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Goodwill And Other Intangible Assets (Changes In Carrying Amount Of Goodwill) (Details) $ in Thousands	Mar. 31, 2017 USD ($)
Goodwill [Line Items]
Goodwill, beginning balance	$ 66,417
Goodwill, ending balance	66,417
Banking [Member]
Goodwill [Line Items]
Goodwill, beginning balance	48,536
Goodwill, ending balance	48,536
Non-Banking [Member]
Goodwill [Line Items]
Goodwill, beginning balance	17,881
Goodwill, ending balance	$ 17,881

X

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

+ Details

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Balance Type:	debit
Period Type:	instant

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Period Type:	duration

X

- Details

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Namespace Prefix:
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X

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=fisi_NonBankingMember
Namespace Prefix:
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Period Type: