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BENEFIT PLANS - Fair Value of Plan Assets by Assets Category (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets $ 1,400    
Total [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets [1] 270,953 $ 226,835  
Total [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets [1] 258,483 213,037  
Total [Member] | Significant Observable Inputs, Level 2 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets [1] 12,470 13,798  
Total [Member] | Significant Unobservable Inputs, Level 3 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets [1] 0 0  
Money Market Accounts / Cash Equivalents [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 68,137 3,983  
Money Market Accounts / Cash Equivalents [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 68,137 3,983  
Money Market Accounts / Cash Equivalents [Member] | Significant Observable Inputs, Level 2 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 0 0  
Money Market Accounts / Cash Equivalents [Member] | Significant Unobservable Inputs, Level 3 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 0 0  
Equity Securities [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 143,779 103,026  
Equity Securities [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 137,691 97,651  
Equity Securities [Member] | Significant Observable Inputs, Level 2 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 6,088 5,375  
Equity Securities [Member] | Significant Unobservable Inputs, Level 3 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 0 0  
U.S. Large-Cap [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 69,195 50,301  
U.S. Large-Cap [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 69,195 50,301  
U.S. Large-Cap [Member] | Significant Observable Inputs, Level 2 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 0 0  
U.S. Large-Cap [Member] | Significant Unobservable Inputs, Level 3 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 0 0  
U.S Mid/Small Cap [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 17,338 10,408  
U.S Mid/Small Cap [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 17,338 10,408  
U.S Mid/Small Cap [Member] | Significant Observable Inputs, Level 2 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 0 0  
U.S Mid/Small Cap [Member] | Significant Unobservable Inputs, Level 3 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 0 0  
CBU Stock [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 7,701 6,522  
CBU Stock [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 7,701 6,522  
CBU Stock [Member] | Significant Observable Inputs, Level 2 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 0 0  
CBU Stock [Member] | Significant Unobservable Inputs, Level 3 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 0 0  
International [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 49,545 35,795  
International [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 43,457 30,420  
International [Member] | Significant Observable Inputs, Level 2 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 6,088 5,375  
International [Member] | Significant Unobservable Inputs, Level 3 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 0 0  
Fixed Income Securities [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 41,424 100,256  
Fixed Income Securities [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 35,042 91,915  
Fixed Income Securities [Member] | Significant Observable Inputs, Level 2 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 6,382 8,341  
Fixed Income Securities [Member] | Significant Unobservable Inputs, Level 3 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 0 0  
U.S. Treasury and Agency Securities [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 12,395 82,039  
U.S. Treasury and Agency Securities [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 6,154 73,698  
U.S. Treasury and Agency Securities [Member] | Significant Observable Inputs, Level 2 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 6,241 8,341  
U.S. Treasury and Agency Securities [Member] | Significant Unobservable Inputs, Level 3 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 0 0  
Investment Grade Bonds [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 23,226 12,938  
Investment Grade Bonds [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 23,085 12,938  
Investment Grade Bonds [Member] | Significant Observable Inputs, Level 2 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 141 0  
Investment Grade Bonds [Member] | Significant Unobservable Inputs, Level 3 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 0 0  
High Yield [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets [2] 5,803 5,279  
High Yield [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets [2] 5,803 5,279  
High Yield [Member] | Significant Observable Inputs, Level 2 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets [2] 0 0  
High Yield [Member] | Significant Unobservable Inputs, Level 3 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets [2] 0 0  
Other Investments [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets [3] 17,613 19,570  
Other Investments [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets [3] 17,613 19,488  
Other Investments [Member] | Significant Observable Inputs, Level 2 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets [3] 0 82  
Other Investments [Member] | Significant Unobservable Inputs, Level 3 [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets [3] 0 0  
Dividends and Interest Receivable [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 200 500  
Pension Benefits [Member]      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 272,600 227,323 $ 203,672
Pension Plans [Abstract]      
Contribution made to defined benefit pension plan by employer 7,993 7,893  
Pension Benefits [Member] | Unfunded Plan Member      
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Fair value of plan assets 3,500    
Pension Benefits [Member] | Nonqualified Plan [Member] | Unfunded Plan Member      
Pension Plans [Abstract]      
Contribution made to defined benefit pension plan by employer $ 3,900 $ 7,300  
[1] Excludes dividends and interest receivable totaling $0.2 million and $0.5 million at December 31, 2020 and 2019, respectively.
[2] This category is exchange-traded funds representing a diversified index of high yield corporate bonds.
[3] This category is comprised of exchange-traded funds and mutual funds holding non-traditional investment classes including private equity funds and alternative exchange funds.