PENSION PLANS AND OTHER POST-RETIREMENT BENEFITS - Components of net periodic pension cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plans
Components of net periodic pension costs
Service cost		$ 11
Interest cost	$ 6,520	7,661
Expected return on plan assets	(8,645)	(8,671)
Net amortization loss (gain)	311	703
Net prior service cost amortization	30	30
Net periodic pension cost (benefit)	(1,784)	(266)
Post-retirement Benefit Obligations
Components of net periodic pension costs
Service cost	258	117
Interest cost	884	1,144
Expected return on plan assets	(46)	(43)
Net amortization loss (gain)	(299)	(108)
Net prior service cost amortization	412	768
Net periodic pension cost (benefit)	$ 1,209	$ 1,878