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STOCKHOLDERS' EQUITY, Stock Options (Details)
12 Months Ended
Dec. 31, 2016
USD ($)
Plan
Installment
$ / shares
shares
Dec. 31, 2015
USD ($)
Director
$ / shares
shares
Dec. 31, 2014
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of stock based compensation plans | Plan 1    
Number of board of directors to whom stock options granted | Director   2  
Assumptions used to estimate the fair values of the stock options granted [Abstract]      
Risk-free interest rate 0.00% 1.41% 1.66%
Expected volatility 0.00% 39.00% 32.00%
Expected life 0 years 6 years 3 months 5 years
Dividend yield 0.00% 0.00% 0.00%
Stock Options Activity [Roll Forward]      
Outstanding, beginning of period (in shares) 562,000 759,958  
Grants (in shares) 0 30,000  
Exercised (in shares) (265,000) (227,958)  
Forfeitures or expirations (in shares) 0 0  
Outstanding, end of period (in shares) 297,000 562,000 759,958
Vested and expected to vest (in shares) 297,000    
Exercisable (in shares) 250,750    
Weighted Average Exercise Price [Roll Forward]      
Outstanding at beginning of year (in dollars per share) | $ / shares $ 11.91 $ 11.04  
Grants (in dollars per share) | $ / shares 0 37.64  
Exercised (in dollars per share) | $ / shares 2.68 12.39  
Forfeitures or expirations (in dollars per share) | $ / shares 0 0  
Outstanding at end of year (in dollars per share) | $ / shares 20.14 $ 11.91 $ 11.04
Vested and expected to vest (in dollars per share) | $ / shares 20.14    
Exercisable (in dollars per share) | $ / shares $ 17.88    
Weighted Average Remaining Contractual Term [Roll Forward]      
Outstanding 3 years 1 month 6 days 2 years 6 months 4 days 3 years 1 month 13 days
Grants 0 years 10 years  
Exercised 0 years 0 years  
Forfeitures or expirations 0 years 0 years  
Vested and expected to vest 3 years 1 month 6 days    
Exercisable 2 years 7 months 6 days    
Aggregate Intrinsic Value [Roll Forward]      
Outstanding, beginning of period | $ $ 17,456,220 $ 23,483,235  
Grants | $ 0 0  
Exercised | $ 9,392,150 9,251,027  
Forfeitures or expirations | $ 0 0  
Outstanding at end of period | $ 10,561,380 17,456,220 $ 23,483,235
Vested and expected to vest | $ 10,561,380    
Exercisable | $ 9,483,518    
Board of Directors [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock granted during period, value of stock options | $ $ 0 $ 450,000 $ 123,000
Stock Options Activity [Roll Forward]      
Grants (in shares) 0 30,000 15,000
2001 Stock Option Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
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) 209,098    
2001 Stock Option Plan [Member] | Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Exercise price per share of common stock 100.00%    
2001 Stock Option Plan [Member] | Qualified Incentive Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
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]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Minimum ownership percentage for qualified incentive stock options 10.00%