Stockholders' Equity and Earnings Per Common Share Treasury Stock Transactions (Details) - USD ($) shares in Millions, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Equity, Class of Treasury Stock
Number of Treasury Shares Repurchased		3.4	0.0	9.0	4.4
Cost of Shares Repurchased	[1]	$ 100.1	$ 0.0	$ 300.3	$ 200.3
Commission Paid on Common Stock Repurchased		0.1		0.3	$ 0.2
Repurchase Program Authorized in May 2019
Equity, Class of Treasury Stock
Authorized Stock Repurchases		750.0		750.0
Remaining Authorized Share Repurchases		616.2		616.2
Repurchase Program Authorized in May 2018
Equity, Class of Treasury Stock
Authorized Stock Repurchases		$ 750.0		$ 750.0

[1] Includes commissions of $0.1 million and $0.3 million for the three and nine months ended September 30, 2019 and $0.2 million for the nine months ended September 30, 2018