PENSION AND OTHER POSTRETIREMENT BENEFITS (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) plan	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012
PENSIONS AND OTHER POSTRETIREMENT BENEFITS
Number of plans | plan		2
Number of plans covering nonunion employees | plan		1
Defined Benefit Plan, Benefit Obligation, Amount Settled			$ 30,200
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements			(6,100)
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements and Curtailments	$ (6,500)	$ 7,100
Change in projected benefit obligation:
Plan amendments		(1,684)
Liability (gain) loss related to curtailment		0	(6,509)	$ 0
Change in plan assets:
Fair value of plan assets at the beginning of the period		210,556	225,862
Fair value of plan assets at the end of the period	210,556		210,556	225,862
Pension Benefits
Change in projected benefit obligation:
Projected benefit obligation at the beginning of period		296,416	261,714
Service cost		7,457	6,937	8,009
Interest cost		12,350	13,341	12,066
Participant contributions		0	0
Actuarial (gain) loss		(21,134)	58,922
Benefits paid		(15,867)	(36,476)
Liability (gain) loss related to curtailment		(5,413)	0
Liability (gain) related to settlement		0	(8,022)
Projected benefit obligation at the end of the period	296,416	273,809	296,416	261,714
Accumulated benefit obligation at end of the period	290,933	273,388	290,933
Change in plan assets:
Fair value of plan assets at the beginning of the period		225,862	244,302
Actual return on plan assets		(50)	18,036
Employer contributions		611	0
Participant contributions		0	0
Benefits paid		(15,867)	(6,270)
Benefits paid related to settlement		0	(30,206)
Fair value of plan assets at the end of the period	225,862	210,556	225,862	$ 244,302
Funded status	$ (70,554)	$ (63,253)	$ (70,554)
Assumptions used in computing the benefit obligation
Discount rate					3.40%
Expected return on plan assets	7.10%	7.10%	7.10%
Rate of compensation increase	2.50%	2.50%	2.50%
Pension Benefits | Maximum
Assumptions used in computing the benefit obligation
Discount rate	5.18%		5.18%	4.40%	5.15%
Pension Benefits | Minimum
Assumptions used in computing the benefit obligation
Discount rate	5.10%		5.10%	4.30%	5.10%
Other Benefits
Change in projected benefit obligation:
Projected benefit obligation at the beginning of period		$ 9,804	$ 10,063
Service cost		5	23	$ 37
Interest cost		289	385	325
Participant contributions		281	326
Actuarial (gain) loss		(1,182)	(362)
Benefits paid		(1,219)	(1,324)
Liability (gain) loss related to curtailment		0	693
Liability (gain) related to settlement		0	0
Projected benefit obligation at the end of the period	$ 9,804	6,294	9,804	10,063
Accumulated benefit obligation at end of the period	0	0	0
Change in plan assets:
Fair value of plan assets at the beginning of the period		0	0
Actual return on plan assets		0	0
Employer contributions		938	998
Participant contributions		281	326
Benefits paid		(1,219)	(1,324)
Benefits paid related to settlement		0	0
Fair value of plan assets at the end of the period	0	0	0	$ 0
Funded status	$ (9,804)	$ (6,294)	$ (9,804)
Assumptions used in computing the benefit obligation
Discount rate					5.00%
Other Benefits | Maximum
Assumptions used in computing the benefit obligation
Discount rate	5.05%		5.05%	4.25%
Other Benefits | Minimum
Assumptions used in computing the benefit obligation
Discount rate	2.69%		2.69%	2.25%