Commitments and Contingencies - Commitments Narrative (Details) (USD $)	3 Months Ended
	Sep. 30, 2014	Mar. 31, 2015	Dec. 31, 2014	Jun. 30, 2012	Mar. 31, 2014
Guarantor Obligations [Line Items]
Accrued environmental liabilities		$ 340,000,000	$ 347,000,000
Exelon Generation Co L L C [Member]
Guarantor Obligations [Line Items]
Accrued environmental liabilities		62,000,000	63,000,000
2015		560,000,000
2014		454,000,000
2017		177,000,000
Minimum future operating lease payments due in one year		0
Minimum future operating lease payments due in three years		12,000,000
Minimum future operating lease payments due in four years		13,000,000
Minimum future operating lease payments due in five years		13,000,000
Minimum future operating lease payments due beyond five years		285,000,000
Other Commitment		3,500,000
Loss Contingency, Loss in Period	44,000,000
Loss Contingency, Estimate of Possible Loss	105,000,000
Operating Leases, Future Minimum Payments, Due in Two Years		5,000,000
Exelon Generation Co L L C [Member] | Perryman Construction [Member]
Guarantor Obligations [Line Items]
Other Commitment		17,000,000
Exelon Generation Co L L C [Member] | Combine-cycle Turbine Units [Member]
Guarantor Obligations [Line Items]
Other Commitment		816,000,000
Exelon Generation Co L L C [Member] | Fair Wind Project [Member]
Guarantor Obligations [Line Items]
Other Commitment		26,000,000
Exelon Generation Co L L C [Member] | Sendero Wind Project [Member]
Guarantor Obligations [Line Items]
Other Commitment		34,000,000
Baltimore Gas and Electric Company [Member]
Guarantor Obligations [Line Items]
Accrued environmental liabilities		3,000,000	1,000,000
Business Acquisition, Direct Investment With State And Local Governments Due To Settlement				1,000,000,000
Minimum [Member] | Exelon Generation Co L L C [Member]
Guarantor Obligations [Line Items]
Business Acquisition, Construction Cost					95,000,000
Business Acquisition, Development Of New Generation Cost		600,000,000
Maximum [Member] | Exelon Generation Co L L C [Member]
Guarantor Obligations [Line Items]
Business Acquisition, Construction Cost					120,000,000
Business Acquisition, Development Of New Generation Cost		650,000,000
Equity Method Investments [Member] | Exelon Generation Co L L C [Member]
Guarantor Obligations [Line Items]
2015		77,000,000
2017		$ 19,000,000