Debt and Credit Agreements - Summary of Short-term Borrowing Activities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Average borrowings	$ 531	$ 823	$ 1,125
Maximum borrowings outstanding	$ 1,237	$ 2,147	$ 3,076
Short-term Debt, Weighted Average Interest Rate, over Time	2.21%	1.32%	0.88%
Average interest rates at year end	2.15%	1.24%	1.12%
Exelon Generation Co L L C [Member]
Short-term Debt [Line Items]
Average borrowings	$ 37	$ 405	$ 536
Maximum borrowings outstanding	$ 583	$ 1,455	$ 1,735
Short-term Debt, Weighted Average Interest Rate, over Time	1.96%	1.23%	0.94%
Average interest rates at year end	1.96%	1.23%	1.14%
Commonwealth Edison Co [Member]
Short-term Debt [Line Items]
Average borrowings	$ 154	$ 200	$ 256
Maximum borrowings outstanding	$ 520	$ 470	$ 755
Short-term Debt, Weighted Average Interest Rate, over Time	2.14%	1.24%	0.77%
Average interest rates at year end	2.14%	1.24%
PECO Energy Co [Member]
Short-term Debt [Line Items]
Average borrowings	$ 68	$ 2	$ 0
Maximum borrowings outstanding	$ 350	$ 60	0
Short-term Debt, Weighted Average Interest Rate, over Time	2.24%	1.13%
Average interest rates at year end	2.24%	1.13%
Baltimore Gas and Electric Company [Member]
Short-term Debt [Line Items]
Average borrowings	$ 65	$ 54	143
Maximum borrowings outstanding	$ 239	$ 165	$ 369
Short-term Debt, Weighted Average Interest Rate, over Time	2.18%	1.28%	0.77%
Average interest rates at year end	2.18%	1.28%	0.95%
Pepco Holdings LLC [Member]
Short-term Debt [Line Items]
Short-term Bank Loans and Notes Payable	$ 500
Average borrowings			$ 153
Maximum borrowings outstanding			$ 559
Short-term Debt, Weighted Average Interest Rate, over Time			1.03%
Potomac Electric Power Company [Member]
Short-term Debt [Line Items]
Average borrowings	22	$ 51	$ 4
Maximum borrowings outstanding	$ 90	$ 197	$ 73
Short-term Debt, Weighted Average Interest Rate, over Time	2.24%	1.06%	0.71%
Average interest rates at year end	2.24%	1.06%	0.90%
Delmarva Power & Light Company [Member]
Short-term Debt [Line Items]
Average borrowings	$ 87	$ 40	$ 33
Maximum borrowings outstanding	$ 245	$ 216	$ 116
Short-term Debt, Weighted Average Interest Rate, over Time	2.07%	1.48%	0.68%
Average interest rates at year end	2.07%	1.48%
Atlantic City Electric Company [Member]
Short-term Debt [Line Items]
Short-term Bank Loans and Notes Payable	$ 125
Average borrowings	95	$ 30	$ 0
Maximum borrowings outstanding	$ 210	$ 133	$ 5
Short-term Debt, Weighted Average Interest Rate, over Time	2.21%	1.43%	0.65%
Average interest rates at year end	2.21%	1.43%