Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Accounts receivable, net of allowance for doubtful accounts	$ (5.9)	$ (7.3)
Accumulated depreciation	$ 338.0	$ 284.3
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 A and B, shares authorized (in shares)	100,000,000.0	100,000,000.0
Common stock A and B, shares issued (in shares)	26,300,000	22,600,000
Common stock A and B, shares outstanding (in shares)	26,300,000	22,600,000
Class B
Common stock A and B, shares authorized (in shares)	25,000,000.0	25,000,000.0
Common stock A and B, shares issued (in shares)	200,000	600,000
Common stock A and B, shares outstanding (in shares)	200,000	600,000