Retirement Benefits - Contributions made to Pension and Other Postretirement Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	$ 601	$ 408	$ 383
Expected qualified pension plan contributions	505
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	51
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	75
Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	255	175	139
Expected qualified pension plan contributions	196
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	27
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	24
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	148	77	42
Expected qualified pension plan contributions	170
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	2
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	23
PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	18	28	28
Expected qualified pension plan contributions	14
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	78	48	54
Expected qualified pension plan contributions	57
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	1
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	39	25	74
Expected qualified pension plan contributions	29
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	11	14	17
Expected qualified pension plan contributions	1
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	2
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	6
Delmarva Power & Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	0	1	0
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	2	1	6
Expected qualified pension plan contributions	3
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	542	356	337
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	236	160	128
Pension Plan, Defined Benefit [Member] | Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	143	72	38
Pension Plan, Defined Benefit [Member] | PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	18	27	28
Pension Plan, Defined Benefit [Member] | Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	56	34	40
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	30	10	62
Pension Plan, Defined Benefit [Member] | Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	2	2	6
Pension Plan, Defined Benefit [Member] | Delmarva Power & Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	0	1	0
Pension Plan, Defined Benefit [Member] | Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	2	0	6
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	59	51	46
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	19	15	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	5	4
Other Postretirement Benefit Plan, Defined Benefit [Member] | PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	0	1	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	22	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	9	15	12
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	9	12	11
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	0
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	$ 0	$ 1	$ 0