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Share-Based Compensation Plans (Share-Based Compensation Expense Included In Consolidated Statements Of Income) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share-based compensation expense	$ 103us-gaap_ShareBasedCompensation	$ 103us-gaap_ShareBasedCompensation
Salaries and Employee Benefits [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share-based compensation expense	78us-gaap_ShareBasedCompensation / us-gaap_IncomeStatementLocationAxis = fisi_SalariesAndEmployeeBenefitsMember	81us-gaap_ShareBasedCompensation / us-gaap_IncomeStatementLocationAxis = fisi_SalariesAndEmployeeBenefitsMember
Other Noninterest Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share-based compensation expense	$ 25us-gaap_ShareBasedCompensation / us-gaap_IncomeStatementLocationAxis = fisi_OtherNoninterestExpensesMember	$ 22us-gaap_ShareBasedCompensation / us-gaap_IncomeStatementLocationAxis = fisi_OtherNoninterestExpensesMember

X

- Definition

The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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Period Type:	duration

X

- Details

Name:	us-gaap_IncomeStatementLocationAxis=fisi_OtherNoninterestExpensesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: