Shareholders' Equity - Share Repurchases (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Aug. 31, 2018	Jul. 31, 2016
Equity, Class of Treasury Stock [Line Items]
Shares repurchased during period (in shares)	3,100,000	1,700,000
Total cost of shares repurchased	$ 299.2	$ 178.0
Common Stock Repurchase 2018 Authorization
Equity, Class of Treasury Stock [Line Items]
Shares authorized to be repurchased (in shares)			6,000,000.0
Shares repurchased during period (in shares)	300,000
Shares remaining authorized for repurchase (in shares)	5,700,000
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)	2,800,000
Shares remaining authorized for repurchase (in shares)	0