Retirement Benefits - Contributions made to Pension and Other Postretirement Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	$ 408	$ 383	$ 405
Expected qualified pension plan contributions	505
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	36
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	42
Constellation Legacy Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	0	0	21
Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	175	139	148
Expected qualified pension plan contributions	227
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	14
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	16
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	77	42	41
Expected qualified pension plan contributions	141
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	2
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	3
PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	28	28	24
Expected qualified pension plan contributions	17
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	1
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	0
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	48	54	53
Expected qualified pension plan contributions	56
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	2
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	16
Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	25	74	99
Expected qualified pension plan contributions	22
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	9
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	7
Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	14	17	72
Expected qualified pension plan contributions	0
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	2
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	7
Delmarva Power & Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	1	0	2
Expected qualified pension plan contributions	0
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	1
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	0
Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	1	6	20
Expected qualified pension plan contributions	2
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	0
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	0
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	356	337	341
Pension Plan, Defined Benefit [Member] | Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	160	128	137
Pension Plan, Defined Benefit [Member] | Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	72	38	36
Pension Plan, Defined Benefit [Member] | PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	27	28	24
Pension Plan, Defined Benefit [Member] | Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	34	40	39
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	10	62	67
Pension Plan, Defined Benefit [Member] | Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	2	6	62
Pension Plan, Defined Benefit [Member] | Delmarva Power & Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	1	0	0
Pension Plan, Defined Benefit [Member] | Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	0	6	0
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	51	46	64
Other Postretirement Benefit Plan, Defined Benefit [Member] | Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	15	11	11
Other Postretirement Benefit Plan, Defined Benefit [Member] | Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	5	4	5
Other Postretirement Benefit Plan, Defined Benefit [Member] | PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	1	0	0
Other Postretirement Benefit Plan, Defined Benefit [Member] | Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	14	14	14
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	15	12	32
Other Postretirement Benefit Plan, Defined Benefit [Member] | Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	12	11	10
Other Postretirement Benefit Plan, Defined Benefit [Member] | Delmarva Power & Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	0	0	2
Other Postretirement Benefit Plan, Defined Benefit [Member] | Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	$ 1	$ 0	$ 20