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BENEFIT PLANS (Tables) | 12 Months Ended
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Dec. 31, 2020
BENEFIT PLANS |
Schedule of funded status of the Company's plans reconciled with amounts reported | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Pension Benefits | ​ | Post-retirement Benefits
(000’s omitted) |  | 2020 |  | 2019 |  | 2020 |  | 2019
Change in benefit obligation: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
Benefit obligation at the beginning of year | ​ | $ | 162,084 | ​ | $ | 144,211 | ​ | $ | 1,677 | ​ | $ | 1,657
Service cost | ​ |  | 5,750 | ​ |  | 5,081 | ​ |  | 0 | ​ |  | 0
Interest cost | ​ |  | 5,657 | ​ |  | 6,264 | ​ |  | 57 | ​ |  | 70
Plan amendment / acquisition | ​ |  | 13,598 | ​ |  | 0 | ​ |  | 0 | ​ |  | 0
Participant contributions | ​ |  | 0 | ​ |  | 0 | ​ |  | 0 | ​ |  | 35
Deferred actuarial (gain)/loss | ​ |  | 13,954 | ​ |  | 16,292 | ​ |  | 114 | ​ |  | 87
Benefits paid | ​ |  | (10,682) | ​ |  | (9,764) | ​ |  | (130) | ​ |  | (172)
Benefit obligation at end of year | ​ |  | 190,361 | ​ |  | 162,084 | ​ |  | 1,718 | ​ |  | 1,677
Change in plan assets: | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​
Fair value of plan assets at beginning of year | ​ |  | 227,323 | ​ |  | 203,672 | ​ |  | 0 | ​ |  | 0
Actual return of plan assets | ​ |  | 33,543 | ​ |  | 25,522 | ​ |  | 0 | ​ |  | 0
Participant contributions | ​ |  | 0 | ​ |  | 0 | ​ |  | 0 | ​ |  | 35
Employer contributions | ​ |  | 7,993 | ​ |  | 7,893 | ​ |  | 130 | ​ |  | 137
Plan acquisition | ​ |  | 14,423 | ​ |  | 0 | ​ |  | 0 | ​ |  | 0
Benefits paid | ​ |  | (10,682) | ​ |  | (9,764) | ​ |  | (130) | ​ |  | (172)
Fair value of plan assets at end of year | ​ |  | 272,600 | ​ |  | 227,323 | ​ |  | 0 | ​ |  | 0
Over/(Under) funded status at year end | ​ | $ | 82,239 | ​ | $ | 65,239 | ​ | $ | (1,718) | ​ | $ | (1,677)
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Amounts recognized in the consolidated statement of condition were: | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |
Other assets | ​ | $ | 101,849 | ​ | $ | 81,930 | ​ | $ | 0 | ​ | $ | 0
Other liabilities | ​ |  | (19,610) | ​ |  | (16,691) | ​ |  | (1,718) | ​ |  | (1,677)
Amounts recognized in accumulated other comprehensive loss/(income) (“AOCI”) were: | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |
Net loss | ​ | $ | 34,290 | ​ | $ | 42,743 | ​ | $ | 715 | ​ | $ | 641
Net prior service cost (credit) | ​ |  | 4,348 | ​ |  | 4,056 | ​ |  | (1,086) | ​ |  | (1,265)
Pre-tax AOCI | ​ |  | 38,638 | ​ |  | 46,799 | ​ |  | (371) | ​ |  | (624)
Taxes | ​ |  | (9,488) | ​ |  | (11,448) | ​ |  | 94 | ​ |  | 155
AOCI at year end | ​ | $ | 29,150 | ​ | $ | 35,351 | ​ | $ | (277) | ​ | $ | (469)
Schedule of Amounts recognized in accumulated other comprehensive income, net of tax | Amounts recognized in accumulated other comprehensive income, net of tax, for the year ended December 31, are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Pension Benefits | ​ | Post-retirement Benefits
(000’s omitted) |  | 2020 |  | 2019 |  | 2020 |  | 2019
Prior service cost/(credit) | ​ | $ | 222 | ​ | $ | (49) | ​ | $ | 136 | ​ | $ | 136
Net (gain) loss | ​ |  | (6,423) | ​ |  | 1,920 | ​ |  | 56 | ​ |  | 38
Total | ​ | $ | (6,201) | ​ | $ | 1,871 | ​ | $ | 192 | ​ | $ | 174
​
Schedule of estimated costs, net of tax, that will be amortized from accumulated other comprehensive (income) loss into net periodic (income) cost over the next fiscal year | The estimated costs, net of tax, that will be amortized from accumulated other comprehensive (income) loss into net periodic (income) cost over the next fiscal year are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Pension |  | Post-retirement
(000’s omitted) | ​ | Benefits | ​ | Benefits
Prior service credit | ​ | $ | 379 | ​ | $ | (179)
Net loss | ​ |  | 3,560 | ​ |  | 48
Total | ​ | $ | 3,939 | ​ | $ | (131)
​
Schedule of weighted-average assumptions used to determine the benefit obligations | The weighted-average assumptions used to determine the benefit obligations as of December 31 are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Pension Benefits | ​ | Post-retirement Benefits |
​ |  | 2020 |  | 2019 |  | 2020 |  | 2019 |
Discount rate |  | 2.80 | % | 3.50 | % | 2.80 | % | 3.60 | %
Expected return on plan assets |  | 7.00 | % | 7.00 | % | N/A |  | N/A | ​
Rate of compensation increase |  | 3.50 | % | 3.50 | % | N/A |  | N/A | ​
​
Schedule of net periodic benefit cost | The net periodic benefit cost as of December 31 is as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Pension Benefits | ​ | Post-retirement Benefits
(000’s omitted) |  | 2020 |  | 2019 |  | 2018 |  | 2020 |  | 2019 |  | 2018
Service cost | ​ | $ | 5,750 | ​ | $ | 5,081 | ​ | $ | 4,561 | ​ | $ | 0 | ​ | $ | 0 | ​ | $ | 0
Interest cost | ​ |  | 5,657 | ​ |  | 6,264 | ​ |  | 5,676 | ​ |  | 57 | ​ |  | 70 | ​ |  | 69
Expected return on plan assets | ​ |  | (16,306) | ​ |  | (14,311) | ​ |  | (14,820) | ​ |  | 0 | ​ |  | 0 | ​ |  | 0
Plan amendment | ​ |  | (637) | ​ |  | 0 | ​ |  | 13 | ​ |  | 0 | ​ |  | 0 | ​ |  | 0
Amortization of unrecognized net loss/(gain) | ​ |  | 3,239 | ​ |  | 2,568 | ​ |  | 1,193 | ​ |  | 40 | ​ |  | 38 | ​ |  | 21
Amortization of prior service cost | ​ |  | 241 | ​ |  | 64 | ​ |  | (293) | ​ |  | (179) | ​ |  | (179) | ​ |  | (179)
Net periodic (benefit) | ​ | $ | (2,056) | ​ | $ | (334) | ​ | $ | (3,670) | ​ | $ | (82) | ​ | $ | (71) | ​ | $ | (89)
​
Schedule of weighted-average assumptions used to determine the net periodic pension cost | The weighted-average assumptions used to determine the net periodic pension cost for the years ended December 31 are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Pension Benefits | ​ | Post-retirement Benefits |
​ |  | 2020 |  | 2019 |  | 2018 |  | 2020 |  | 2019 |  | 2018 |
Discount rate |  | 3.50 | % | 4.50 | % | 4.00 | % | 3.60 | % | 4.45 | % | 4.00 | %
Expected return on plan assets |  | 7.00 | % | 7.00 | % | 7.00 | % | N/A |  | N/A |  | N/A | ​
Rate of compensation increase |  | 3.50 | % | 3.50 | % | 3.50 | % | N/A |  | N/A |  | N/A | ​
Schedule of amount of benefit payments | The amount of benefit payments that are expected to be paid over the next ten years are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Pension | ​ | Post-retirement
(000’s omitted) |  | Benefits |  | Benefits
2021 | ​ | $ | 10,588 | ​ | $ | 174
2022 | ​ |  | 11,061 | ​ |  | 130
2023 | ​ |  | 12,004 | ​ |  | 128
2024 | ​ |  | 12,404 | ​ |  | 125
2025 | ​ |  | 12,238 | ​ |  | 122
2026-2030 | ​ |  | 64,468 | ​ |  | 560
​
Schedule of fair value of defined benefit plan assets by asset category | The fair values of the Company’s defined benefit pension plan assets at December 31, 2020 by asset category are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Quoted Prices | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | in Active | ​ | Significant | ​ | Significant | ​ |  | ​
​ | ​ | Markets for | ​ | Observable | ​ | Unobservable | ​ |  | ​
