Significant Accounting Policies Balance Sheet Classification of Deferred Taxes (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Other noncurrent assets	$ 1,322	$ 1,472
Other noncurrent liabilities	2,097	1,827
Commonwealth Edison Co [Member]
Other noncurrent assets	238	325
Other noncurrent liabilities	562	529
Exelon Generation Co L L C [Member]
Other noncurrent assets	662	688
Other noncurrent liabilities	658	595
PECO Energy Co [Member]
Other noncurrent assets	12	20
Other noncurrent liabilities	86	85
Baltimore Gas and Electric Company [Member]
Other noncurrent assets	4	9
Other noncurrent liabilities	$ 56	$ 61