Significant Accounting Policies Balance Sheet Classification of Deferred Taxes (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Other noncurrent assets	$ 1,575	$ 1,330
Other noncurrent liabilities	2,130	2,097
Commonwealth Edison Co [Member]
Other noncurrent assets	395	238
Other noncurrent liabilities	630	562
Exelon Generation Co L L C [Member]
Other noncurrent assets	755	670
Other noncurrent liabilities	610	658
PECO Energy Co [Member]
Other noncurrent assets	27	12
Other noncurrent liabilities	76	86
Baltimore Gas and Electric Company [Member]
Other noncurrent assets	5	4
Other noncurrent liabilities	$ 73	$ 56