PENSION AND OTHER POST-RETIREMENT BENEFIT PLANS - SCHEDULE OF COMPONENTS OF NET PERIODIC BENEFIT COST (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Amounts recognized in net periodic benefit cost
Service cost	$ 21,734	$ 20,878	$ 21,223
Interest cost	26,112	35,756	31,943
Expected return on plan assets	(52,907)	(54,520)	(58,612)
Amortization of prior service (credit) cost	(7,308)	(7,230)	(7,202)
Amortization of net loss (gain)	26,952	32,647	26,875
Curtailment credit	0	0	(299)
Settlement loss	13,421	5,498	20,211
Total net periodic benefit cost	28,004	33,029	34,139
Change in plan assets and benefit obligations recognized in AOCI, pre-tax
Actuarial net (gain) loss	(15,606)	(52,028)	3,715
Prior service cost (credit)	7,310	7,232	7,198
Total recognized in other comprehensive (income) loss, pre-tax	(8,296)	(44,796)	10,913
Net amounts recognized in periodic benefit cost and AOCI	19,708	(11,767)	45,052
Other Benefits
Amounts recognized in net periodic benefit cost
Service cost	159	151	230
Interest cost	6,029	7,837	6,923
Expected return on plan assets	0	0	0
Amortization of prior service (credit) cost	300	811	836
Amortization of net loss (gain)	(39)	(385)	0
Curtailment credit	0	0	0
Settlement loss	0	0	0
Total net periodic benefit cost	6,449	8,414	7,989
Change in plan assets and benefit obligations recognized in AOCI, pre-tax
Actuarial net (gain) loss	15,266	23,956	(10,771)
Prior service cost (credit)	(300)	(810)	(838)
Total recognized in other comprehensive (income) loss, pre-tax	14,966	23,146	(11,609)
Net amounts recognized in periodic benefit cost and AOCI	$ 21,415	$ 31,560	$ (3,620)