Income Taxes Income Tax Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	$ 769	$ 916
Exelon Generation Co L L C [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	470	490
Commonwealth Edison Co [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	(12)	(12)
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] | Successor [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	112	172
Delmarva Power and Light Company [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	21	37
Potomac Electric Power Company [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	59	80
Atlantic City Electric Company [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	$ 0	$ 22