CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Millions, $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance (in shares) at Dec. 31, 2016		52.7
Balance at Dec. 31, 2016	$ 868.9	$ 0.5	$ 562.9	$ 315.6	$ (10.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units and restricted stock awards (in shares)		0.4
Vesting of restricted stock units and restricted stock awards	(15.8)	$ 0.0	(15.8)
Employee stock purchase plan (in shares)		0.2
Employee stock purchase plan	7.4	$ 0.0	7.4
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		0.0
Exercise of common stock options	0.4	$ 0.0	0.4
Stock-based compensation expense	24.4		24.4
Stock Repurchased and Retired During Period, Shares		(1.1)
Stock Repurchased and Retired During Period, Value	(58.1)	$ 0.0	(13.2)	(44.9)
Translation adjustments	6.5				6.5
Net income	157.7			157.7
Balance (in shares) at Dec. 31, 2017		52.2
Balance at Dec. 31, 2017	991.4	$ 0.5	566.1	428.4	(3.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units and restricted stock awards (in shares)		0.1
Vesting of restricted stock units and restricted stock awards	(4.7)	$ 0.0	(4.7)
Employee stock purchase plan (in shares)		0.2
Employee stock purchase plan	8.8	$ 0.0	8.8
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		0.0
Exercise of common stock options	0.6		0.6
Stock-based compensation expense	31.0		31.0
Stock Repurchased and Retired During Period, Shares		0.0
Translation adjustments	(2.7)				(2.7)
Net income	$ 157.7			157.7
Balance (in shares) at Dec. 31, 2018	52.5	52.5
Balance at Dec. 31, 2018	$ 1,182.1	$ 0.5	601.8	586.1	(6.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units and restricted stock awards (in shares)		0.4
Vesting of restricted stock units and restricted stock awards	(12.1)	$ 0.0	(12.1)
Employee stock purchase plan (in shares)		0.2
Employee stock purchase plan	12.6	$ 0.0	12.6
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		0.1
Exercise of common stock options	0.1		0.1
Stock-based compensation expense	39.5		39.5
Stock Repurchased and Retired During Period, Shares		(0.3)
Stock Repurchased and Retired During Period, Value	(20.0)	$ 0.0	(3.9)	(16.1)
Translation adjustments	(0.7)				(0.7)
Net income	$ 174.7			174.7
Balance (in shares) at Dec. 31, 2019	52.9	52.9
Balance at Dec. 31, 2019	$ 1,376.2	$ 0.5	$ 638.0	$ 744.7	$ (7.0)