POSTRETIREMENT BENEFIT PLANS - Composition of Plan Assets, Pension Plans (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 80,483,000	$ 72,064,000
Principal pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	58,843,000	52,633,000	$ 50,009,000
Principal pension | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	29,164,000	27,657,000
Principal pension | Global equities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,552,000	6,826,000
Principal pension | Global equities | Plan assets measured at net asset value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16,259,000	14,616,000
Principal pension | Fixed income and cash investment funds | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,831,000	4,398,000
Principal pension | U.S. corporate | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,512,000	8,025,000
Principal pension | Other debt securities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,505,000	6,076,000
Principal pension | Debt securities | Plan assets measured at net asset value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,445,000	3,744,000
Principal pension | Real estate | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,274,000	2,309,000
Principal pension | Real estate | Plan assets measured at net asset value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,324,000	1,167,000
Principal pension | Private equities & other investments | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	490,000	23,000
Principal pension | Private equities & other investments | Plan assets measured at net asset value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,651,000	5,449,000
Other pension
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21,506,000	19,142,000	$ 17,537,000
Other pension | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15,107,000	12,954,000
Other pension | Global equities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,674,000	3,484,000
Other pension | Global equities | Plan assets measured at net asset value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,415,000	1,450,000
Other pension | Fixed income and cash investment funds | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10,003,000	8,089,000
Other pension | U.S. corporate | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	410,000	365,000
Other pension | Other debt securities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	440,000	424,000
Other pension | Debt securities | Plan assets measured at net asset value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,268,000	914,000
Other pension | Real estate | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	81,000	140,000
Other pension | Real estate | Plan assets measured at net asset value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,978,000	1,930,000
Other pension | Private equities & other investments | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	499,000	452,000
Other pension | Private equities & other investments | Plan assets measured at net asset value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,738,000	$ 1,894,000