Mergers, Acquisitions and Dispositions - Purchase Price Allocation of Merger with Constellation (Details) (Constellation Energy Group LLC [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Business Acquisition [Line Items]
Current assets	$ 4,936
Property, plant and equipment	9,342
Unamortized energy contracts	3,218
Other intangibles, trade name and retail relationships	457
Investment in affiliates	1,942
Asset retirement obligation	740
Other assets	2,265
Total assets	22,900
Current liabilities	3,408
Unamortized energy contract liabilities	1,722
Long-term debt, including current maturities	5,632
Noncontrolling interest	90
Other liabilities	4,683
Total liabilities	15,535
Total net assets	7,365
Exelon Generation Co L L C [Member]
Business Acquisition [Line Items]
Current assets	3,638
Property, plant and equipment	4,054
Unamortized energy contracts	3,218
Other intangibles, trade name and retail relationships	457
Investment in affiliates	1,942
Asset retirement obligation	0
Other assets	1,266
Total assets	14,575
Current liabilities	2,804
Unamortized energy contract liabilities	1,512
Long-term debt, including current maturities	2,972
Noncontrolling interest	90
Other liabilities	1,933
Total liabilities	9,311
Total net assets	$ 5,264