Stockholders' Equity and Earnings Per Common Share Treasury Stock Transactions (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Equity, Class of Treasury Stock
Number of Treasury Shares Repurchased	1.9	2.1
Cost of Shares Repurchased	$ 100.2	$ 100.0
Commission Paid on Common Stock Repurchased	0.2	$ 0.0
Repurchase Program Authorized in May 2017
Equity, Class of Treasury Stock
Authorized Stock Repurchases	750.0
Remaining Authorized Share Repurchases	412.8
Repurchase Program Authorized in May 2016
Equity, Class of Treasury Stock
Authorized Stock Repurchases	$ 750.0