EMPLOYEE BENEFIT PLANS - Pension plan assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Total Carrying Value
Employee benefit plan disclosures
Fair value of the pension plan assets	$ 64,621	$ 70,424
Quoted prices in active market (Level 1)
Employee benefit plan disclosures
Fair value of the pension plan assets	39,862	42,628
Significant other observable inputs (Level 2)
Employee benefit plan disclosures
Fair value of the pension plan assets	24,759	27,796
Cash and Cash Equivalents | Total Carrying Value
Employee benefit plan disclosures
Fair value of the pension plan assets	289	300
Cash and Cash Equivalents | Quoted prices in active market (Level 1)
Employee benefit plan disclosures
Fair value of the pension plan assets	289	300
U.S. Treasury Securities | Total Carrying Value
Employee benefit plan disclosures
Fair value of the pension plan assets	1,452	1,615
U.S. Treasury Securities | Quoted prices in active market (Level 1)
Employee benefit plan disclosures
Fair value of the pension plan assets	1,452	1,615
Equity Securities - U.S. | Total Carrying Value
Employee benefit plan disclosures
Fair value of the pension plan assets	16,884	18,513
Equity Securities - U.S. | Quoted prices in active market (Level 1)
Employee benefit plan disclosures
Fair value of the pension plan assets	16,884	18,513
Equity Securities - International | Total Carrying Value
Employee benefit plan disclosures
Fair value of the pension plan assets	9,888	10,109
Equity Securities - International | Quoted prices in active market (Level 1)
Employee benefit plan disclosures
Fair value of the pension plan assets	9,888	10,109
Collective trust fund | Total Carrying Value
Employee benefit plan disclosures
Fair value of the pension plan assets	15,771	17,485
Collective trust fund | Significant other observable inputs (Level 2)
Employee benefit plan disclosures
Fair value of the pension plan assets	15,771	17,485
Private Real Estate | Total Carrying Value
Employee benefit plan disclosures
Fair value of the pension plan assets	8,988	10,311
Private Real Estate | Significant other observable inputs (Level 2)
Employee benefit plan disclosures
Fair value of the pension plan assets	8,988	10,311
Bond market fund | Total Carrying Value
Employee benefit plan disclosures
Fair value of the pension plan assets	8,526	9,173
Bond market fund | Quoted prices in active market (Level 1)
Employee benefit plan disclosures
Fair value of the pension plan assets	8,526	9,173
Commodities broad basket fund | Total Carrying Value
Employee benefit plan disclosures
Fair value of the pension plan assets	2,823	2,918
Commodities broad basket fund | Quoted prices in active market (Level 1)
Employee benefit plan disclosures
Fair value of the pension plan assets	$ 2,823	2,918
Pension Benefits
Employee benefit plan disclosures
Target Allocation (as a percent)	100.00%
Fair value of the pension plan assets	$ 64,621	$ 70,424	$ 73,658
Pension Benefits | Mutual Fund Fixed Income
Employee benefit plan disclosures
Target Allocation (as a percent)	15.00%
Pension Benefits | Equity Securities - U.S.
Employee benefit plan disclosures
Target Allocation (as a percent)	25.00%
Pension Benefits | Equity Securities - International
Employee benefit plan disclosures
Target Allocation (as a percent)	15.00%
Pension Benefits | Collective trust fund
Employee benefit plan disclosures
Target Allocation (as a percent)	25.00%
Pension Benefits | Private Real Estate
Employee benefit plan disclosures
Target Allocation (as a percent)	15.00%
Pension Benefits | Commodities broad basket fund
Employee benefit plan disclosures
Target Allocation (as a percent)	5.00%