Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Current assets:
Allowances for accounts receivable	$ 3.0	$ 2.9
Stockholders’ equity:
Preferred stock, par value (USD per share)	$ 1.00	$ 1.00
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (USD per share)	$ 0.001	$ 0.001
Common stock, shares authorized	187,500,000	187,500,000
Common stock, shares, issued	75,599,584	73,212,545
Common stock, shares outstanding	72,586,441	71,019,563
Treasury stock, shares	3,013,143	2,192,982