PENSION AND OTHER POSTRETIREMENT BENEFITS - NET PERIODIC BENEFIT COST (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Percentage of employees entitled to annuity payments upon retirement	38.20%
Pension Benefits
Net periodic benefit cost [Abstract]
Service Cost	$ 4,843	$ 5,337	$ 4,510
Interest Cost	12,125	11,168	10,735
Expected Return on Plan Assets	(13,508)	(14,789)	(13,491)
Amortizations:
Prior Service (Credits) Cost	212	212	175
Actuarial Loss	9,394	10,608	5,716
Net Periodic Benefit Cost	13,066	12,536	7,645
Capitalized Benefit Costs	(4,645)	(4,805)	(3,047)
Deferred Benefit Costs	(645)	(1,007)	0
Total Net Periodic Benefit Expense	7,776	6,724	4,598
Other Postretirement Benefits
Net periodic benefit cost [Abstract]
Service Cost	851	1,116	891
Interest Cost	2,615	2,973	2,852
Expected Return on Plan Assets	(3,104)	(2,993)	(2,603)
Amortizations:
Prior Service (Credits) Cost	(344)	608	152
Actuarial Loss	1,109	1,342	974
Net Periodic Benefit Cost	1,127	3,046	2,266
Capitalized Benefit Costs	(277)	(1,043)	(722)
Deferred Benefit Costs	0	(256)	0
Total Net Periodic Benefit Expense	$ 850	$ 1,747	$ 1,544