Investment in Constellation Energy Nuclear Group, LLC - Schedule of Assets and Liabilities of CENG (Details) (Exelon Generation Co L L C [Member], Constellation Energy Nuclear Group [Member], USD $) In Millions, unless otherwise specified	Apr. 01, 2014
Exelon Generation Co L L C [Member] | Constellation Energy Nuclear Group [Member]
Business Acquisition [Line Items]
Current assets	$ 499
Nuclear decommissioning trust fund	1,955
Property, plant and equipment	3,017
Nuclear fuel	482
Other assets	10
Total assets	5,963
Current liabilities	237
Asset retirement obligation	1,760
Pension and other employee benefit obligations	281
Unamortized energy contract liabilities	171
Other liabilities	114
Total liabilities	2,563
Total net assets	$ 3,400