CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts	$ 4.0	$ 2.3
Preferred stock, par value per share (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	20,000,000	20,000,000
Preferred stock, shares issued (in shares)	1,704,339	0
Preferred stock, shares outstanding (in shares)	1,704,339	0
Treasury stock, shares (in shares)	99,446	0
Common Class A
Common stock, par value per share (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	120,000,000	120,000,000
Common stock, shares issued (in shares)	13,235,082	12,993,118
Common stock, shares outstanding (in shares)	13,135,636	12,993,118
Common Class B
Common stock, par value per share (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	60,000,000	60,000,000
Common stock, shares issued (in shares)	21,485,126	20,449,484
Common stock, shares outstanding (in shares)	21,485,126	20,449,484