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COMMON STOCK Schedule of Changes in Restricted Stock - TIme Based Awards (Details) - Restricted Stock [Member] - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Terms of Award The fair value of the time-based RSUs is amortized ratably over the requisite service period, primarily three years. The time-based RSUs generally vest ratably on each anniversary following the grant date that a participant is continuously employed.    
Number of Shares      
Outstanding beginning of year, January 1, 479,642    
Granted 273,941    
Vested (266,809)    
Forfeited (14,642)    
Outstanding end of year, December 31, 472,132 479,642  
Weighted-Average Grant-Date Fair Value      
Outstanding at beginning of year, January 1, $ 56.09    
Weighted-average grant date fair value per share 65.14 $ 58.52 $ 48.88
Vested 57.67    
Forfeited 62.92    
Outstanding at end of year, December 31, $ 60.23 $ 56.09  
Total intrinsic value of time based awards vested $ 16,303 $ 18,973 $ 17,077
Total intrinsic value of time-based awards non-vested $ 24,334 $ 34,812 $ 28,029
Market price per common share as of December 31, $ 51.54 $ 72.58 $ 53.38
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized $ 18,500    
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition 1 year 9 months 23 days