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LOANS - Allowance for Credit Losses (Details) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | $ 49,911 | $ 49,284 |
Charge-offs |  | (13,727) |
Recoveries |  | 5,924 |
Provision | 14,212 | 8,430 | $ 10,837
Balance, end of period | 60,869 | 49,911 | 49,284
Liabilities for off-balance-sheet credit exposures [Roll Forward] |  |  |
Beginning balance | 0 |  |
Ending balance |  | 0 |
Total allowance for credit losses [Roll Forward] |  |  |
Beginning balance | $ 49,911 |  |
Ending balance |  | $ 49,911 |
Allowance for Credit Losses |  |  |
Allowance for credit losses to total loans ratio | 0.82 |  |
Increase in allowance for credit losses to total loans ratio over the same quarter in prior year |  | 0.10% |
Minimum [Member] | COVID-19 [Member] |  |  |
Allowance for Credit Losses |  |  |
Percentage of peak unemployment | 6.00% |  |
Maximum [Member] | COVID-19 [Member] |  |  |
Allowance for Credit Losses |  |  |
Percentage of peak unemployment | 10.00% |  |
ASU 2016-13 [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | $ 51,268 |  |
Charge-offs | (10,664) |  |
Recoveries | 5,689 |  |
Steuben acquisition | 3,662 |  |
Provision | 10,914 |  |
Balance, end of period | 60,869 | $ 51,268 |
Liabilities for off-balance-sheet credit exposures [Roll Forward] |  |  |
Beginning balance | 1,185 |  |
Charge-offs | 0 |  |
Recoveries | 0 |  |
Steuben acquisition | 67 |  |
Provision | 237 |  |
Ending balance | 1,489 | 1,185 |
Total allowance for credit losses [Roll Forward] |  |  |
Beginning balance | 52,453 |  |
Charge-offs | (10,664) |  |
Recoveries | 5,689 |  |
Steuben acquisition | 3,729 |  |
Provision | 11,151 |  |
Ending balance | 62,358 | 52,453 |
ASU 2016-13 [Member] | Cumulative Effect Adjustment [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | (1,357) |  |
Balance, end of period | (3,100) | (1,357) |
Liabilities for off-balance-sheet credit exposures [Roll Forward] |  |  |
Beginning balance | 1,185 |  |
Ending balance |  | 1,185 |
Total allowance for credit losses [Roll Forward] |  |  |
Beginning balance | 2,542 |  |
Ending balance |  | 2,542 |
Acquired Impaired Loans [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 163 | 33 |
Charge-offs |  | 0 |
Recoveries |  | 0 |
Provision |  | 130 |
Balance, end of period |  | 163 | 33
Acquired Impaired Loans [Member] | ASU 2016-13 [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 0 |  |
Charge-offs | 0 |  |
Recoveries | 0 |  |
Steuben acquisition | 0 |  |
Provision | 0 |  |
Balance, end of period | 0 | 0 |
Acquired Impaired Loans [Member] | ASU 2016-13 [Member] | Cumulative Effect Adjustment [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 163 |  |
Balance, end of period |  | 163 |
Acquired Deteriorated |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 0 |  |
Balance, end of period |  | 0 |
Acquired Deteriorated | ASU 2016-13 [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 3,072 |  |
Charge-offs | (91) |  |
Recoveries | 440 |  |
Steuben acquisition | 668 |  |
Provision | (2,207) |  |
Balance, end of period | 1,882 | 3,072 |
Acquired Deteriorated | ASU 2016-13 [Member] | Cumulative Effect Adjustment [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | (3,072) |  |
Balance, end of period |  | (3,072) |
Consumer Mortgage [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 10,269 |  |
Balance, end of period |  | 10,269 |
Consumer Mortgage [Member] | ASU 2016-13 [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 9,218 |  |
Charge-offs | (862) |  |
Recoveries | 130 |  |
Steuben acquisition | 146 |  |
Provision | 2,040 |  |
Balance, end of period | 10,672 | 9,218 |
Consumer Mortgage [Member] | ASU 2016-13 [Member] | Cumulative Effect Adjustment [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 1,051 |  |
Balance, end of period |  | 1,051 |
