EMPLOYEE BENEFIT PLANS - Changes in Plan Assets, Benefit Obligations, and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets:
Other adjustments	$ 4,988	$ 5,733	$ 9,308
Pension Benefits
Change in projected benefit obligation:
Beginning of year	812,029	639,921
Service cost	36,002	26,718	29,027
Interest cost	25,741	26,966	23,994
Actuarial (gain) loss (1)	(23,470)	133,230
Plan amendment	(833)	0
Benefits paid, net of retiree premiums	(15,530)	(14,806)
End of year	833,939	812,029	639,921
Change in plan assets:
Fair value of plan assets at beginning of year	573,575	469,774
Actual return on plan assets	121,751	97,811
Employer contributions	36,998	20,796
Retiree contributions and Medicare part D subsidies	0	0
Benefits paid	(15,530)	(14,806)
Other adjustments	0	0
Fair value of plan assets at end of year	716,794	573,575	469,774
Reconciliation of funded status
Funded status	(117,145)	(238,454)
Unrecognized actuarial loss	52,816	177,750
Unrecognized prior service cost	5,181	10,242
Net amount recognized	(59,148)	(50,462)
Short-term portion of the pension benefits	2,100
Other Benefits
Change in projected benefit obligation:
Beginning of year	150,515	127,204
Service cost	7,945	7,475	8,317
Interest cost	4,305	5,441	4,873
Actuarial (gain) loss (1)	(30,485)	11,701
Plan amendment	0	0
Benefits paid, net of retiree premiums	(1,622)	(1,306)
End of year	130,658	150,515	127,204
Change in plan assets:
Fair value of plan assets at beginning of year	128,554	102,625
Actual return on plan assets	13,272	19,730
Employer contributions	6,853	7,505
Retiree contributions and Medicare part D subsidies	2,075	1,874
Benefits paid	(3,849)	(3,180)
Other adjustments	152	0
Fair value of plan assets at end of year	147,057	128,554	$ 102,625
Reconciliation of funded status
Funded status	16,399	(21,961)
Unrecognized actuarial loss	(20,699)	15,822
Unrecognized prior service cost	1,932	2,129
Net amount recognized	$ (2,368)	$ (4,010)