Nuclear Decommissioning - Rollforward of Nuclear Decommissioning ARO (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018	Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Net increase due to changes in, and timing of, estimated future cash flows		$ 330		$ 84
Accretion expense	[1]	124	$ 120
Liabilities held for sale		799			$ 777
Nuclear Decommissioning [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Nuclear decommissioning ARO, beginning balance	[2],[3]	10,005
Net increase due to changes in, and timing of, estimated future cash flows		223
Accretion expense		120
Costs incurred related to decommissioning plants		(19)
Nuclear decommissioning ARO, ending balance	[2],[3]	10,329
Current portion of ARO		41			22
Oyster Creek [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Liabilities held for sale		765			777
Exelon Generation Co L L C [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Net increase due to changes in, and timing of, estimated future cash flows		330
Accretion expense	[1]	123	$ 120
Liabilities held for sale		799			777
Exelon Generation Co L L C [Member] | Nuclear Decommissioning [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Nuclear decommissioning ARO, beginning balance	[2],[3]	10,005
Net increase due to changes in, and timing of, estimated future cash flows		223
Accretion expense		120
Costs incurred related to decommissioning plants		(19)
Nuclear decommissioning ARO, ending balance	[2],[3]	10,329
Current portion of ARO		41			22
Exelon Generation Co L L C [Member] | Oyster Creek [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Liabilities held for sale		765			777
Nuclear Decommissioning Asset Retirement Obligation [Member] | Oyster Creek [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Liabilities held for sale		760			772
Nuclear Decommissioning Asset Retirement Obligation [Member] | Exelon Generation Co L L C [Member] | Oyster Creek [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Liabilities held for sale		$ 760			$ 772

[1] Included in Operating and maintenance expense in the Registrants’ Consolidated Statements of Operations and Comprehensive Income.

[2] Includes $41 million and $22 million as the current portion of the ARO at March 31, 2019 and December 31, 2018, respectively, which is included in Other current liabilities in Exelon’s and Generation’s Consolidated Balance Sheets.

[3] Includes $760 million and $772 million of ARO related to Oyster Creek which is classified as Liabilities held for sale in Exelon's and Generation's Consolidated Balance Sheets at March 31, 2019 and December 31, 2018, respectively. See Note 3 — Mergers, Acquisitions and Dispositions for additional information.