Shareholders' Equity - Share Repurchases (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jul. 28, 2016	Oct. 29, 2015
Equity, Class of Treasury Stock [Line Items]
Shares repurchased during period (in shares)	1,700,000	6,400,000
Total cost of shares repurchased	$ 178.0	$ 462.6
2016 Authorization
Equity, Class of Treasury Stock [Line Items]
Shares authorized to be repurchased (in shares)			6,000,000
Shares repurchased during period (in shares)		1,100,000
Shares remaining authorized for repurchase (in shares)	3,000,000
2015 Authorization
Equity, Class of Treasury Stock [Line Items]
Shares authorized to be repurchased (in shares)				6,000,000
Shares repurchased during period (in shares)		5,300,000
Shares remaining authorized for repurchase (in shares)	0