Income Taxes Income Tax Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	$ 733	$ 743
Exelon Generation Co L L C [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	464	468
Commonwealth Edison Co [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	2	2
PECO Energy Co [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	0	0
Baltimore Gas and Electric Company [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	120	120
Pepco Holdings LLC [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	125	125
Potomac Electric Power Company [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	59	59
Delmarva Power and Light Company [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	21	21
Atlantic City Electric Company [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	$ 14	$ 14