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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 ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
Jul. 15, 2015
Dec. 31, 2014
Debt                      
Letters of credit               $ 7,900      
Revolving Credit Facility                      
Debt                      
Interest rate during period (as a percent)               3.92%      
Maximum borrowing capacity under revolving credit facility               $ 650,000      
Uncommitted option to increase credit facility               250,000      
Unused portion of credit facility               $ 304,000      
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                      
Debt covenant, ratio of debt to consolidated EBITDA                 1.0    
Ratio of consolidated EBITDA to interest expense               3.50 1.0    
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               $ 7,900      
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              
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        
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