Debt and Credit Agreements - Summary of Short-term Borrowing Activities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 23, 2017	Jan. 13, 2016	Jun. 30, 2015
Exelon Corporate [Member]
Short-term Debt [Line Items]
Short-term Bank Loans and Notes Payable				$ 500
Average borrowings	$ 823	$ 1,125	$ 499
Maximum borrowings outstanding	$ 2,147	$ 3,076	$ 739
Short-term Debt, Weighted Average Interest Rate, over Time	1.32%	0.88%	0.53%
Average interest rates at year end	1.24%	1.12%	0.88%
Exelon Generation Co L L C [Member]
Short-term Debt [Line Items]
Average borrowings	$ 405	$ 536	$ 1
Maximum borrowings outstanding	$ 1,455	$ 1,735	$ 50
Short-term Debt, Weighted Average Interest Rate, over Time	1.23%	0.94%	0.49%
Average interest rates at year end	1.23%	1.14%
Commonwealth Edison Co [Member]
Short-term Debt [Line Items]
Average borrowings	$ 200	$ 256	$ 461
Maximum borrowings outstanding	$ 470	$ 755	$ 684
Short-term Debt, Weighted Average Interest Rate, over Time	1.24%	0.77%	0.53%
Average interest rates at year end	1.24%		0.89%
PECO Energy Co [Member]
Short-term Debt [Line Items]
Average borrowings	$ 2	$ 0	$ 0
Maximum borrowings outstanding	$ 60	0	0
Short-term Debt, Weighted Average Interest Rate, over Time	1.13%
Average interest rates at year end	1.13%
Baltimore Gas and Electric Company [Member]
Short-term Debt [Line Items]
Average borrowings	$ 54	143	37
Maximum borrowings outstanding	$ 165	$ 369	$ 210
Short-term Debt, Weighted Average Interest Rate, over Time	1.28%	0.77%	0.48%
Average interest rates at year end	1.28%	0.95%	0.87%
Pepco Holdings LLC [Member]
Short-term Debt [Line Items]
Short-term Bank Loans and Notes Payable					$ 500	$ 300
Average borrowings		$ 153	$ 444
Maximum borrowings outstanding		$ 559	$ 784
Short-term Debt, Weighted Average Interest Rate, over Time		1.03%	0.90%
Average interest rates at year end			1.22%
Potomac Electric Power Company [Member]
Short-term Debt [Line Items]
Average borrowings	$ 51	$ 4	$ 34
Maximum borrowings outstanding	$ 197	$ 73	$ 190
Short-term Debt, Weighted Average Interest Rate, over Time	1.06%	0.71%	0.44%
Average interest rates at year end	1.06%	0.90%	0.68%
Delmarva Power and Light Company [Member]
Short-term Debt [Line Items]
Average borrowings	$ 40	$ 33	$ 81
Maximum borrowings outstanding	$ 216	$ 116	$ 179
Short-term Debt, Weighted Average Interest Rate, over Time	1.48%	0.68%	0.47%
Average interest rates at year end	1.48%		0.79%
Atlantic City Electric Company [Member]
Short-term Debt [Line Items]
Average borrowings	$ 30	$ 0	$ 175
Maximum borrowings outstanding	$ 133	$ 5	$ 253
Short-term Debt, Weighted Average Interest Rate, over Time	1.43%	0.65%	0.46%
Average interest rates at year end	1.43%		0.65%