Commitments And Contingencies (Narrative) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Commitments And Contingencies [Abstract]
Forward sales commitments	$ 413	$ 0

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