Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Equity [Abstract]
Number of shares authorized to be repurchased	6,300,000		6,300,000
Purchase of treasury stock	$ 43.0	$ 13.0	$ 45.9	$ 13.0
Treasury Stock, Shares, Acquired	800,000	300,000	800,000	300,000
Treasury Stock Acquired, Average Cost Per Share	$ 56.28	$ 43.83	$ 56.32	$ 43.83
Shares issued			1,700,000
Common stock reissued			400,000