Intangible Assets - Renewable and Alternative Energy Credits (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Renewable energy credit current [Member]
Finite-Lived Intangible Assets [Line Items]
Acquired Finite-lived Intangible Asset, Residual Value	$ 345	$ 279
Renewable energy credit noncurrent [Member]
Finite-Lived Intangible Assets [Line Items]
Acquired Finite-lived Intangible Asset, Residual Value	86	52
Exelon Generation Co L L C [Member] | Renewable energy credit current [Member]
Finite-Lived Intangible Assets [Line Items]
Acquired Finite-lived Intangible Asset, Residual Value	336	270
Exelon Generation Co L L C [Member] | Renewable energy credit noncurrent [Member]
Finite-Lived Intangible Assets [Line Items]
Acquired Finite-lived Intangible Asset, Residual Value	$ 86	$ 52