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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. 30, 2015
USD ($)
Jul. 31, 2015
USD ($)
Jan. 31, 2014
USD ($)
Aug. 31, 2004
USD ($)
Sep. 30, 2003
USD ($)
Jun. 30, 2012
USD ($)
Dec. 31, 2017
Dec. 31, 2016
USD ($)
Dec. 31, 2012
USD ($)
Dec. 31, 2015
USD ($)
Debt                        
Letters of credit                   $ 11,200    
Debt maturities                        
2017                   31,800    
2018                   30,700    
2019                   42,100    
2020                   48,400    
2021                   54,200    
Thereafter                   531,700    
Total Debt                   $ 738,900   $ 429,900
Minimum                        
Debt                        
Ratio of consolidated EBITDA to interest expense                   3.50    
Maximum                        
Debt                        
Debt covenant, required ratio of debt to consolidated EBITDA                   3.25    
Revolving Credit Facility                        
Debt                        
Long-term Line of Credit                   $ 55,000   0
Debt instrument term                     5 years  
Maximum borrowing capacity under revolving credit facility     $ 400,000               $ 375,000  
Uncommitted option to increase credit facility     $ 150,000               75,000  
Unused portion of credit facility                   $ 333,800    
Interest rate applicable to any borrowings                   1.97%    
Debt maturities                        
Total Debt                   $ 55,000    
Revolving Credit Facility | Forecast                        
Debt                        
Debt covenant, required principal amount of priority debt as a percentage of consolidated tangible assets                 17.50%      
Revolving Credit Facility | Extension of maturity                        
Debt                        
Debt instrument term     5 years                  
Revolving Credit Facility | Minimum                        
Debt                        
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, 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%    
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                        
Maximum borrowing capacity under revolving credit facility                     100,000  
Letters of credit                   $ 11,200    
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          
Debt maturities                        
Total Debt                   $ 26,400   28,600
5.79%, payable through 2020                        
Debt                        
Interest rate (as a percent)                   5.79%    
Annual principal payments                   $ 3,500    
Debt maturities                        
Total Debt                   $ 24,500   31,500
5.27%, payable through 2029                        
Debt                        
Debt issued           $ 55,000            
Interest rate (as a percent)           5.27%       5.27%    
Semi-annual payments           $ 1,100            
Debt maturities                        
Total Debt                   $ 28,600   30,800
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%    
Debt maturities                        
Total Debt                   $ 100,000   $ 100,000
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