Earnings Per Share Stock Buyback (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
	Apr. 30, 2018	Aug. 31, 2017	Mar. 31, 2018	Sep. 16, 2015
Equity, Class of Treasury Stock [Line Items]
Authorized amount			$ 50.0	$ 150.0
Treasury stock, common, value			12.8
Treasury stock, shares, acquired (in shares)		180,942
Treasury stock acquired, average cost per share (in dollars per share)		$ 70.47
Remaining authorized repurchase amount			57.3
Stock Repurchase Plan and Agreement [Member]
Equity, Class of Treasury Stock [Line Items]
Remaining authorized repurchase amount			$ 37.2
Subsequent Event [Member]
Equity, Class of Treasury Stock [Line Items]
Treasury stock, common, value	$ 15.7
Treasury stock, shares, acquired (in shares)	254,015
Treasury stock acquired, average cost per share (in dollars per share)	$ 62.01