XML 46 R35.htm IDEA: XBRL DOCUMENT
v3.5.0.2

Basis of Presentation And Summary Of Significant Accounting Policies (Details) | 9 Months Ended
--- | ---
Sep. 30, 2016 state
Number of states in which services are provided | 44
Courier Capital [Member] |
Number of states in which services are provided | 9

X
- Definition Number of States in Which Entity Provides Services + References No definition available. + Details
Name: | fisi_NumberOfStatesInWhichEntityProvidesServices
Namespace Prefix: | fisi_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=fisi_CourierCapitalMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |