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Basis of Presentation And Summary Of Significant Accounting Policies (Details) | 3 Months Ended
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Mar. 31, 2017 state
Number of states in which services are provided | 45
Courier Capital [Member] |
Number of states in which services are provided | 9

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

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |