Unaudited Condensed Consolidated Statement Of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]
BALANCE at period start (in shares) at Dec. 31, 2017		102,562,149
BALANCE at period start at Dec. 31, 2017	$ 2,369.3	$ 1.0	$ 4.0	$ 2,832.2	$ (467.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	93.7			93.7
Repurchases of common stock	(26.6)				(26.6)
Stock-based compensation expense	14.3		14.3
Shares awarded under stock-based compensation plans, net of shares withheld for taxes	11.1		(2.2)		13.3
Cumulative effect of change in accounting principle - revenue recognition	10.1			10.1
BALANCE at period end (in shares) at Mar. 31, 2018		102,562,149
BALANCE at period end at Mar. 31, 2018	2,471.9	$ 1.0	16.1	2,936.0	(481.2)
BALANCE at period start (in shares) at Dec. 31, 2018		102,562,149
BALANCE at period start at Dec. 31, 2018	2,716.0	$ 1.0	20.8	3,238.3	(544.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	92.0			92.0
Repurchases of common stock	(33.5)				(33.5)
Stock-based compensation expense	13.7		13.7
Shares awarded under stock-based compensation plans, net of shares withheld for taxes	(2.4)		(9.8)		7.4
BALANCE at period end (in shares) at Mar. 31, 2019		102,562,149
BALANCE at period end at Mar. 31, 2019	$ 2,785.8	$ 1.0	$ 24.7	$ 3,330.3	$ (570.2)