Commitments And Contingencies (Narrative) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Forward sales commitments	$ 8,700	$ 566

Commitments to sell residential real estate loans.

No definition available.

Name:	fisi_ForwardSalesCommitments
Namespace Prefix:	fisi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant