Fair Value of Financial Assets and Liabilities - Fair Value of Financial Liabilities Recorded at the Carrying Amount (Details) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt to financing trust		$ 390	$ 389
Spent Nuclear Fuel Obligation, Noncurrent		1,164	1,147
Unamortized debt issuance costs		219	201
Financing Trusts [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Unamortized debt issuance costs		1
Reported Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	834	929
Long-term debt (including amounts due within one year)	[2],[3]	35,290	34,264
Long-term debt to financing trust	[4]	390	389
Spent Nuclear Fuel Obligation, Noncurrent		1,164	1,147
Estimate of Fair Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	834	929
Long-term debt (including amounts due within one year)	[2],[3]	35,687	36,705
Long-term debt to financing trust	[4]	411	431
Spent Nuclear Fuel Obligation, Noncurrent		993	936
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	0	0
Long-term debt (including amounts due within one year)	[2],[3]	0	0
Long-term debt to financing trust	[4]	0	0
Spent Nuclear Fuel Obligation, Noncurrent		0	0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	834	929
Long-term debt (including amounts due within one year)	[2],[3]	33,608	34,735
Long-term debt to financing trust	[4]	0	0
Spent Nuclear Fuel Obligation, Noncurrent		993	936
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	0	0
Long-term debt (including amounts due within one year)	[2],[3]	2,079	1,970
Long-term debt to financing trust	[4]	411	431
Spent Nuclear Fuel Obligation, Noncurrent		0	0
Exelon Generation Co L L C [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Spent Nuclear Fuel Obligation, Noncurrent		1,164	1,147
Unamortized debt issuance costs		53	60
Exelon Generation Co L L C [Member] | Reported Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]		2
Long-term debt (including amounts due within one year)	[2],[3]	8,842	8,990
Spent Nuclear Fuel Obligation, Noncurrent		1,164	1,147
Exelon Generation Co L L C [Member] | Estimate of Fair Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]		2
Long-term debt (including amounts due within one year)	[2],[3]	9,024	9,512
Spent Nuclear Fuel Obligation, Noncurrent		993	936
Exelon Generation Co L L C [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]		0
Long-term debt (including amounts due within one year)	[2],[3]	0	0
Spent Nuclear Fuel Obligation, Noncurrent		0	0
Exelon Generation Co L L C [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]		2
Long-term debt (including amounts due within one year)	[2],[3]	7,563	7,839
Spent Nuclear Fuel Obligation, Noncurrent		993	936
Exelon Generation Co L L C [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]		0
Long-term debt (including amounts due within one year)	[2],[3]	1,461	1,673
Spent Nuclear Fuel Obligation, Noncurrent		0	0
Commonwealth Edison Co [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt to financing trust		205	205
Unamortized debt issuance costs		64	52
Commonwealth Edison Co [Member] | Financing Trusts [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Unamortized debt issuance costs		1
Commonwealth Edison Co [Member] | Reported Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt (including amounts due within one year)	[2],[3]	8,100	7,601
Long-term debt to financing trust	[4]	205	205
Commonwealth Edison Co [Member] | Estimate of Fair Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt (including amounts due within one year)	[2],[3]	8,317	8,418
Long-term debt to financing trust	[4]	214	227
Commonwealth Edison Co [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt (including amounts due within one year)	[2],[3]	0	0
Long-term debt to financing trust	[4]	0	0
Commonwealth Edison Co [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt (including amounts due within one year)	[2],[3]	8,317	8,418
Long-term debt to financing trust	[4]	0	0
Commonwealth Edison Co [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt (including amounts due within one year)	[2],[3]	0	0
Long-term debt to financing trust	[4]	214	227
PECO Energy Co [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt to financing trust		184	184
Unamortized debt issuance costs		23	17
PECO Energy Co [Member] | Reported Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt (including amounts due within one year)	[2],[3]	3,083	2,903
Long-term debt to financing trust	[4]	184	184
PECO Energy Co [Member] | Estimate of Fair Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt (including amounts due within one year)	[2],[3]	3,180	3,194
Long-term debt to financing trust	[4]	196	204
PECO Energy Co [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt (including amounts due within one year)	[2],[3]	0	0
Long-term debt to financing trust	[4]	0	0
PECO Energy Co [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt (including amounts due within one year)	[2],[3]	3,130	3,194
Long-term debt to financing trust	[4]	0	0
PECO Energy Co [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt (including amounts due within one year)	[2],[3]	50	0
Long-term debt to financing trust	[4]	196	204
Baltimore Gas and Electric Company [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Unamortized debt issuance costs		19	17
Baltimore Gas and Electric Company [Member] | Reported Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]		77
Long-term debt (including amounts due within one year)	[2],[3]	2,876	2,577
Baltimore Gas and Electric Company [Member] | Estimate of Fair Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]		77
Long-term debt (including amounts due within one year)	[2],[3]	2,933	2,825
Baltimore Gas and Electric Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]		0
Long-term debt (including amounts due within one year)	[2],[3]	0	0
Baltimore Gas and Electric Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]		77
Long-term debt (including amounts due within one year)	[2],[3]	2,933	2,825
Baltimore Gas and Electric Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]		0
Long-term debt (including amounts due within one year)	[2],[3]	0	0
Pepco Holdings LLC [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Unamortized debt issuance costs		11	6
Pepco Holdings LLC [Member] | Reported Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	334	350
Long-term debt (including amounts due within one year)	[2],[3]	6,089	5,874
Pepco Holdings LLC [Member] | Estimate of Fair Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	334	350
Long-term debt (including amounts due within one year)	[2],[3]	5,891	6,019
Pepco Holdings LLC [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	0	0
Long-term debt (including amounts due within one year)	[2],[3]	0	0
Pepco Holdings LLC [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	334	350
Long-term debt (including amounts due within one year)	[2],[3]	5,323	5,722
Pepco Holdings LLC [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	0	0
Long-term debt (including amounts due within one year)	[2],[3]	568	297
Potomac Electric Power Company [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Unamortized debt issuance costs		34	32
Potomac Electric Power Company [Member] | Reported Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	64	26
Long-term debt (including amounts due within one year)	[2],[3]	2,625	2,540
Potomac Electric Power Company [Member] | Estimate of Fair Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	64	26
Long-term debt (including amounts due within one year)	[2],[3]	2,991	3,123
Potomac Electric Power Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	0	0
Long-term debt (including amounts due within one year)	[2],[3]	0	0
Potomac Electric Power Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	64	26
Long-term debt (including amounts due within one year)	[2],[3]	2,890	3,114
Potomac Electric Power Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	0	0
Long-term debt (including amounts due within one year)	[2],[3]	101	9
Delmarva Power and Light Company [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Unamortized debt issuance costs		12	11
Delmarva Power and Light Company [Member] | Reported Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]		216
Long-term debt (including amounts due within one year)	[2],[3]	1,494	1,300
Delmarva Power and Light Company [Member] | Estimate of Fair Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]		216
Long-term debt (including amounts due within one year)	[2],[3]	1,495	1,393
Delmarva Power and Light Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]		0
Long-term debt (including amounts due within one year)	[2],[3]	0	0
Delmarva Power and Light Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]		216
Long-term debt (including amounts due within one year)	[2],[3]	1,299	1,393
Delmarva Power and Light Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]		0
Long-term debt (including amounts due within one year)	[2],[3]	196	0
Atlantic City Electric Company [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Unamortized debt issuance costs		4	5
Atlantic City Electric Company [Member] | Reported Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	270	108
Long-term debt (including amounts due within one year)	[2],[3]	1,100	1,121
Atlantic City Electric Company [Member] | Estimate of Fair Value Measurement [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	270	108
Long-term debt (including amounts due within one year)	[2],[3]	1,158	1,237
Atlantic City Electric Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	0	0
Long-term debt (including amounts due within one year)	[2],[3]	0	0
Atlantic City Electric Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	270	108
Long-term debt (including amounts due within one year)	[2],[3]	887	949
Atlantic City Electric Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Short-term liabilities	[1]	0	0
Long-term debt (including amounts due within one year)	[2],[3]	$ 271	$ 288

[1] Level 1 securities consist of dividends payable (included in other current liabilities). Level 2 securities consist of short term borrowings.

[2] Includes unamortized debt issuance costs which are not fair valued of $219 million, $53 million, $64 million, $23 million, $19 million, $11 million, $34 million, $12 million and $4 million for Exelon, Generation, ComEd, PECO, BGE, PHI, Pepco, DPL and ACE, respectively, as of September 30, 2018. Includes unamortized debt issuance costs which are not fair valued of $201 million, $60 million, $52 million, $17 million, $17 million, $6 million, $32 million, $11 million and $5 million for Exelon, Generation, ComEd, PECO, BGE, PHI, Pepco, DPL and ACE, respectively, as of December 31, 2017.

[3] Level 2 securities consist of fixed-rate taxable debt securities, fixed-rate tax-exempt debt, variable rate tax-exempt debt and variable rate non-recourse debt. Level 3 securities consist of fixed-rate private placement taxable debt securities, fixed rate nonrecourse debt, government-backed fixed rate non-recourse debt and loan agreements.

[4] Includes unamortized debt issuance costs which are not fair valued of $1 million and $1 million for Exelon and ComEd, respectively, as of September 30, 2018 and December 31, 2017.