Income Taxes - Components of Income Tax Expense (Benefit) from Continuing Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Income taxes	$ 774	$ 118	$ (126)
Exelon Generation Co L L C [Member]
Income Taxes [Line Items]
Income taxes	516	(108)	(1,376)
Commonwealth Edison Co [Member]
Income Taxes [Line Items]
Income taxes	163	168	417
PECO Energy Co [Member]
Income Taxes [Line Items]
Income taxes	65	6	104
Baltimore Gas and Electric Company [Member]
Income Taxes [Line Items]
Income taxes	79	74	218
Pepco Holdings LLC [Member]
Income Taxes [Line Items]
Income taxes	38	33	217
Potomac Electric Power Company [Member]
Income Taxes [Line Items]
Income taxes	16	11	105
Delmarva Power & Light Company [Member]
Income Taxes [Line Items]
Income taxes	22	22	71
Atlantic City Electric Company [Member]
Income Taxes [Line Items]
Income taxes	0	12	26
Internal Revenue Service (IRS) [Member]
Income Taxes [Line Items]
Current	85	226	194
Deferred	489	(99)	(470)
Investment Tax Credit	(72)	(24)	(25)
Internal Revenue Service (IRS) [Member] | Exelon Generation Co L L C [Member]
Income Taxes [Line Items]
Current	147	337	584
Deferred	346	(347)	(2,005)
Investment Tax Credit	(69)	(21)	(21)
Internal Revenue Service (IRS) [Member] | Commonwealth Edison Co [Member]
Income Taxes [Line Items]
Current	59	(63)	(191)
Deferred	15	145	523
Investment Tax Credit	(2)	(2)	(2)
Internal Revenue Service (IRS) [Member] | PECO Energy Co [Member]
Income Taxes [Line Items]
Current	45	11	71
Deferred	20	10	28
Investment Tax Credit	0	0	0
Internal Revenue Service (IRS) [Member] | Baltimore Gas and Electric Company [Member]
Income Taxes [Line Items]
Current	(51)	(5)	74
Deferred	95	47	101
Investment Tax Credit	0	0	(1)
Internal Revenue Service (IRS) [Member] | Pepco Holdings LLC [Member]
Income Taxes [Line Items]
Current	43	(4)	(60)
Deferred	(34)	23	251
Investment Tax Credit	(1)	(1)	(1)
Internal Revenue Service (IRS) [Member] | Potomac Electric Power Company [Member]
Income Taxes [Line Items]
Current	16	28	(20)
Deferred	(6)	(22)	115
Investment Tax Credit	0	0	0
Internal Revenue Service (IRS) [Member] | Delmarva Power & Light Company [Member]
Income Taxes [Line Items]
Current	29	(3)	(24)
Deferred	(21)	13	82
Investment Tax Credit	0	0	0
Internal Revenue Service (IRS) [Member] | Atlantic City Electric Company [Member]
Income Taxes [Line Items]
Current	(3)	(14)	(12)
Deferred	(6)	18	34
Investment Tax Credit	0	0	0
State and Local Jurisdiction [Member]
Income Taxes [Line Items]
Current	5	(1)	14
Deferred	267	16	161
State and Local Jurisdiction [Member] | Exelon Generation Co L L C [Member]
Income Taxes [Line Items]
Current	10	6	65
Deferred	82	(83)	1
State and Local Jurisdiction [Member] | Commonwealth Edison Co [Member]
Income Taxes [Line Items]
Current	(5)	(29)	(49)
Deferred	96	117	136
State and Local Jurisdiction [Member] | PECO Energy Co [Member]
Income Taxes [Line Items]
Current	0	1	14
Deferred	0	(16)	(9)
State and Local Jurisdiction [Member] | Baltimore Gas and Electric Company [Member]
Income Taxes [Line Items]
Current	0	0	(5)
Deferred	35	32	49
State and Local Jurisdiction [Member] | Pepco Holdings LLC [Member]
Income Taxes [Line Items]
Current	3	7	(4)
Deferred	27	8	31
State and Local Jurisdiction [Member] | Potomac Electric Power Company [Member]
Income Taxes [Line Items]
Current	0	0	(2)
Deferred	6	5	12
State and Local Jurisdiction [Member] | Delmarva Power & Light Company [Member]
Income Taxes [Line Items]
Current	0	0	0
Deferred	14	12	13
State and Local Jurisdiction [Member] | Atlantic City Electric Company [Member]
Income Taxes [Line Items]
Current	0	0	0
Deferred	$ 9	$ 8	$ 4