BENEFIT PLANS, Fair Value of Plan Assets by Assets Category (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Total [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets	[1]	$ 226,835	$ 202,943
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]	218,412	191,507
Total [Member] | Significant Observable Inputs, Level 2 [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets	[1]	8,423	11,436
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		3,983	4,856
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		3,983	4,856
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		105,221	89,201
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		105,221	89,201
Equity Securities [Member] | Significant Observable Inputs, Level 2 [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		0	0
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		50,301	39,122
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		50,301	39,122
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		10,408	9,881
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		10,408	9,881
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		6,522	7,692
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		6,522	7,692
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		37,990	32,506
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		37,990	32,506
International [Member] | Significant Observable Inputs, Level 2 [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		0	0
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		101,693	94,553
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		93,352	83,183
Fixed Income Securities [Member] | Significant Observable Inputs, Level 2 [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		8,341	11,370
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
Government Securities [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		82,039	75,787
Government 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		73,698	64,417
Government Securities [Member] | Significant Observable Inputs, Level 2 [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		8,341	11,370
Government 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		12,884	12,054
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		12,884	12,054
Investment Grade Bonds [Member] | Significant Observable Inputs, Level 2 [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		0	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]	6,770	6,712
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]	6,770	6,712
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]	15,938	14,333
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]	15,856	14,267
Other Investments [Member] | Significant Observable Inputs, Level 2 [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets	[3]	82	66
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		500	700
Pension Benefits [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		227,323	203,672	$ 217,107
Pension Plans [Abstract]
Contribution made to defined benefit pension plan by employer		7,893	$ 605
Pension Benefits [Member] | Nonqualified Plan [Member] | us-gaap_UnfundedPlanMember
Pension Plans [Abstract]
Contribution made to defined benefit pension plan by employer		$ 7,300

[1] Excludes dividends and interest receivable totaling $0.5 million and $0.7 million at December 31, 2019 and 2018, 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.