BENEFIT PLANS - Fair Value of Plan Assets by Assets Category (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
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]	$ 290,247	270,953
Total [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets	[1]	284,137	258,483
Total [Member] | Significant Observable Inputs, Level 2
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets	[1]	6,110	12,470
Total [Member] | Significant Unobservable Inputs, Level 3
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		19,605	68,137
Money Market Accounts / Cash Equivalents [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		19,605	68,137
Money Market Accounts / Cash Equivalents [Member] | Significant Observable Inputs, Level 2
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
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		169,818	143,779
Equity Securities [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		169,818	137,691
Equity Securities [Member] | Significant Observable Inputs, Level 2
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		0	6,088
Equity Securities [Member] | Significant Unobservable Inputs, Level 3
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		75,180	69,195
U.S. Large-Cap [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		75,180	69,195
U.S. Large-Cap [Member] | Significant Observable Inputs, Level 2
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
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		12,052	17,338
U.S Mid/Small Cap [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		12,052	17,338
U.S Mid/Small Cap [Member] | Significant Observable Inputs, Level 2
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
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		0	0
CBU common stock [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		7,604	7,701
CBU common stock [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		7,604	7,701
CBU common stock [Member] | Significant Observable Inputs, Level 2
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		0	0
CBU common stock [Member] | Significant Unobservable Inputs, Level 3
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		73,891	49,545
International [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		73,891	43,457
International [Member] | Significant Observable Inputs, Level 2
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		0	6,088
International [Member] | Significant Unobservable Inputs, Level 3
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		0	0
Other
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		1,091
Other | Quoted Prices in Active Markets For Identical Assets, Level 1
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		1,091
Other | Significant Observable Inputs, Level 2
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		0
Other | Significant Unobservable Inputs, Level 3
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		0
Fixed Income Securities [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		76,097	41,424
Fixed Income Securities [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		69,987	35,042
Fixed Income Securities [Member] | Significant Observable Inputs, Level 2
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		6,110	6,382
Fixed Income Securities [Member] | Significant Unobservable Inputs, Level 3
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		0	0
U.S. Treasury and Agency Securities
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		25,962	12,395
U.S. Treasury and Agency Securities | Quoted Prices in Active Markets For Identical Assets, Level 1
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		19,988	6,154
U.S. Treasury and Agency Securities | Significant Observable Inputs, Level 2
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		5,974	6,241
U.S. Treasury and Agency Securities | Significant Unobservable Inputs, Level 3
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		41,510	23,226
Investment Grade Bonds [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		41,374	23,085
Investment Grade Bonds [Member] | Significant Observable Inputs, Level 2
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		136	141
Investment Grade Bonds [Member] | Significant Unobservable Inputs, Level 3
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]	8,625	5,803
High Yield [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets	[2]	8,625	5,803
High Yield [Member] | Significant Observable Inputs, Level 2
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
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]	24,727	17,613
Other Investments [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets	[3]	24,727	17,613
Other Investments [Member] | Significant Observable Inputs, Level 2
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets	[3]	0	0
Other Investments [Member] | Significant Unobservable Inputs, Level 3
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		400	200
Pension Benefits [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Fair value of plan assets		290,687	272,600	$ 227,323
Pension Plans [Abstract]
Contribution made to defined benefit pension plan by employer		3,639	7,993
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		$ 2,900	$ 3,900

[1] Excludes dividends and interest receivable totaling $0.4 million and $0.2 million at December 31, 2021 and 2020, 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.