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STOCKHOLDERS' EQUITY, Stock Options (Details)
12 Months Ended
Dec. 31, 2018
USD ($)
Plan
Installment
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
Director
$ / shares
shares
Additional General Disclosures [Abstract]      
Number of stock based compensation plans | Plan 1    
Stock granted during period, value of stock options | $ $ 81,500 $ 0 $ 0
Number of board of directors to whom stock options granted | Director     2
Weighted average grant date fair value of stock granted (in dollars per share) | $ / shares $ 22.31    
Assumptions used to estimate the fair values of the stock options granted [Abstract]      
Risk-free interest rate   0.00% 0.00%
Expected volatility   0.00% 0.00%
Expected life 6 years 3 months 0 years 0 years
Dividend yield 0.00% 0.00% 0.00%
Stock Options Activity [Roll Forward]      
Outstanding, beginning of period (in shares) 232,000 297,000  
Grants (in shares) 81,500 0  
Exercised (in shares) (138,000) (45,000)  
Forfeitures or expirations (in shares)   (20,000)  
Outstanding, end of period (in shares) 175,500 232,000 297,000
Vested and expected to vest (in shares) 175,500    
Exercisable, end of period (in shares) 86,500    
Weighted Average Exercise Price [Roll Forward]      
Outstanding, beginning of period (in dollars per share) | $ / shares $ 21.56 $ 20.14  
Grants (in dollars per share) | $ / shares 51.42 0  
Exercised (in dollars per share) | $ / shares 18.63 8.79  
Forfeitures or expirations (in dollars per share) | $ / shares   29.21  
Outstanding at end of year (in dollars per share) | $ / shares 37.73 $ 21.56 $ 20.14
Vested and expected to vest (in dollars per share) | $ / shares 37.73    
Exercisable (in dollars per share) | $ / shares $ 24.84    
Weighted Average Remaining Contractual Term [Roll Forward]      
Outstanding 6 years 3 months 29 days 2 years 6 months 7 days 3 years 1 month 6 days
Grants 0 years 0 years  
Exercised 0 years 0 years  
Forfeitures or expirations   0 years  
Vested and expected to vest 6 years 3 months 29 days    
Exercisable 3 years 2 months 19 days    
Aggregate Intrinsic Value [Roll Forward]      
Outstanding, beginning of period | $ $ 5,050,990 $ 10,561,380  
Grants | $ 0 0  
Exercised | $ 3,720,149 1,554,100  
Forfeitures or expirations | $   0  
Outstanding, end of period | $ 4,014,235 $ 5,050,990 $ 10,561,380
Vested and expected to vest | $ 4,014,235    
Exercisable | $ $ 3,093,460    
Minimum [Member]      
Assumptions used to estimate the fair values of the stock options granted [Abstract]      
Risk-free interest rate 2.62%    
Expected volatility 39.40%    
Maximum [Member]      
Assumptions used to estimate the fair values of the stock options granted [Abstract]      
Risk-free interest rate 2.94%    
Expected volatility 39.90%    
2001 Stock Option Plan [Member]      
Additional General Disclosures [Abstract]      
Number of shares authorized for issuance (in shares) 2,050,000    
Number of installments for options vest and become exercisable | Installment 4    
Options expiration period 10 years    
Number of shares available for grant (in shares) 147,598    
2001 Stock Option Plan [Member] | Minimum [Member]      
Additional General Disclosures [Abstract]      
Exercise price per share of common stock 100.00%    
2001 Stock Option Plan [Member] | Qualified Incentive Stock Options [Member]      
Additional General Disclosures [Abstract]      
Exercise price per share of common stock 110.00%    
Options expiration period 5 years    
2001 Stock Option Plan [Member] | Qualified Incentive Stock Options [Member] | Minimum [Member]      
Additional General Disclosures [Abstract]      
Minimum ownership percentage for qualified incentive stock options 10.00%