Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Millions	Total	$0.001 Par Value Common Stock	Additional Paid-In Capital	Treasury Stock	Accumulated Other Comprehensive Loss	Retained Earnings
Balance (in shares) at Dec. 31, 2017		50,600,000		1,200,000
Balance at Dec. 31, 2017	$ 912.2	$ 0.1	$ 618.3	$ (39.5)	$ (3.8)	$ 337.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Employee equity plans activity (in shares)		800,000
Employee equity plans activity	21.7		21.8	$ (0.1)
Net income	66.2					66.2
Other comprehensive income	(1.0)				(1.0)
Balance (in shares) at Sep. 30, 2018		51,400,000		1,200,000
Balance at Sep. 30, 2018	966.6	$ 0.1	640.1	$ (39.6)	(4.8)	370.8
Balance (in shares) at Jun. 30, 2018		51,200,000		1,200,000
Balance at Jun. 30, 2018	938.4	$ 0.1	632.6	$ (39.6)	(4.6)	349.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Employee equity plans activity (in shares)		200,000
Employee equity plans activity	7.5		7.5
Net income	20.9					20.9
Other comprehensive income	(0.2)				(0.2)
Balance (in shares) at Sep. 30, 2018		51,400,000		1,200,000
Balance at Sep. 30, 2018	$ 966.6	$ 0.1	640.1	$ (39.6)	(4.8)	370.8
Balance (in shares) at Dec. 31, 2018	52,400,000	52,400,000		1,200,000
Balance at Dec. 31, 2018	$ 1,010.9	$ 0.1	688.6	$ (39.6)	(5.5)	367.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Employee equity plans activity (in shares)		400,000
Employee equity plans activity	20.1		20.2	$ (0.1)
Net income	7.6					7.6
Other comprehensive income	$ (3.6)				(3.6)
Balance (in shares) at Sep. 30, 2019	52,800,000	52,800,000		1,200,000
Balance at Sep. 30, 2019	$ 1,035.0	$ 0.1	708.8	$ (39.7)	(9.1)	374.9
Balance (in shares) at Jun. 30, 2019		52,700,000		1,200,000
Balance at Jun. 30, 2019	988.9	$ 0.1	701.8	$ (39.7)	(5.0)	331.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Employee equity plans activity (in shares)		100,000
Employee equity plans activity	7.0		7.0
Net income	43.2					43.2
Other comprehensive income	$ (4.1)				(4.1)
Balance (in shares) at Sep. 30, 2019	52,800,000	52,800,000		1,200,000
Balance at Sep. 30, 2019	$ 1,035.0	$ 0.1	$ 708.8	$ (39.7)	$ (9.1)	$ 374.9