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Regulatory Matters (Actual And Required Capital Ratios) (Details) - USD ($) $ in Thousands | Dec. 31, 2016 | Dec. 31, 2015
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Parent Company [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Tier 1 leverage, Actual Amount | $ 265,246 | $ 243,452
Tier 1 leverage, Actual Ratio | 7.36% | 7.41%
Tier 1 leverage, Well Capitalized, Amount | $ 180,119 | $ 164,382
Tier 1 leverage, Well Capitalized, Ratio | 5.00% | 5.00%
CET1 capital, Actual Amount | $ 247,906 | $ 226,413
CET1 capital, Actual Ratio | 9.59% | 9.77%
CET1 capital, Well Capitalized, Amount | $ 167,970 | $ 150,705
CET1 capital, Well Capitalized, Ratio | 6.50% | 6.50%
Tier 1 capital, Actual Amount | $ 265,246 | $ 243,452
Tier 1 capital, Actual Ratio | 10.26% | 10.50%
Tier 1 capital, Well Capitalized, Amount | $ 206,733 | $ 185,483
Tier 1 capital, Well Capitalized, Ratio | 8.00% | 8.00%
Total risk-based capital, Actual Amount | $ 335,241 | $ 309,527
Total risk-based capital, Actual Ratio | 12.97% | 13.35%
Total risk-based capital, Well Capitalized, Amount | $ 258,416 | $ 231,854
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 | $ 144,095 | $ 131,506
Tier 1 leverage, For Capital Adequacy Purposes, Ratio | 4.00% | 4.00%
CET1 capital, For Capital Adequacy Purposes, Amount | $ 132,438 | $ 104,334
CET1 capital, For Capital Adequacy Purposes, Ratio | 5.13% | 4.50%
Tier 1 capital, For Capital Adequacy Purposes, Amount | $ 171,201 | $ 139,112
Tier 1 capital, For Capital Adequacy Purposes, Ratio | 6.63% | 6.00%
Total risk-based capital, For Capital Adequacy Purposes, Amount | $ 222,884 | $ 185,483
Total risk-based capital, For Capital Adequacy Purposes, Ratio | 8.63% | 8.00%
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 | $ 144,095 | $ 131,506
Tier 1 leverage, For Capital Adequacy Purposes, Ratio | 4.00% | 4.00%
CET1 capital, For Capital Adequacy Purposes, Amount | $ 180,891 | $ 162,297
CET1 capital, For Capital Adequacy Purposes, Ratio | 7.00% | 7.00%
Tier 1 capital, For Capital Adequacy Purposes, Amount | $ 219,654 | $ 197,076
Tier 1 capital, For Capital Adequacy Purposes, Ratio | 8.50% | 8.50%
Total risk-based capital, For Capital Adequacy Purposes, Amount | $ 271,337 | $ 243,446
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 | $ 284,765 | $ 265,487
Tier 1 leverage, Actual Ratio | 7.92% | 8.09%
Tier 1 leverage, Well Capitalized, Amount | $ 179,828 | $ 163,985
Tier 1 leverage, Well Capitalized, Ratio | 5.00% | 5.00%
CET1 capital, Actual Amount | $ 284,765 | $ 265,487
CET1 capital, Actual Ratio | 11.06% | 11.49%
CET1 capital, Well Capitalized, Amount | $ 167,432 | $ 150,180
CET1 capital, Well Capitalized, Ratio | 6.50% | 6.50%
Tier 1 capital, Actual Amount | $ 284,765 | $ 265,487
Tier 1 capital, Actual Ratio | 11.06% | 11.49%
Tier 1 capital, Well Capitalized, Amount | $ 206,070 | $ 184,837
Tier 1 capital, Well Capitalized, Ratio | 8.00% | 8.00%
Total risk-based capital, Actual Amount | $ 315,699 | $ 292,572
Total risk-based capital, Actual Ratio | 12.26% | 12.66%
Total risk-based capital, Well Capitalized, Amount | $ 257,588 | $ 231,046
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 | $ 143,862 | $ 131,188
Tier 1 leverage, For Capital Adequacy Purposes, Ratio | 4.00% | 4.00%
CET1 capital, For Capital Adequacy Purposes, Amount | $ 132,014 | $ 103,971
CET1 capital, For Capital Adequacy Purposes, Ratio | 5.13% | 4.50%
Tier 1 capital, For Capital Adequacy Purposes, Amount | $ 170,652 | $ 138,628
Tier 1 capital, For Capital Adequacy Purposes, Ratio | 6.63% | 6.00%
Total risk-based capital, For Capital Adequacy Purposes, Amount | $ 222,170 | $ 184,837
Total risk-based capital, For Capital Adequacy Purposes, Ratio | 8.63% | 8.00%
Bank [Member] | Fully Phased-in [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Tier 1 leverage, For Capital Adequacy Purposes, Amount | $ 143,862 | $ 131,188
Tier 1 leverage, For Capital Adequacy Purposes, Ratio | 4.00% | 4.00%
CET1 capital, For Capital Adequacy Purposes, Amount | $ 180,312 | $ 161,733
CET1 capital, For Capital Adequacy Purposes, Ratio | 7.00% | 7.00%
Tier 1 capital, For Capital Adequacy Purposes, Amount | $ 218,950 | $ 196,389
Tier 1 capital, For Capital Adequacy Purposes, Ratio | 8.50% | 8.50%
Total risk-based capital, For Capital Adequacy Purposes, Amount | $ 270,467 | $ 242,599
Total risk-based capital, For Capital Adequacy Purposes, Ratio | 10.50% | 10.50%

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- Definition The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958570-112826 + Details
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- Definition The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition The minimum amount of Tangible Capital divided by tangible assets (Tangible Capital Ratio) as defined in the regulations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958570-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 + Details
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- Definition The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958570-112826 + Details
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- Definition The amount of Tier 1 Leverage Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition Tier 1 Leverage Capital divided by average assets as defined in the regulations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 + Details
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- Definition Tier 1 Risk Based Capital as defined in the regulations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 + Details
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- Definition The minimum amount of Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958570-112826 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 + Details
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- Definition The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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Data Type: | na
Balance Type: |
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- Details
Name: | dei_LegalEntityAxis=fisi_BankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |