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LONG-TERM DEBT - Additional Information (Details)
 12 Months Ended
Apr. 15, 2016
Apr. 06, 2015
USD ($)
Aug. 15, 2014
$ / shares
May 21, 2014
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Debt Instrument [Line Items]              
Aggregate principal amount issued         $ 175,000,000 $ 177,684,000  
Net proceeds from issuance of long-term debt         $ 0 0 $ 172,304,000
Debt extinguishment costs           2,700,000  
Senior Notes              
Debt Instrument [Line Items]              
Debt instrument, optional redemption period prior to maturity         90 days    
Minimum redemption period notice required         30 days    
Maximum redemption period notice required         60 days    
Redemption price, percentage         100.00%    
Senior Notes | 5.25% Senior Notes Due April 15, 2025              
Debt Instrument [Line Items]              
Aggregate principal amount issued   $ 175,000,000     $ 175,000,000 175,000,000  
Stated interest rate   5.25%          
Net proceeds from issuance of long-term debt   $ 172,800,000          
Junior Subordinated Debt | 7.50% Amortizing Note Due May 15, 2017              
Debt Instrument [Line Items]              
Aggregate principal amount issued         $ 0 $ 2,684,000  
Stated interest rate 7.50%   7.50% 7.50%      
Redemption price, percentage 100.00%            
Gross proceeds from issuance of tangible equity units       $ 69,000,000      
Tangible equity units dividend rate     8.00% 8.00%      
Tangible equity units issued during period, shares | shares       1,380,000      
Tangible equity units issued to underwriter during period, shares | shares       180,000      
Tangible equity units, price per share (in dollars per share) | $ / shares     $ 50 $ 50.00      
Tangible equity units, convertible, conversion ratio       10.604556      
Fair value of debt instrument       $ 14,600,000      
Adjustments to additional paid in capital, issuance of tangible equity units       54,400,000      
Tangible equity units issued, issuance costs       4,000,000      
Tangible equity units issued, issuance costs, underwriter discount       3,300,000      
Debt instrument, deferred finance cost       857,000      
Adjustments to additional paid in capital, tangible equity units issued, issuance costs       $ 3,200,000      
Equal quarterly cash installments per Amortizing Note       1      
Installment payment due on August 15, 2014 per Amortizing Note       0.933333