CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDER'S EQUITY - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Shares Outstanding at Dec. 31, 2017		52.2
Total stockholders’ equity at Dec. 31, 2017	$ 991.4	$ 0.5	$ 566.1	$ 428.4	$ (3.6)
Vesting of restricted stock units, value	(1.0)		(1.0)
Employee stock purchase plan, shares		0.1
Employee stock purchase plan, value	4.3		4.3
Exercise of stock options, value	0.1		0.1
Stock-based compensation expense	4.9		4.9
Translation adjustments	1.5				1.5
Net income	29.1			29.1
Shares Outstanding at Mar. 31, 2018		52.3
Total stockholders’ equity at Mar. 31, 2018	1,030.3	$ 0.5	574.4	457.5	(2.1)
Shares Outstanding at Dec. 31, 2018		52.5
Total stockholders’ equity at Dec. 31, 2018	1,182.1	$ 0.5	601.8	586.1	(6.3)
Vesting of restricted stock units, shares		0.2
Vesting of restricted stock units, value	(5.5)		(5.5)
Employee stock purchase plan, shares		0.1
Employee stock purchase plan, value	6.9		6.9
Stock-based compensation expense	10.0		10.0
Translation adjustments	(1.1)				(1.1)
Net income	34.9			34.9
Shares Outstanding at Mar. 31, 2019		52.8
Total stockholders’ equity at Mar. 31, 2019	$ 1,227.3	$ 0.5	$ 613.2	$ 621.0	$ (7.4)