Equity - Share Repurchases, Dividends, Stock-Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions			1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 23, 2018	Oct. 20, 2015	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share Repurchase Plan
Increase in authorized number of shares to be repurchased	5,000,000	7,500,000
Number of shares authorized by the Board of Directors to be repurchased under share repurchase plan	10,700,000
Number of shares authorized by the Board of Directors to be repurchased under share repurchase plan, expressed as a percentage of outstanding shares (as a percent)	15.00%
Dividends
Common stock quarterly dividend per share (in dollars per share)	$ 0.50
Common stock dividend quarterly declared and paid per share (in dollars per share)				$ 0.50	$ 0.45	$ 1.50	$ 1.35
Dividends paid				$ 35.9	$ 32.8	$ 108.5	$ 98.4
Common Stock and Additional Paid in Capital
Share Repurchase Plan
Share repurchases (in shares)				(918,352)		(1,510,916)
Average costs per share						$ 86.48
Value of shares repurchased				$ 80.7		$ 130.7
Common Stock and Additional Paid-In (in Shares)
Stock-based compensation (in shares)				3,984		288,991
Stock options exercised (in shares)						48,275
Share repurchases (in shares)				(918,352)		(1,510,916)
Total (in shares)				(914,368)		(1,173,650)
Stock-based compensation expense						$ 33.8
Payments made to tax authorities on employees' behalf						$ 5.5
Repurchase of common shares pending settlement (in shares)				7,231		7,231
Repurchase value of common shares pending settlement				$ 0.6		$ 0.6
Common Stock and Additional Paid-In (in Amount)
Stock-based compensation				14.4		28.3
Stock options exercised						2.8
Repurchase of common shares				(80.7)		(130.7)
Total				$ (66.3)		$ (99.6)
Weighted Average Exercise Price Per Share
Stock options exercised (in dollars per share)						$ 57.91
RSU's
Dividends
Dividend equivalents paid						$ 2.0	$ 0.9
Stock-Based Compensation
Vesting period (in years)						3 years
RSU changes
Granted (in shares)			474,715
RSU's | Performance-based
RSU changes
Granted (in shares)			178,970
Unvested RSUs and Restricted Stock
RSU changes
Unvested at the beginning of the period (in shares)						924,575
Granted (in shares)						474,715
Vested (in shares)						(7,790)
Cancelled or forfeited (in shares)						(20,160)
Unvested at the end of the period (in shares)				1,371,340		1,371,340
Shares reserved for future grants (all plans)				1,376,110		1,376,110
Weighted Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share)						$ 74.09
Granted (in dollars per shares)						84.26
Vested (in dollars per shares)						72.43
Cancelled or forfeited (in dollars per shares)						75.36
Unvested at the end of the period (in dollars per shares)				$ 77.60		$ 77.60