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]
Schedule of Equity Method Investments [Line Items]
Current assets	$ 499
Nuclear decommissioning trust fund	1,955
Property, plant and equipment	2,941
Nuclear fuel	482
Other assets	10
Total assets	5,887
Current liabilities	237
Asset retirement obligation	1,684
Pension and other employee benefit obligations	281
Unamortized energy contract liabilities	171
Other liabilities	114
Total liabilities	2,487
Total net assets	$ 3,400