v3.7.0.1
EMPLOYEE BENEFIT PLANS - Pension plan assets (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Pension Benefits      
Employee benefit plan disclosures      
Target Allocation (as a percent) 100.00%    
Fair value of the pension plan assets $ 65,354 $ 64,621 $ 70,424
Pension Benefits | Carrying Value      
Employee benefit plan disclosures      
Fair value of the pension plan assets 65,354 64,621  
Pension Benefits | Quoted Prices in Active Market (Level 1)      
Employee benefit plan disclosures      
Fair value of the pension plan assets 40,256 39,862  
Pension Benefits | Significant Other Observable Inputs (Level 2)      
Employee benefit plan disclosures      
Fair value of the pension plan assets $ 25,098 24,759  
Pension Benefits | Mutual Fund Fixed Income      
Employee benefit plan disclosures      
Target Allocation (as a percent) 15.00%    
Pension Benefits | Cash and Cash Equivalents | Carrying Value      
Employee benefit plan disclosures      
Fair value of the pension plan assets $ 258 289  
Pension Benefits | Cash and Cash Equivalents | Quoted Prices in Active Market (Level 1)      
Employee benefit plan disclosures      
Fair value of the pension plan assets $ 258 289  
Pension Benefits | U.S. Treasury Securities | Carrying Value      
Employee benefit plan disclosures      
Fair value of the pension plan assets   1,452  
Pension Benefits | U.S. Treasury Securities | Quoted Prices in Active Market (Level 1)      
Employee benefit plan disclosures      
Fair value of the pension plan assets   1,452  
Pension Benefits | Equity Securities - U.S.      
Employee benefit plan disclosures      
Target Allocation (as a percent) 30.00%    
Pension Benefits | Equity Securities - U.S. | Carrying Value      
Employee benefit plan disclosures      
Fair value of the pension plan assets $ 19,929 16,884  
Pension Benefits | Equity Securities - U.S. | Quoted Prices in Active Market (Level 1)      
Employee benefit plan disclosures      
Fair value of the pension plan assets $ 19,929 16,884  
Pension Benefits | Equity Securities - International      
Employee benefit plan disclosures      
Target Allocation (as a percent) 15.00%    
Pension Benefits | Equity Securities - International | Carrying Value      
Employee benefit plan disclosures      
Fair value of the pension plan assets $ 9,953 9,888  
Pension Benefits | Equity Securities - International | Quoted Prices in Active Market (Level 1)      
Employee benefit plan disclosures      
Fair value of the pension plan assets 9,953 9,888  
Pension Benefits | Bond market fund | Carrying Value      
Employee benefit plan disclosures      
Fair value of the pension plan assets 10,116 8,526  
Pension Benefits | Bond market fund | Quoted Prices in Active Market (Level 1)      
Employee benefit plan disclosures      
Fair value of the pension plan assets $ 10,116 8,526  
Pension Benefits | Collective trust fund      
Employee benefit plan disclosures      
Target Allocation (as a percent) 25.00%    
Pension Benefits | Collective trust fund | Carrying Value      
Employee benefit plan disclosures      
Fair value of the pension plan assets $ 15,866 15,771  
Pension Benefits | Collective trust fund | Significant Other Observable Inputs (Level 2)      
Employee benefit plan disclosures      
Fair value of the pension plan assets $ 15,866 15,771  
Pension Benefits | Commodities broad basket fund | Carrying Value      
Employee benefit plan disclosures      
Fair value of the pension plan assets   2,823  
Pension Benefits | Commodities broad basket fund | Quoted Prices in Active Market (Level 1)      
Employee benefit plan disclosures      
Fair value of the pension plan assets   2,823  
Pension Benefits | Private Real Estate      
Employee benefit plan disclosures      
Target Allocation (as a percent) 15.00%    
Pension Benefits | Private Real Estate | Carrying Value      
Employee benefit plan disclosures      
Fair value of the pension plan assets $ 9,232 8,988  
Pension Benefits | Private Real Estate | Significant Other Observable Inputs (Level 2)      
Employee benefit plan disclosures      
Fair value of the pension plan assets 9,232 $ 8,988  
Other Benefits      
Employee benefit plan disclosures      
Fair value of the pension plan assets 111,071    
Other Benefits | Carrying Value      
Employee benefit plan disclosures      
Fair value of the pension plan assets 111,071    
Other Benefits | Quoted Prices in Active Market (Level 1)      
Employee benefit plan disclosures      
Fair value of the pension plan assets 105,387    
Other Benefits | Significant Other Observable Inputs (Level 2)      
Employee benefit plan disclosures      
Fair value of the pension plan assets 5,684    
Other Benefits | Cash and Cash Equivalents | Carrying Value      
Employee benefit plan disclosures      
Fair value of the pension plan assets 20    
Other Benefits | Cash and Cash Equivalents | Quoted Prices in Active Market (Level 1)      
Employee benefit plan disclosures      
Fair value of the pension plan assets 20    
Other Benefits | Equity Securities - U.S. | Carrying Value      
Employee benefit plan disclosures      
Fair value of the pension plan assets 638    
Other Benefits | Equity Securities - U.S. | Quoted Prices in Active Market (Level 1)      
Employee benefit plan disclosures      
Fair value of the pension plan assets 638    
Other Benefits | Equity Securities - International | Carrying Value      
Employee benefit plan disclosures      
Fair value of the pension plan assets 26,793    
Other Benefits | Equity Securities - International | Quoted Prices in Active Market (Level 1)      
Employee benefit plan disclosures      
Fair value of the pension plan assets 26,793    
Other Benefits | Bond market fund | Carrying Value      
Employee benefit plan disclosures      
Fair value of the pension plan assets 77,936    
Other Benefits | Bond market fund | Quoted Prices in Active Market (Level 1)      
Employee benefit plan disclosures      
Fair value of the pension plan assets 77,936    
Other Benefits | Private Real Estate | Carrying Value      
Employee benefit plan disclosures      
Fair value of the pension plan assets 5,596    
Other Benefits | Private Real Estate | Significant Other Observable Inputs (Level 2)      
Employee benefit plan disclosures      
Fair value of the pension plan assets $ 5,596