Retirement Benefits Retirement Benefits - Estimated Future Benefit Payments (Details) $ in Millions	Dec. 31, 2017 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2018	$ 30	[1]
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2018	1,166
2019	1,165
2020	1,210
2021	1,236
2022	1,265
2022 through 2026	6,671
Total estimated future benefit payments through 2027	12,713
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2018	256
2019	262
2020	270
2021	276
2022	284
2022 through 2026	1,509
Total estimated future benefit payments through 2027	$ 2,857

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