			$ 124	[1],[2]
Business Combination, Integration Related Costs	19	28
Payments	24
Constellation Energy Nuclear Group LLC [Member]
Restructuring Cost and Reserve [Line Items]
Severance costs					2
Exelon Generation Co L L C [Member]
Restructuring Cost and Reserve [Line Items]
Severance costs			80	[1]		16
Business Combination, Integration Related Costs					19
Operating And Maintenance Expense [Member] | Exelon Generation Co L L C [Member]
Restructuring Cost and Reserve [Line Items]
Severance costs	$ 3	$ 2

[1] The amounts above include $46 million at Generation, $14 million at ComEd, $7 million at PECO, and $7 million at BGE, for amounts billed by BSC through intercompany allocations for the year ended December 31, 2012.

[2] Exelon, ComEd and BGE established regulatory assets of $35 million, $16 million and $19 million, respectively, for severance benefits costs for the year ended December 31, 2012. The majority of these costs are expected to be recovered over a five-year period.