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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 ($)
item
Dec. 31, 2019
USD ($)
Jul. 15, 2015
Dec. 31, 2014
Debt                    
Letters of credit               $ 7,700    
Revolving Credit Facility                    
Debt                    
Interest rate during period (as a percent)               3.33%    
Maximum borrowing capacity under revolving credit facility               $ 650,000    
Uncommitted option to increase credit facility               250,000    
Unused portion of credit facility               $ 75,100    
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               $ 7,700    
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                    
Interest rate (as a percent)               5.34%    
5.34%, payable through 2028 | MatNav                    
Debt                    
Debt issued           $ 55,000        
Interest rate (as a percent)           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                    
Interest rate (as a percent)               5.27%    
5.27%, payable through 2029 | MatNav                    
Debt                    
Debt issued         $ 55,000          
Interest rate (as a percent)         5.27%          
Semi-annual payments         $ 1,100          
Senior Unsecured Long Term Debt                    
Debt                    
Debt issued       $ 100,000            
Debt instrument term       30 years            
Interest rate (as a percent)                   4.35%
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      
Number of tranches | item             3      
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              
Interest rate (as a percent)                 3.92%  
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