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LONG-TERM DEBT - Additional Information (Detail)
 9 Months Ended
Apr. 06, 2015
USD ($)
Aug. 15, 2014
$ / shares
May 21, 2014
USD ($)
$ / shares
shares
Sep. 30, 2018
USD ($)
Dec. 31, 2017
USD ($)
Debt Instrument [Line Items]          
Par Value       $ 175,000,000 $ 175,000,000
Senior Notes          
Debt Instrument [Line Items]          
Redemption period       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]          
Par Value $ 175,000,000     $ 175,000,000 $ 175,000,000
Stated interest rate 5.25%     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]          
Stated interest rate   7.50% 7.50%    
Tangible equity units     $ 69,000,000    
Tangible equity units dividend rate   8.00% 8.00%    
Tangible equity units issued during period (in shares) | shares     1,380,000    
Tangible equity units issued to underwriter during period (in shares) | shares     180,000    
Tangible equity units, price per share (in dollars per share) | $ / shares     $ 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    
Tangible equity units issued, stated value per unit (In dollars per unit) | $ / shares   $ 50