Fair Value of Financial Assets and Liabilities Fair Value of Financial Assets and Liabilities - Narrative (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
EquitySecuritiesWithoutReadilyDeterminableFairValueAmount	$ 68,000,000
Derivative Instruments Not Designated as Hedging Instruments, Asset, at Fair Value	0	$ 0
Spent Nuclear Fuel Obligation, Noncurrent	1,157,000,000	1,147,000,000
Forward Power Basis	3.17
Forward Gas Basis	0.47
Exelon Generation Co L L C [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Spent Nuclear Fuel Obligation, Noncurrent	1,157,000,000	$ 1,147,000,000
Fixed Income Securities [Member] | Exelon Generation Co L L C [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	62,000,000
Middle Market Lending [Member] | Exelon Generation Co L L C [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	302,000,000
Private Equity Funds [Member] | Exelon Generation Co L L C [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	178,000,000
Real Estate Funds [Member] | Exelon Generation Co L L C [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	$ 100,000,000