​ | ​ | Identical Assets | ​ | Inputs | ​ | Inputs | ​ |  | ​
Asset category (000’s omitted) |  | Level 1 |  | Level 2 |  | Level 3 |  | Total
Cash equivalents | ​ | $ | 68,137 | ​ | $ | 0 | ​ | $ | 0 | ​ | $ | 68,137
Equity securities: | ​ |  | ​ | ​ |  | ​ | ​ |  |  | ​ |  | ​
U.S. large-cap | ​ |  | 69,195 | ​ |  | 0 | ​ |  | 0 | ​ |  | 69,195
U.S mid/small cap | ​ |  | 17,338 | ​ |  | 0 | ​ |  | 0 | ​ |  | 17,338
CBU stock | ​ |  | 7,701 | ​ |  | 0 | ​ |  | 0 | ​ |  | 7,701
International | ​ |  | 43,457 | ​ |  | 6,088 | ​ |  | 0 | ​ |  | 49,545
​ | ​ |  | 137,691 | ​ |  | 6,088 | ​ |  | 0 | ​ |  | 143,779
​ | ​ |  | ​ | ​ |  | ​ | ​ |  |  | ​ |  | ​
Fixed income securities: | ​ |  | ​ | ​ |  | ​ | ​ |  |  | ​ |  | ​
Government securities | ​ |  | 6,154 | ​ |  | 6,241 | ​ |  | 0 | ​ |  | 12,395
Investment grade bonds | ​ |  | 23,085 | ​ |  | 141 | ​ |  | 0 | ​ |  | 23,226
High yield(a) | ​ |  | 5,803 | ​ |  | 0 | ​ |  | 0 | ​ |  | 5,803
​ | ​ |  | 35,042 | ​ |  | 6,382 | ​ |  | 0 | ​ |  | 41,424
​ | ​ |  | ​ | ​ |  | ​ | ​ |  |  | ​ |  | ​
Other investments (b) | ​ |  | 17,613 | ​ |  | 0 | ​ |  | 0 | ​ |  | 17,613
​ | ​ |  | ​ | ​ |  | ​ | ​ |  |  | ​ |  | ​
Total (c)(d) | ​ | $ | 258,483 | ​ | $ | 12,470 | ​ | $ | 0 | ​ | $ | 270,953
​ The fair values of the Company’s defined benefit pension plan assets at December 31, 2019 by asset category are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Quoted Prices | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | in Active | ​ | Significant | ​ | Significant | ​ | ​ | ​
​ | ​ | Markets for | ​ | Observable | ​ | Unobservable | ​ | ​ | ​
​ | ​ | Identical Assets | ​ | Inputs | ​ | Inputs | ​ | ​ | ​
Asset category (000’s omitted) |  | Level 1 |  | Level 2 |  | Level 3 |  | Total
Cash equivalents | ​ | $ | 3,983 | ​ | $ | 0 | ​ | $ | 0 | ​ | $ | 3,983
Equity securities: | ​ |  | ​ | ​ |  | ​ | ​ |  |  | ​ |  | ​
U.S. large-cap | ​ |  | 50,301 | ​ |  | 0 | ​ |  | 0 | ​ |  | 50,301
U.S mid/small cap | ​ |  | 10,408 | ​ |  | 0 | ​ |  | 0 | ​ |  | 10,408
CBU stock | ​ |  | 6,522 | ​ |  | 0 | ​ |  | 0 | ​ |  | 6,522
International | ​ |  | 30,420 | ​ |  | 5,375 | ​ |  | 0 | ​ |  | 35,795
​ | ​ |  | 97,651 | ​ |  | 5,375 | ​ |  | 0 | ​ |  | 103,026
​ | ​ |  | ​ | ​ |  | ​ | ​ |  |  | ​ |  | ​
Fixed income securities: | ​ |  | ​ | ​ |  | ​ | ​ |  |  | ​ |  | ​
Government securities | ​ |  | 73,698 | ​ |  | 8,341 | ​ |  | 0 | ​ |  | 82,039
Investment grade bonds | ​ |  | 12,938 | ​ |  | 0 | ​ |  | 0 | ​ |  | 12,938
High yield(a) | ​ |  | 5,279 | ​ |  | 0 | ​ |  | 0 | ​ |  | 5,279
​ | ​ |  | 91,915 | ​ |  | 8,341 | ​ |  | 0 | ​ |  | 100,256
​ | ​ |  | ​ | ​ |  | ​ | ​ |  |  | ​ |  | ​
Other investments (b) | ​ |  | 19,488 | ​ |  | 82 | ​ |  | 0 | ​ |  | 19,570
​ | ​ |  | ​ | ​ |  | ​ | ​ |  |  | ​ |  | ​
Total (c) | ​ | $ | 213,037 | ​ | $ | 13,798 | ​ | $ | 0 | ​ | $ | 226,835
​
(a) | This category is exchange-traded funds representing a diversified index of high yield corporate bonds.
(b) | This category is comprised of exchange-traded funds and mutual funds holding non-traditional investment classes including private equity funds and alternative exchange funds.
(c) | Excludes dividends and interest receivable totaling $0.2 million and $0.5 million at December 31, 2020 and 2019, respectively.
(d) | Excludes certain investments that were measured at net asset value per share (or its equivalents) totaling $1.4 million at December 31, 2020.

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