CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDER'S EQUITY - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Paid-in Capital	Retained Earnings	Other
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		0.3
Vesting of restricted stock units, value	(7.6)		(7.6)
Employee stock purchase plan, shares		0.2
Employee stock purchase plan, value	12.6		12.6
Exercise of stock options	0.1		0.1
Stock-based compensation expense	31.4		31.4
Stock Repurchased and Retired During Period, Shares		(0.3)
Stock Repurchased and Retired During Period, Value	(20.0)		(3.9)	(16.1)
Foreign currency translation adjustments	(2.5)				(2.5)
Net income	135.4			135.4
Shares Outstanding at Sep. 30, 2019		52.7
Total stockholders’ equity at Sep. 30, 2019	1,331.5	$ 0.5	634.4	705.4	(8.8)
Shares Outstanding at Jun. 30, 2019		52.8
Total stockholders’ equity at Jun. 30, 2019	1,283.7	$ 0.5	625.5	664.1	(6.4)
Vesting of restricted stock units, shares		0.1
Vesting of restricted stock units, value	(0.8)		(0.8)
Employee stock purchase plan, shares		0.1
Employee stock purchase plan, value	5.7		5.7
Exercise of stock options	0.1		0.1
Stock-based compensation expense	7.8		7.8
Stock Repurchased and Retired During Period, Shares		(0.3)
Stock Repurchased and Retired During Period, Value	(20.0)		(3.9)	(16.1)
Foreign currency translation adjustments	(2.4)				(2.4)
Net income	57.4			57.4
Shares Outstanding at Sep. 30, 2019		52.7
Total stockholders’ equity at Sep. 30, 2019	1,331.5	$ 0.5	634.4	705.4	(8.8)
Shares Outstanding at Dec. 31, 2019		52.9
Total stockholders’ equity at Dec. 31, 2019	1,376.2	$ 0.5	638.0	744.7	(7.0)
Vesting of restricted stock units, shares		0.2
Vesting of restricted stock units, value	(7.0)		(7.0)
Employee stock purchase plan, shares		0.5
Employee stock purchase plan, value	11.1		11.1
Exercise of stock options	0.6		0.6
Stock-based compensation expense	24.2		24.2
Stock Repurchased and Retired During Period, Shares		(0.8)
Stock Repurchased and Retired During Period, Value	(27.9)		(9.2)	(18.7)
Foreign currency translation adjustments	2.1				2.1
Net income	144.9			144.9
Shares Outstanding at Sep. 30, 2020		52.8
Total stockholders’ equity at Sep. 30, 2020	1,524.2	$ 0.5	657.7	870.9	(4.9)
Shares Outstanding at Jun. 30, 2020		52.5
Total stockholders’ equity at Jun. 30, 2020	1,457.1	$ 0.5	645.9	818.6	(7.9)
Vesting of restricted stock units, value	(1.1)		(1.1)
Employee stock purchase plan, shares		0.3
Employee stock purchase plan, value	5.2		5.2
Exercise of stock options	0.6		0.6
Stock-based compensation expense	7.1		7.1
Foreign currency translation adjustments	3.0				3.0
Net income	52.3			52.3
Shares Outstanding at Sep. 30, 2020		52.8
Total stockholders’ equity at Sep. 30, 2020	$ 1,524.2	$ 0.5	$ 657.7	$ 870.9	$ (4.9)