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Basis of Presentation And Summary Of Significant Accounting Policies (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) state
Basis of Presentation And Summary Of Significant Accounting Policies [Abstract]
Number of states in which services are provided | state	45
Reclassification from AOCI to retained earnings | $	$ 2.8

X

- Definition

Number of States in Which Entity Provides Services

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No definition available.

+ Details

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X

- Definition

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.

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No definition available.

+ Details

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Namespace Prefix:	fisi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- References

No definition available.

+ Details

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