OTHER COMPREHENSIVE INCOME (Changes in Accumulated Other Comprehensive Income, Net of Tax) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI, Beginning balance	$ (139,376)	$ (120,069)	$ (161,714)
Other comprehensive income (loss) before reclassifications	(14,106)	(20,583)	23,832
Amounts reclassified from AOCI	4,016	1,276	17,813
Other comprehensive income (loss)	(10,090)	(19,307)	41,645
AOCI, Ending balance	(149,466)	(139,376)	(120,069)
Reclassification Adjustment for Cash Flow Hedge Losses [Member]
AOCI, Beginning balance	(13,300)	(14,494)	(20,322)
Other comprehensive income (loss) before reclassifications	0	0	0
Amounts reclassified from AOCI	1,862	1,194	5,828
Other comprehensive income (loss)	1,862	1,194	5,828
AOCI, Ending balance	(11,438)	(13,300)	(14,494)
Amortization of Pension and Postretirement Plan Actuarial Loss and Prior Service Cost [Member]
AOCI, Beginning balance	(126,076)	(105,575)	(141,392)
Other comprehensive income (loss) before reclassifications	(14,106)	(20,583)	23,832
Amounts reclassified from AOCI	2,154	82	11,985
Other comprehensive income (loss)	(11,952)	(20,501)	35,817
AOCI, Ending balance	$ (138,028)	$ (126,076)	$ (105,575)