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Severance - Severance Liabilities related to CENG (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Restructuring Reserve [Roll Forward]  
Payments $ (24)us-gaap_PaymentsForRestructuring
Severance [Member]  
Restructuring Reserve [Roll Forward]  
Beginning Balance 2us-gaap_RestructuringReserve
/ us-gaap_RestructuringCostAndReserveAxis
= us-gaap_EmployeeSeveranceMember
Severance charges 3exc_SeveranceChargesExpense
/ us-gaap_RestructuringCostAndReserveAxis
= us-gaap_EmployeeSeveranceMember
Payments (11)us-gaap_PaymentsForRestructuring
/ us-gaap_RestructuringCostAndReserveAxis
= us-gaap_EmployeeSeveranceMember
Ending Balance 13us-gaap_RestructuringReserve
/ us-gaap_RestructuringCostAndReserveAxis
= us-gaap_EmployeeSeveranceMember
Constellation Energy Group LLC [Member] | Severance [Member]  
Restructuring Reserve [Roll Forward]  
Severance charges $ 19exc_SeveranceChargesExpense
/ us-gaap_BusinessAcquisitionAxis
= exc_ConstellationEnergyGroupLLCMember
/ us-gaap_RestructuringCostAndReserveAxis
= us-gaap_EmployeeSeveranceMember		 [1]
[1] Includes the fair value of the CENG integration-related obligation as of April 1, 2014, the date of consolidation. Note this includes an additional $3 million of severance charges incurred in the first quarter of 2014 by CENG. See Note 5 - Investment in Constellation Energy Nuclear Group, LLC for additional information.