Significant Accounting Policies Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net Cash Provided by (Used in) Operating Activities	$ 1,502	$ 1,074
Quantifying Misstatement in Current Year Financial Statements, Amount		(127)
Net Cash Provided by (Used in) Investing Activities	(1,857)	(2,283)
Scenario, Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net Cash Provided by (Used in) Operating Activities		1,074
Net Cash Provided by (Used in) Investing Activities		2,283
Exelon Generation Co L L C [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net Cash Provided by (Used in) Operating Activities	855	420
Quantifying Misstatement in Current Year Financial Statements, Amount		(320)
Net Cash Provided by (Used in) Investing Activities	(615)	(910)
Exelon Generation Co L L C [Member] | Scenario, Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net Cash Provided by (Used in) Operating Activities		420
Net Cash Provided by (Used in) Investing Activities		910
Commonwealth Edison Co [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net Cash Provided by (Used in) Operating Activities	134	236
Quantifying Misstatement in Current Year Financial Statements, Amount		91
Net Cash Provided by (Used in) Investing Activities	(523)	(619)
Commonwealth Edison Co [Member] | Scenario, Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net Cash Provided by (Used in) Operating Activities		236
Net Cash Provided by (Used in) Investing Activities		619
PECO Energy Co [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net Cash Provided by (Used in) Operating Activities	19	106
Quantifying Misstatement in Current Year Financial Statements, Amount		42
Net Cash Provided by (Used in) Investing Activities	(215)	(69)
PECO Energy Co [Member] | Scenario, Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net Cash Provided by (Used in) Operating Activities		106
Net Cash Provided by (Used in) Investing Activities		69
Baltimore Gas and Electric Company [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net Cash Provided by (Used in) Operating Activities	313	208
Quantifying Misstatement in Current Year Financial Statements, Amount		40
Net Cash Provided by (Used in) Investing Activities	$ (223)	(202)
Baltimore Gas and Electric Company [Member] | Scenario, Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net Cash Provided by (Used in) Operating Activities		208
Net Cash Provided by (Used in) Investing Activities		$ 202