PENSION AND OTHER POSTRETIREMENT BENEFITS (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	50.40%
Pension Benefits
Net periodic benefit cost [Abstract]
Service Cost	$ 4,144	$ 4,430	$ 3,697
Interest Cost	10,292	9,357	8,952
Expected Return on Plan Assets	(11,029)	(11,914)	(10,818)
Amortization:
Prior Service Cost (Credits)	203	203	177
Actuarial Loss	7,975	8,969	4,864
Net Periodic Benefit Cost	11,585	11,045	6,872
Capitalized Benefit Costs	(4,645)	(4,805)	(3,047)
Affiliate SERP Allocations	(1,960)	(1,688)	(1,313)
Deferred Benefit Costs	(644)	(1,007)	0
Total Net Periodic Benefit Expense	4,336	3,545	2,512
Other Postretirement Benefits
Net periodic benefit cost [Abstract]
Service Cost	576	726	585
Interest Cost	2,120	2,406	2,297
Expected Return on Plan Assets	(2,823)	(2,708)	(2,467)
Amortization:
Prior Service Cost (Credits)	(257)	499	133
Actuarial Loss	945	1,107	770
Net Periodic Benefit Cost	561	2,030	1,318
Capitalized Benefit Costs	(277)	(1,043)	(722)
Affiliate SERP Allocations	0	0	0
Deferred Benefit Costs	0	(256)	0
Total Net Periodic Benefit Expense	$ 284	$ 731	$ 596