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Consolidated Balance Sheets (Parenthetical) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Current assets:    
Allowance for Doubtful Accounts Receivable, Current $ 0 $ 0
Stockholders' Equity:    
Preferred stock, par value (USD per share) $ 0.01 $ 0.01
Preferred stock, shares authorized 10,000,000 10,000,000
Preferred stock, shares outstanding 0 0
Common stock, par value (USD per share) $ 0.01 $ 0.01
Common stock, shares authorized 20,000,000 20,000,000
Common stock, shares outstanding 7,711,277 7,635,093
Treasury Stock, Shares 3,773,128 3,753,595