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BENEFIT PLANS, Fair Value of Plan Assets by Assets Category (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | [1] | $ 216,476 | $ 180,086 |
Total dividends and interest receivable excluded from plan assets |  | 600 | 300 |
Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | [1] | 196,336 | 166,117 |
Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | [1] | 20,140 | 13,969 |
Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | [1] | 0 | 0 |
Money Market Accounts [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 10,381 | 8,151 |
Money Market Accounts [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 0 | 103 |
Money Market Accounts [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 10,381 | 8,048 |
Money Market Accounts [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
Equity Securities [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 135,530 | 96,748 |
Equity Securities [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 135,530 | 96,748 |
Equity Securities [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
Equity Securities [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
U.S. Large-Cap [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 53,154 | 43,235 |
U.S. Large-Cap [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 53,154 | 43,235 |
U.S. Large-Cap [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
U.S. Large-Cap [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
U.S Mid/Small Cap [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 26,309 | 19,032 |
U.S Mid/Small Cap [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 26,309 | 19,032 |
U.S Mid/Small Cap [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
U.S Mid/Small Cap [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
CBU Stock [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 8,344 | 7,417 |
CBU Stock [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 8,344 | 7,417 |
CBU Stock [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
CBU Stock [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
International [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 47,723 | 27,064 |
International [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 47,723 | 27,064 |
International [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
International [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
Fixed Income Securities [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 59,190 | 63,106 |
Fixed Income Securities [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 49,504 | 57,243 |
Fixed Income Securities [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 9,686 | 5,863 |
Fixed Income Securities [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
Government Securities [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 23,615 | 31,238 |
Government Securities [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 13,929 | 25,375 |
Government Securities [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 9,686 | 5,863 |
Government Securities [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
Investment Grade Bonds [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 19,278 | 15,253 |
Investment Grade Bonds [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 19,278 | 15,253 |
Investment Grade Bonds [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
Investment Grade Bonds [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
High Yield [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | [2] | 16,297 | 16,615 |
High Yield [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | [2] | 16,297 | 16,615 |
High Yield [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | [2] | 0 | 0 |
High Yield [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | [2] | 0 | 0 |
Other Investments [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | [3] | 11,375 | 12,081 |
Other Investments [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | [3] | 11,302 | 12,023 |
Other Investments [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | [3] | 73 | 58 |
Other Investments [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | [3] | 0 | 0 |
Pension Benefits [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 217,107 | 180,400 | $ 172,026
Employer contributions |  | 9,839 | $ 2,146 |
Pension Benefits [Member] | Merchants Bank Pension Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Employer contributions |  | 2,000 |  |
Pension Benefits [Member] | Defined Benefit Pension Plan Excluding Merchant Bank Pension Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Employer contributions |  | $ 7,200 |  |
[1] | Excludes dividends and interest receivable totaling $0.6 million and $0.3 million at December 31, 2017 and 2016, 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.

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- Definition Represents the total of interest and dividend receivable, including any amortization and accretion (as applicable) of discounts and premiums, earned from (1) loans and leases whether held-for-sale or held-in-portfolio; (2) investment securities; (3) federal funds sold; (4) securities purchased under agreements to resell; (5) investments in banker's acceptances, commercial paper, or certificates of deposit; (6) dividend income; or (7) other investments not otherwise specified herein. + References No definition available. + Details
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