Shareholders' Equity - Share Repurchases (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Aug. 31, 2018	Jul. 31, 2016
Equity, Class of Treasury Stock [Line Items]
Total cost of shares repurchased	$ 101.0	$ 50.1
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)	1,200,000
Total cost of shares repurchased	$ 101.0
Shares remaining authorized for repurchase (in shares)	1,900,000
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)		400,000
Total cost of shares repurchased		$ 50.1
Shares remaining authorized for repurchase (in shares)	0