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Consolidated Balance Sheets (Parentheticals) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Allowance for doubtful accounts $ 7,733 $ 7,386
Accumulated depreciation $ 219,271 $ 197,802
Preferred stock, par value (in dollars per share) $ 0 $ 0
Preferred stock, shares authorized (in shares) 15,000,000 15,000,000
Preferred stock, shares outstanding (in shares) 0 0
Class A common stock    
Common stock A and B, par value (in dollars per share) $ 0 $ 0
Common stock A and B, shares authorized (in shares) 100,000,000 100,000,000
Common stock A and B, shares issued (in shares) 23,433,000 23,968,000
Common stock A and B, shares outstanding (in shares) 23,433,000 23,968,000
Class B common stock    
Common stock A and B, par value (in dollars per share) $ 0 $ 0
Common stock A and B, shares authorized (in shares) 25,000,000 25,000,000
Common stock A and B, shares issued (in shares) 1,000,000 1,000,000
Common stock A and B, shares outstanding (in shares) 1,000,000 1,000,000