Shareholders' Equity - Share Repurchases (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Aug. 03, 2018	Jul. 31, 2016
Equity, Class of Treasury Stock [Line Items]
Shares repurchased during period (in shares)	1,000,000	1,100,000
Total cost of shares repurchased	$ 113.2	$ 115.8
2016 Authorization
Equity, Class of Treasury Stock [Line Items]
Shares authorized to be repurchased (in shares)				6,000,000
Shares remaining authorized for repurchase (in shares)	1,800,000
Subsequent Event | Common Stock Repurchase 2018 Authorization
Equity, Class of Treasury Stock [Line Items]
Shares authorized to be repurchased (in shares)			6,000,000.0