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DEBT - DESCRIPTION (Details)
$ in Thousands
1 Months Ended 3 Months Ended 12 Months Ended
Dec. 21, 2016
USD ($)
Sep. 14, 2016
USD ($)
Jul. 31, 2015
USD ($)
Jan. 31, 2014
USD ($)
Aug. 31, 2004
USD ($)
Sep. 30, 2003
USD ($)
Jun. 30, 2012
USD ($)
Dec. 31, 2017
USD ($)
Debt                
Letters of credit               $ 10,400
Revolving Credit Facility                
Debt                
Interest rate during period (as a percent)               2.98%
Maximum borrowing capacity under revolving credit facility               $ 650,000
Uncommitted option to increase credit facility               250,000
Unused portion of credit facility               $ 294,700
Debt covenant, required principal amount of priority debt as a percentage of consolidated tangible assets               20.00%
Debt covenant, required principal amount of priority debt (excluding Title XI) as a percentage of consolidated tangible assets               10.00%
Revolving Credit Facility | Minimum                
Debt                
Ratio of consolidated EBITDA to interest expense               3.50
Credit facility commitment fee percentage               0.15%
Revolving Credit Facility | Minimum | London Interbank Offered Rate (LIBOR)                
Debt                
Variable rate margin               1.00%
Revolving Credit Facility | Minimum | Base rate                
Debt                
Variable rate margin               0.00%
Revolving Credit Facility | Maximum                
Debt                
Debt covenant, ratio of debt to consolidated EBITDA               3.25
Credit facility commitment fee percentage               0.30%
Revolving Credit Facility | Maximum | London Interbank Offered Rate (LIBOR)                
Debt                
Variable rate margin               1.75%
Revolving Credit Facility | Maximum | Base rate                
Debt                
Variable rate margin               0.75%
Standby and commercial letters of credit                
Debt                
Long-term Line of Credit               $ 10,400
Maximum borrowing capacity under revolving credit facility               $ 100,000
Standby and commercial letters of credit | Minimum                
Debt                
Line of credit fees percentage               1.00%
Standby and commercial letters of credit | Maximum                
Debt                
Line of credit fees percentage               1.75%
Swing Line Loans                
Debt                
Maximum borrowing capacity under revolving credit facility               $ 50,000
5.34%, payable through 2028                
Debt                
Debt issued           $ 55,000    
Interest rate (as a percent)           5.34%   5.34%
Semi-annual payments           $ 1,100    
5.79%, payable through 2020                
Debt                
Interest rate (as a percent)               5.79%
Annual principal payments               $ 3,500
5.27%, payable through 2029                
Debt                
Debt issued         $ 55,000      
Interest rate (as a percent)         5.27%     5.27%
Semi-annual payments         $ 1,100      
Senior Unsecured Long Term Debt                
Debt                
Debt issued       $ 100,000        
Debt instrument term       30 years        
Senior Unsecured Long Term Debt | Debt Instrument Redemption 2021                
Debt                
Annual principal payments       $ 5,000        
Senior Unsecured Long Term Debt | Debt Instrument Redemption 2022 and 2023                
Debt                
Annual principal payments       7,500        
Senior Unsecured Long Term Debt | Debt Instrument Redemption 2024 to 2027                
Debt                
Annual principal payments       10,000        
Senior Unsecured Long Term Debt | Debt Instrument Redemption 2028                
Debt                
Annual principal payments       8,000        
Senior Unsecured Long Term Debt | Debt Instrument Redemption 2029 and thereafter until 2044                
Debt                
Annual principal payments       $ 2,000        
4.35%, payable through 2044                
Debt                
Interest rate (as a percent)               4.35%
Unsecured debt                
Debt                
Debt issued             $ 170,000  
Unsecured debt | Debt Instrument Redemption 2015 through 2016                
Debt                
Semi-annual payments             4,600  
Unsecured debt | Debt Instrument Redemption 2017 through mid-year 2023                
Debt                
Semi-annual payments             8,400  
Unsecured debt | Debt Instrument Redemption Mid-year 2023 through mid-year 2027                
Debt                
Semi-annual payments             3,800  
Unsecured debt | Debt Instrument Redemption After mid-year 2027                
Debt                
Semi-annual payments             1,200  
Unsecured debt, tranche maturing in 2023                
Debt                
Debt issued             $ 77,500  
Interest rate (as a percent)             3.66%  
Unsecured debt, tranche maturing in 2027                
Debt                
Debt issued             $ 55,000  
Interest rate (as a percent)             4.16%  
Unsecured debt, tranche maturing in 2032                
Debt                
Debt issued             $ 37,500  
Interest rate (as a percent)             4.31%  
Notes 30 years                
Debt                
Debt issued     $ 75,000          
Debt instrument term     30 years          
Notes 30 years | Debt Instrument Redemption After 2026                
Debt                
Annual principal payments     $ 1,500          
Notes 30 years | Debt Instrument Redemption 2017 through 2019                
Debt                
Annual principal payments     1,800          
Notes 30 years | Minimum | Debt Instrument Redemption 2020 through 2026                
Debt                
Annual principal payments     1,300          
Notes 30 years | Maximum | Debt Instrument Redemption 2020 through 2026                
Debt                
Annual principal payments     $ 8,000          
Senior Unsecured Series A Notes                
Debt                
Debt issued $ 75,000              
Debt instrument term 11 years              
Interest rate (as a percent) 3.37%              
Senior Unsecured Series A Notes | Debt Instrument Redemption 2021                
Debt                
Semi-annual payments $ 5,800              
Senior Unsecured Series A Notes | Debt Instrument Redemption 2022 through 2027                
Debt                
Semi-annual payments $ 11,500              
The Senior Unsecured Notes (The "Series D Notes")                
Debt                
Debt issued   $ 200,000            
Debt instrument term   15 years            
Interest rate (as a percent)   3.14%            
Maturities of Long-term Debt                
Semi-annual principal payments in year 2019   $ 6,000            
Semi-annual principal payments in 2020 to 2023   9,200            
Semi-annual principal payments in 2024 to 2031   $ 7,150