Commercial Portfolio Segment [Member] | Business Lending [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 19,426 | 18,522 |
Charge-offs |  | (2,334) |
Recoveries |  | 826 |
Provision |  | 2,412 |
Balance, end of period |  | 19,426 | 18,522
Commercial Portfolio Segment [Member] | Business Lending [Member] | ASU 2016-13 [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 19,714 |  |
Charge-offs | (1,497) |  |
Recoveries | 356 |  |
Steuben acquisition | 2,343 |  |
Provision | 7,274 |  |
Balance, end of period | 28,190 | 19,714 |
Commercial Portfolio Segment [Member] | Business Lending [Member] | ASU 2016-13 [Member] | Cumulative Effect Adjustment [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 288 |  |
Balance, end of period |  | 288 |
Residential Portfolio Segment [Member] | Consumer Mortgage [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 10,269 | 10,124 |
Charge-offs |  | (1,372) |
Recoveries |  | 60 |
Provision |  | 1,457 |
Balance, end of period |  | 10,269 | 10,124
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 13,712 | 14,366 |
Charge-offs |  | (7,631) |
Recoveries |  | 4,180 |
Provision |  | 2,797 |
Balance, end of period |  | 13,712 | 14,366
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | ASU 2016-13 [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 12,715 |  |
Charge-offs | (6,382) |  |
Recoveries | 3,992 |  |
Steuben acquisition | 183 |  |
Provision | 3,188 |  |
Balance, end of period | 13,696 | 12,715 |
Consumer Portfolio Segment [Member] | Consumer Indirect [Member] | ASU 2016-13 [Member] | Cumulative Effect Adjustment [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 997 |  |
Balance, end of period |  | 997 |
Consumer Portfolio Segment [Member] | Consumer Direct [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 3,255 | 3,095 |
Charge-offs |  | (1,945) |
Recoveries |  | 710 |
Provision |  | 1,395 |
Balance, end of period |  | 3,255 | 3,095
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | ASU 2016-13 [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 2,612 |  |
Charge-offs | (1,633) |  |
Recoveries | 743 |  |
Steuben acquisition | 87 |  |
Provision | 1,398 |  |
Balance, end of period | 3,207 | 2,612 |
Consumer Portfolio Segment [Member] | Consumer Direct [Member] | ASU 2016-13 [Member] | Cumulative Effect Adjustment [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 643 |  |
Balance, end of period |  | 643 |
Consumer Portfolio Segment [Member] | Home Equity [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 2,129 | 2,144 |
Charge-offs |  | (445) |
Recoveries |  | 148 |
Provision |  | 282 |
Balance, end of period |  | 2,129 | 2,144
Consumer Portfolio Segment [Member] | Home Equity [Member] | ASU 2016-13 [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 2,937 |  |
Charge-offs | (199) |  |
Recoveries | 28 |  |
Steuben acquisition | 235 |  |
Provision | (779) |  |
Balance, end of period | 2,222 | 2,937 |
Consumer Portfolio Segment [Member] | Home Equity [Member] | ASU 2016-13 [Member] | Cumulative Effect Adjustment [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | (808) |  |
Balance, end of period |  | (808) |
Unallocated Financing Receivables [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 957 | 1,000 |
Charge-offs |  | 0 |
Recoveries |  | 0 |
Provision |  | (43) |
Balance, end of period |  | 957 | $ 1,000
Unallocated Financing Receivables [Member] | ASU 2016-13 [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | 1,000 |  |
Charge-offs | 0 |  |
Recoveries | 0 |  |
Steuben acquisition | 0 |  |
Provision | 0 |  |
Balance, end of period | 1,000 | 1,000 |
Unallocated Financing Receivables [Member] | ASU 2016-13 [Member] | Cumulative Effect Adjustment [Member] |  |  |
Allowance for loan losses [Roll Forward] |  |  |
Balance, beginning of period | $ (43) |  |
Balance, end of period |  | $ (43) |

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