STOCKHOLDERS' EQUITY - AOCI Reclassifications (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amortization of pension and other post-retirement liability adjustments
Tax (benefit) expense	$ (1,600)	$ 45,424	$ 37,471
Net of tax	80,192	82,114	65,948
Amounts reclassified from accumulated other comprehensive income (loss)
Amortization of pension and other post-retirement liability adjustments
Prior service credits	1,485	1,120	852
Actuarial losses	(707)	(244)	(6,167)
Loss recognized during settlement	(2,162)	0	0
Total before tax	(1,384)	876	(5,315)
Tax (benefit) expense	(548)	312	(1,929)
Net of tax	$ (836)	$ 564	$ (3,386)