BENEFIT PLANS, Fair Value of Plan Assets by Assets Category (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Total [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets	[1]	$ 202,943	$ 216,476
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]	191,507	196,336
Total [Member] | Significant Observable Inputs, Level 2 [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets	[1]	11,436	20,140
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		4,856	10,381
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		4,856	0
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	10,381
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		89,201	135,530
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		89,201	135,530
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		39,122	53,154
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		39,122	53,154
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		9,881	26,309
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		9,881	26,309
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		7,692	8,344
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		7,692	8,344
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		32,506	47,723
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		32,506	47,723
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		94,553	59,190
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		83,183	49,504
Fixed Income Securities [Member] | Significant Observable Inputs, Level 2 [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		11,370	9,686
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		75,787	23,615
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		64,417	13,929
Government Securities [Member] | Significant Observable Inputs, Level 2 [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		11,370	9,686
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		12,054	19,278
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		12,054	19,278
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]	6,712	16,297
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]	6,712	16,297
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]	14,333	11,375
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]	14,267	11,302
Other Investments [Member] | Significant Observable Inputs, Level 2 [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets	[3]	66	73
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		700	600
Pension Benefits [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		$ 203,672	$ 217,107	$ 180,400

[1] Excludes dividends and interest receivable totaling $0.7 million and $0.6 million at December 31, 2018 and 2017, 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.