Treasury Stock Transactions (Details) - USD ($) shares in Millions, $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity, Class of Treasury Stock
Expiration Date of Share Repurchase Program Authorized in May 2017		Nov. 25, 2018
Number of Treasury Shares Repurchased		8.2	11.9	12.3
Cost of Shares Repurchased	[1]	$ 400.4	$ 403.3	$ 426.7
Commission Paid on Stock Repurchases		0.3	$ 0.2	$ 0.3
Repurchase Program Authorized in May 2017
Equity, Class of Treasury Stock
Authorized Amount of Treasury Share Repurchases Under Stock Repurchase Program		750.0
Remaining Amount of Treasury Share Repurchases Authorized Under Stock Repurchase Program		512.8
Repurchase Program Authorized in May 2016
Equity, Class of Treasury Stock
Authorized Amount of Treasury Share Repurchases Under Stock Repurchase Program		750.0
Remaining Amount of Treasury Share Repurchases Authorized Under Stock Repurchase Program		0.0
Repurchase Program Authorized in May 2015
Equity, Class of Treasury Stock
Authorized Amount of Treasury Share Repurchases Under Stock Repurchase Program		750.0
Remaining Amount of Treasury Share Repurchases Authorized Under Stock Repurchase Program		$ 0.0

[1] Includes commissions of $0.3 million, $0.2 million, and $0.3 million for the years ended December 31, 2017, 2016, and 2015, respectively.