PENSION PLANS AND OTHER POST-RETIREMENT BENEFITS - Components of net periodic pension cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Plans
Components of net periodic pension costs
Service cost		$ 12		$ 23
Interest cost	$ 6,519	7,661	$ 13,039	15,322
Expected return on plan assets	(8,646)	(8,672)	(17,291)	(17,343)
Net amortization loss (gain)	311	703	622	1,406
Net prior service cost amortization	31	31	61	61
Net periodic pension cost (benefit)	(1,785)	(265)	(3,569)	(531)
Post-retirement Benefit Obligations
Components of net periodic pension costs
Service cost	258	116	516	233
Interest cost	885	1,145	1,769	2,289
Expected return on plan assets	(46)	(44)	(92)	(87)
Net amortization loss (gain)	(299)	(107)	(598)	(215)
Net prior service cost amortization	411	768	823	1,536
Net periodic pension cost (benefit)	$ 1,209	$ 1,878	$ 2,418	$ 3,756