Unaudited Condensed Consolidated Statement Of Shareholders' Equity - 3 months ended Mar. 31, 2018 - 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)