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RETIREMENT PLANS: (Details 7) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Net loss (gain) during the period | $ 3,188 | $ 6,362 | $ (1,192)
Amortization of prior service cost | (1) | (1) | (1)
Amortization of unrecognized gain (loss) | (1,967) | (1,558) | (1,447)
Total recognized in other comprehensive income (loss) | 1,220 | 4,803 | (2,640)
Supplemental Employee Retirement Plans, Defined Benefit |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Net loss (gain) during the period | 1,459 | 1,357 | 260
Amortization of prior service cost | 0 | 0 | 0
Amortization of unrecognized gain (loss) | (246) | (75) | (51)
Total recognized in other comprehensive income (loss) | $ 1,213 | $ 1,282 | $ 209

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