Shareholders' Equity (Shares Repurchased Under Share Repurchase Program) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Apr. 30, 2018
Class of Stock [Line Items]
Aggregate purchase price	$ 26.6
Stock Repurchase Program Board Authorized Repurchases [Member]
Class of Stock [Line Items]
Shares repurchased (in shares)	0.5	0.0
Aggregate purchase price	$ 26.6	$ 0.0
Average purchase price per share (in dollars per share)	$ 48.38	$ 0.00
Remaining amount available for share repurchase	$ 87.1
Subsequent Event [Member] | Stock Repurchase Program Board Authorized Repurchases [Member]
Class of Stock [Line Items]
Additional board authorized share repurchases			$ 250.0