Severance - Severance Benefit Costs associated with Job Reductions (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended						3 Months Ended
	Dec. 31, 2012		Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2013
Restructuring Cost and Reserve [Line Items]
Stock compensation	$ 7	[1],[2]
Total severance benefits	138	[2],[3]
Severance costs	124	[1],[2]
Severance Costs [Member]
Restructuring Cost and Reserve [Line Items]
Net regulatory assets	35
Other Severance Charges [Member]
Restructuring Cost and Reserve [Line Items]
Severance charges	19	[4]	7	[4]	18	[4]
Stock compensation	7	[1],[2]
Exelon Generation Co L L C [Member]
Restructuring Cost and Reserve [Line Items]
Stock compensation	4	[1]
Total severance benefits	88	[3]
Severance costs	80	[1]					16
Exelon Generation Co L L C [Member] | Corporate, Non-Segment [Member]
Restructuring Cost and Reserve [Line Items]
Severance costs	46
Exelon Generation Co L L C [Member] | Other Severance Charges [Member]
Restructuring Cost and Reserve [Line Items]
Severance charges	14	[4]	5	[4]	16	[4]
Stock compensation	4	[1]
Commonwealth Edison Co [Member]
Restructuring Cost and Reserve [Line Items]
Stock compensation	1	[1],[2]
Total severance benefits	16	[2],[3]
Severance costs	14	[1],[2]
Commonwealth Edison Co [Member] | Corporate, Non-Segment [Member]
Restructuring Cost and Reserve [Line Items]
Severance costs	14
Commonwealth Edison Co [Member] | Severance Costs [Member]
Restructuring Cost and Reserve [Line Items]
Net regulatory assets	16
Commonwealth Edison Co [Member] | Other Severance Charges [Member]
Restructuring Cost and Reserve [Line Items]
Severance charges	2	[4]	1	[4]	2	[4]
Stock compensation	1	[1],[2]
Baltimore Gas and Electric Company [Member]
Restructuring Cost and Reserve [Line Items]
Stock compensation	1	[1],[2]
Total severance benefits	19	[2],[3]
Severance costs	17	[1],[2]
Baltimore Gas and Electric Company [Member] | Corporate, Non-Segment [Member]
Restructuring Cost and Reserve [Line Items]
Severance costs	7
Baltimore Gas and Electric Company [Member] | Severance Costs [Member]
Restructuring Cost and Reserve [Line Items]
Net regulatory assets	19
Baltimore Gas and Electric Company [Member] | Other Severance Charges [Member]
Restructuring Cost and Reserve [Line Items]
Severance charges	3	[4]	1	[4]	0	[4]
Stock compensation	1	[1],[2]
PECO Energy Co [Member]
Restructuring Cost and Reserve [Line Items]
Stock compensation	0	[1]
Total severance benefits	7	[3]
Severance costs	7	[1]
PECO Energy Co [Member] | Corporate, Non-Segment [Member]
Restructuring Cost and Reserve [Line Items]
Severance costs	7
PECO Energy Co [Member] | Other Severance Charges [Member]
Restructuring Cost and Reserve [Line Items]
Severance charges	1	[4]	0	[4]	0	[4]
Stock compensation	$ 0	[1]

[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.

[3] Includes the fair value of the CENG integration-related obligation as of April 1, 2014, the date of consolidation. Note this includes an additional $3 million of severance charges incurred in the first quarter of 2014 by CENG. See Note 5 - Investment in Constellation Energy Nuclear Group, LLC for additional information.

[4] The amounts above for Generation include $1 million, $2 million, and $0 million for amounts billed by BSC through intercompany allocations for the years ended December 31, 2014, December 31, 2013, and December 31, 2012, respectively. Amounts billed by BSC to ComEd, PECO and BGE were not material