Retirement Benefits Retirement Benefits - Contributions made to Pension and Other Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended	12 Months Ended
		Mar. 23, 2016		Dec. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					$ 405		$ 397		$ 502
Expected qualified pension plan contributions	[1]				301
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[2]				30
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	[3]				42
Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected qualified pension plan contributions	[1]				36
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[2]				7
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	[3]				1
Constellation Legacy Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[4]				21		25		36
Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					148		152		245
Expected qualified pension plan contributions	[1]				119
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[2]				11
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	[3]				13
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					41		38		150
Expected qualified pension plan contributions	[1]				38
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[2]				2
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	[3]				3
PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					24		30		40
Expected qualified pension plan contributions	[1]				17
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[2]				1
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	[3]				0
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					53		49		17
Expected qualified pension plan contributions	[1]				41
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[2]				1
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	[3]				16
Pepco Holdings LLC [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected qualified pension plan contributions	[1]				50
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[2]				8
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	[3]				9
Pepco Holdings LLC [Member] | Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected qualified pension plan contributions	[1]				40
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[2]				5
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	[3]				1
Pepco Holdings LLC [Member] | Successor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions				$ 86	99
Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions		$ 4							21
Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					72		32		14
Expected qualified pension plan contributions	[1]				4
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[2]				2
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	[3]				8
Delmarva Power and Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					2		22		0
Expected qualified pension plan contributions	[1]				0
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[2]				1
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	[3]				0
Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					20		17		2
Expected qualified pension plan contributions	[1]				6
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	[2]				0
Defined Other Postretirement Benefit Plan Estimated Future Employer Contributions In Next Fiscal Year	[3]				0
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[5]				341		347		462
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months					1,166
Pension Plan, Defined Benefit [Member] | Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[4],[5]				38		39		47
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	[5]				137		140		231
Pension Plan, Defined Benefit [Member] | Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[5]				36		33		143
Pension Plan, Defined Benefit [Member] | PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[5]				24		30		40
Pension Plan, Defined Benefit [Member] | Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[5]				39		31		1
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions		4	[4]		5	[5],[6]	17	[5],[6]	0	[5],[6]
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Successor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions				74	67
Pension Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions		4					0
Pension Plan, Defined Benefit [Member] | Exelon Legacy Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					4	[4]	6		5
Pension Plan, Defined Benefit [Member] | Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[5]				62		24		0
Pension Plan, Defined Benefit [Member] | Delmarva Power and Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[5]				0		22		0
Pension Plan, Defined Benefit [Member] | Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[5]				0		15		0
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					64		50		40
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months					256
Other Postretirement Benefit Plan, Defined Benefit [Member] | Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[4]				2		3		3
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					11		12		14
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		7
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		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		18		16
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions	[6]				0		2		0
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Successor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions				$ 12	32
Other Postretirement Benefit Plan, Defined Benefit [Member] | Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions		$ 0					5
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					10		8		2
Other Postretirement Benefit Plan, Defined Benefit [Member] | Delmarva Power and Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and non-pension postretirement benefit contributions					2		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					$ 20		$ 2		$ 3

[1] Qualified Pension PlansNon-Qualified Pension PlansOtherPostretirementBenefitsExelon$301$30$42Generation1191113ComEd3823PECO171—BGE41116BSC3671PHI5089Pepco428DPL—1—ACE6——PHISCO4051

[2] Qualified Pension PlansNon-Qualified Pension PlansOtherPostretirementBenefitsExelon$301$30$42Generation1191113ComEd3823PECO171—BGE41116BSC3671PHI5089Pepco428DPL—1—ACE6——PHISCO4051

[3] Qualified Pension PlansNon-Qualified Pension PlansOtherPostretirementBenefitsExelon$301$30$42Generation1191113ComEd3823PECO171—BGE41116BSC3671PHI5089Pepco428DPL—1—ACE6——PHISCO4051

[4] Includes $4 million, $6 million, and $5 million of pension contributions funded by Exelon Corporate, for the years ended December 31, 2017, 2016, and 2015, respectively.

[5] Exelon's and Generation's pension contributions include $21 million, $25 million and $36 million related to the legacy CENG plans that was funded by CENG as provided in an Employee Matters Agreement (EMA) between Exelon and CENG for the years ended December 31, 2017, 2016 and 2015, respectively.

[6] PHISCO’s pension contributions for the year ended December 31, 2016 include $4 million of contributions made prior to the closing of Exelon’s merger with PHI on March 23, 2016.