TAX EFFECT ALLOCATED TO EACH COMPONENT OF OTHER COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Income and Comprehensive Income
Adjustment of pension and postretirement benefit plans	$ 3,387	$ 8,203	$ 6,044
Amortization of net losses of pension and postretirement benefit plans	(8,042)	(8,817)	(8,401)
Amortization of prior service credit of pension and postretirement benefit plans	0	0	1,354
Unrealized holding gains (losses) on debt securities arising during the period	(51,213)	(20,113)	219
Reclassification adjustment for (gains) losses included in net income	6	(4)	0
Unrealized net (losses) gains on cash flow hedges	2,472	1,302	(210)
Reclassification adjustment for net losses (gains) included in net income	(2,084)	(1,496)	433
Income tax expense	$ (55,474)	$ (20,925)	$ (561)

Amount of tax (expense) benefit of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) of defined benefit plan.

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Amount, before reclassification adjustment, of tax (expense) benefit for (increase) decrease in accumulated other comprehensive income for defined benefit plan.

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income of accumulated gain (loss) realized from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's deferred hedging gain (loss).

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan.

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Amount of tax expense (benefit) allocated to other comprehensive income (loss) attributable to parent entity.

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Amount of tax expense (benefit), before reclassification adjustments, related to increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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Amount of tax expense (benefit) before reclassification adjustments of unrealized holding gain (loss) on available-for-sale securities.

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