Significant Accounting Policies Balance Sheet Classification of Deferred Taxes (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Other noncurrent assets	$ 1,472	$ 1,445
Other noncurrent liabilities	1,827	1,491
Commonwealth Edison Co [Member]
Other noncurrent assets	325	324
Other noncurrent liabilities	529	507
Exelon Generation Co L L C [Member]
Other noncurrent assets	688	627
Other noncurrent liabilities	595	602
PECO Energy Co [Member]
Other noncurrent assets	20	21
Other noncurrent liabilities	85	90
Baltimore Gas and Electric Company [Member]
Other noncurrent assets	9	8
Other noncurrent liabilities	$ 61	$ 61