New Accounting Standards New Accounting Standards - Narrative (Details) $ in Millions	9 Months Ended
Sep. 30, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings	$ 539

Amount of increase (decrease) in accumulated other comprehensive income (AOCI) for reclassification to retained earnings of tax effect from remeasurement of deferred tax pursuant to Tax Cuts and Jobs Act of 2017.

No definition available.

Name:	exc_TaxCutsandJobsActof2017ReclassificationfromAOCItoRetainedEarnings
Namespace Prefix:	exc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration