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COMMON STOCK Schedule of Changes in Restricted Stock - Market Based Awards (Details) - Restricted Stock - Market Based Awards [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, Options, Vested and Expected to Vest, Outstanding, Aggregate Intrinsic Value $ 2,687 $ 6,562 $ 4,293
Share-based Compensation Arrangement by Share-based Payment Award, Terms of Award The fair value of the market-based PSUs is amortized ratably over the requisite service period, primarily three years. The market-based PSUs vest if the participant is continuously employed throughout the performance period and the market-based performance measure is achieved, with a maximum vesting period of three years. All compensation cost related to the market-based awards will be recognized if the requisite service period is fulfilled, even if the market condition is not achieved.    
Time based shares granted to executives 28,069    
Number of Shares      
Outstanding beginning of year, January 1, 48,420    
Granted 28,069    
Vested (24,140)    
Outstanding end of year, December 31, 52,349 48,420  
Weighted-Average Grant-Date Fair Value      
Outstanding at beginning of year, January 1, $ 64.97    
Weighted-average grant date fair value per share 94.02 $ 72.54 $ 66.16
Vested 57.35    
Outstanding at end of year, December 31, $ 84.06 $ 64.97  
Total intrinsic value of market-based awards non-vested $ 2,698 $ 3,514 $ 3,819
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 $ 2,400    
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition 1 year 9 months