Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Accounts receivable allowances	$ 7.1	$ 2.9
Stockholders’ equity:
Preferred stock, par value (usd per share)	$ 1.00	$ 1.00
Preferred stock, authorized (in shares)	5,000,000	5,000,000
Preferred stock, issued (in shares)	0	0
Preferred stock, outstanding (in shares)	0	0
Common stock, par value (usd per share)	$ 0.001	$ 0.001
Common stock, authorized (in shares)	187,500,000	187,500,000
Common stock, issued (in shares)	76,191,958	73,212,545
Common stock, outstanding (in shares)	73,178,815	71,019,563
Treasury stock (in shares)	3,013,143	2,192,982