Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]
BALANCE, Shares at Dec. 31, 2016		120,562,149
BALANCE, Amount at Dec. 31, 2016	$ 2,310.3	$ 1.2	$ 18.2	$ 3,133.3	$ (842.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	434.6			434.6
Repurchases of common stock	(434.9)				(434.9)
Treasury stock cancellation, Shares		(18,000,000)
Treasury stock cancellation	0.0	$ (0.2)	(30.2)	(735.6)	766.0
Stock-based compensation expense	20.6		20.6
Shares awarded under stock-based compensation plans, net of shares withheld for taxes	38.6		(4.8)		43.4
Other	0.1		0.2	(0.1)
BALANCE, Shares at Dec. 31, 2017		102,562,149
BALANCE, Amount 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	396.0			396.0
Repurchases of common stock	(100.0)				(100.0)
Stock-based compensation expense	25.5		25.5
Shares awarded under stock-based compensation plans, net of shares withheld for taxes	15.1		(8.7)		23.8
Cumulative effect of change in accounting principle - revenue recognition	10.1			10.1
BALANCE, Shares at Dec. 31, 2018		102,562,149
BALANCE, Amount 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	450.0			450.0
Repurchases of common stock	(44.7)				(44.7)
Stock-based compensation expense	31.1		31.1
Shares awarded under stock-based compensation plans, net of shares withheld for taxes	9.7		(16.0)		25.7
BALANCE, Shares at Dec. 31, 2019		102,562,149
BALANCE, Amount at Dec. 31, 2019	$ 3,162.1	$ 1.0	$ 35.9	$ 3,688.3	$ (563.1)