Mergers, Acquisitions and Dispositions - Summary of Integrys Energy Group Acquisition (Details) - Exelon Generation Co L L C [Member] - Intergrys Energy Group Inc [Member] $ in Millions	Nov. 01, 2014 USD ($)
Business Acquisition [Line Items]
Total consideration transferred	$ 332
Working capital assets	390
Mark-to-market derivative assets	184
Unamortized energy contracts	115
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	50
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities	(196)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Derivative Liabilities	(57)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Financial Liabilities	(110)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Noncurrent	(16)
Total net assets	360
Bargain purchase gain (after-tax)	$ 28