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Consolidated Balance Sheets (Parentheticals) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Allowance for doubtful accounts $ 7.2 $ 7.4
Accumulated depreciation $ 240.5 $ 197.8
Preferred stock, shares authorized (in shares) 15,000,000.0 15,000,000.0
Preferred stock, shares outstanding (in shares) 0 0
Class A common stock    
Common stock A and B, shares authorized (in shares) 100,000,000.0 100,000,000.0
Common stock A and B, shares issued (in shares) 22,000,000 24,000,000
Common stock A and B, shares outstanding (in shares) 22,000,000 24,000,000
Class B common stock    
Common stock A and B, shares authorized (in shares) 25,000,000.0 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