EMPLOYEE BENEFIT PLANS - Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Prior service cost amortization	$ (114)	$ (176)	$ 0
Pension Plans
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Net actuarial gain experienced during the year	0	0	(394)
Net actuarial loss recognized as expense	0	(3,352)	(30,478)
Total recognized in other comprehensive income (loss)	0	(3,352)	(30,872)
Postretirement Benefits
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Net actuarial gain experienced during the year	925	682	(645)
Net actuarial loss recognized as expense	(10)	(9)	0
Prior service cost recognized as income	0	780	0
Prior service credit during the year	0	(421)	(4,071)
Prior service cost amortization	454	465	0
Total recognized in other comprehensive income (loss)	$ 1,369	$ 1,497	$ (4,716)