Income Taxes - Summary of Interest Expense (Income) (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
Income Taxes [Line Items]
Document Period End Date			Dec. 31, 2017
Unrecognized Tax Benefits, Income Tax Penalties Expense			$ (2)	$ 106	$ 0
Net interest (income) expense recognized related to uncertain tax positions			37	165	(13)
Exelon Generation Co L L C [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Income Tax Penalties Expense			0	0	0
Net interest (income) expense recognized related to uncertain tax positions			(1)	(13)	(31)
Commonwealth Edison Co [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Income Tax Penalties Expense			0	86	0
Net interest (income) expense recognized related to uncertain tax positions			11	117	7
PECO Energy Co [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Income Tax Penalties Expense			0	0	0
Net interest (income) expense recognized related to uncertain tax positions			0	0	0
Baltimore Gas and Electric Company [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Income Tax Penalties Expense			0	0	0
Net interest (income) expense recognized related to uncertain tax positions			0	0	0
Pepco Holdings LLC [Member] | Successor [Member]
Income Taxes [Line Items]
Net interest (income) expense recognized related to uncertain tax positions		$ (2)	0
Pepco Holdings LLC [Member] | Predecessor [Member]
Income Taxes [Line Items]
Net interest (income) expense recognized related to uncertain tax positions	$ 0				(34)
Potomac Electric Power Company [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Income Tax Penalties Expense			0	0	0
Net interest (income) expense recognized related to uncertain tax positions			0	6	(4)
Delmarva Power and Light Company [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Income Tax Penalties Expense			0	0	0
Net interest (income) expense recognized related to uncertain tax positions			0	0	0
Atlantic City Electric Company [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Income Tax Penalties Expense			0	0	0
Net interest (income) expense recognized related to uncertain tax positions			$ 0	$ (1)	$ 0