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Regulatory Matters (Actual And Required Capital Ratios) (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Parent Company [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Tier 1 leverage, Actual Amount | $ 381,473 | $ 344,283
Tier 1 leverage, Actual Ratio | 9.00% | 8.16%
Tier 1 leverage, Well Capitalized, Amount | $ 211,880 | $ 210,949
Tier 1 leverage, Well Capitalized, Ratio | 5.00% | 5.00%
CET1 capital, Actual Amount | $ 364,145 | $ 326,955
CET1 capital, Actual Ratio | 10.31% | 9.70%
CET1 capital, Well Capitalized, Amount | $ 229,663 | $ 219,150
CET1 capital, Well Capitalized, Ratio | 6.50% | 6.50%
Tier 1 capital, Actual Amount | $ 381,473 | $ 344,283
Tier 1 capital, Actual Ratio | 10.80% | 10.21%
Tier 1 capital, Well Capitalized, Amount | $ 282,663 | $ 269,723
Tier 1 capital, Well Capitalized, Ratio | 8.00% | 8.00%
Total risk-based capital, Actual Amount | $ 451,228 | $ 417,399
Total risk-based capital, Actual Ratio | 12.77% | 12.38%
Total risk-based capital, Well Capitalized, Amount | $ 353,328 | $ 337,154
Total risk-based capital, Well Capitalized, Ratio | 10.00% | 10.00%
Parent Company [Member] | Phase-in Schedule [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Tier 1 leverage, For Capital Adequacy Purposes, Amount | $ 169,504 | $ 168,759
Tier 1 leverage, For Capital Adequacy Purposes, Ratio | 4.00% | 4.00%
CET1 capital, For Capital Adequacy Purposes, Amount | $ 247,330 | $ 214,936
CET1 capital, For Capital Adequacy Purposes, Ratio | 7.00% | 6.38%
Tier 1 capital, For Capital Adequacy Purposes, Amount | $ 300,329 | $ 265,509
Tier 1 capital, For Capital Adequacy Purposes, Ratio | 8.50% | 7.88%
Total risk-based capital, For Capital Adequacy Purposes, Amount | $ 370,995 | $ 332,940
Total risk-based capital, For Capital Adequacy Purposes, Ratio | 10.50% | 9.88%
Parent Company [Member] | Fully Phased-in [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Tier 1 leverage, For Capital Adequacy Purposes, Amount | $ 169,504 | $ 168,759
Tier 1 leverage, For Capital Adequacy Purposes, Ratio | 4.00% | 4.00%
CET1 capital, For Capital Adequacy Purposes, Amount | $ 247,330 | $ 236,008
CET1 capital, For Capital Adequacy Purposes, Ratio | 7.00% | 7.00%
Tier 1 capital, For Capital Adequacy Purposes, Amount | $ 300,329 | $ 286,581
Tier 1 capital, For Capital Adequacy Purposes, Ratio | 8.50% | 8.50%
Total risk-based capital, For Capital Adequacy Purposes, Amount | $ 370,995 | $ 354,012
Total risk-based capital, For Capital Adequacy Purposes, Ratio | 10.50% | 10.50%
Bank [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Tier 1 leverage, Actual Amount | $ 409,031 | $ 372,939
Tier 1 leverage, Actual Ratio | 9.67% | 8.86%
Tier 1 leverage, Well Capitalized, Amount | $ 211,486 | $ 210,419
Tier 1 leverage, Well Capitalized, Ratio | 5.00% | 5.00%
CET1 capital, Actual Amount | $ 409,031 | $ 372,939
CET1 capital, Actual Ratio | 11.61% | 11.09%
CET1 capital, Well Capitalized, Amount | $ 229,055 | $ 218,488
CET1 capital, Well Capitalized, Ratio | 6.50% | 6.50%
Tier 1 capital, Actual Amount | $ 409,031 | $ 372,939
Tier 1 capital, Actual Ratio | 11.61% | 11.09%
Tier 1 capital, Well Capitalized, Amount | $ 281,914 | $ 268,908
Tier 1 capital, Well Capitalized, Ratio | 8.00% | 8.00%
Total risk-based capital, Actual Amount | $ 439,514 | $ 406,853
Total risk-based capital, Actual Ratio | 12.47% | 12.10%
Total risk-based capital, Well Capitalized, Amount | $ 352,392 | $ 336,135
Total risk-based capital, Well Capitalized, Ratio | 10.00% | 10.00%
Bank [Member] | Phase-in Schedule [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Tier 1 leverage, For Capital Adequacy Purposes, Amount | $ 169,189 | $ 168,335
Tier 1 leverage, For Capital Adequacy Purposes, Ratio | 4.00% | 4.00%
CET1 capital, For Capital Adequacy Purposes, Amount | $ 246,674 | $ 214,286
CET1 capital, For Capital Adequacy Purposes, Ratio | 7.00% | 6.38%
Tier 1 capital, For Capital Adequacy Purposes, Amount | $ 299,533 | $ 264,706
Tier 1 capital, For Capital Adequacy Purposes, Ratio | 8.50% | 7.88%
Total risk-based capital, For Capital Adequacy Purposes, Amount | $ 370,011 | $ 331,933
Total risk-based capital, For Capital Adequacy Purposes, Ratio | 10.50% | 9.88%
Bank [Member] | Fully Phased-in [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Tier 1 leverage, For Capital Adequacy Purposes, Amount | $ 169,189 | $ 168,335
Tier 1 leverage, For Capital Adequacy Purposes, Ratio | 4.00% | 4.00%
CET1 capital, For Capital Adequacy Purposes, Amount | $ 246,674 | $ 235,294
CET1 capital, For Capital Adequacy Purposes, Ratio | 7.00% | 7.00%
Tier 1 capital, For Capital Adequacy Purposes, Amount | $ 299,533 | $ 285,715
Tier 1 capital, For Capital Adequacy Purposes, Ratio | 8.50% | 8.50%
Total risk-based capital, For Capital Adequacy Purposes, Amount | $ 370,011 | $ 352,942
Total risk-based capital, For Capital Adequacy Purposes, Ratio | 10.50% | 10.50%

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- Definition Tier 1 Risk Based Capital as defined in the regulations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=117337116&loc=d3e65071-112826 + Details
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- Definition The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
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Name: | srt_ConsolidatedEntitiesAxis=srt_ParentCompanyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_StatementScenarioAxis=fisi_PhaseInScheduleMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_StatementScenarioAxis=fisi_FullyPhasedInMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ConsolidatedEntitiesAxis=fisi_BankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |