Stockholders' Equity and Earnings Per Common Share Treasury Stock Transactions (Details) - USD ($) shares in Millions, $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Equity, Class of Treasury Stock
Number of Treasury Shares Repurchased		2.9	2.5	5.6	4.4
Cost of Shares Repurchased	[1]	$ 100.2	$ 100.1	$ 200.2	$ 200.3
Commission Paid on Common Stock Repurchased		0.2		0.2	$ 0.2
Repurchase Program Authorized in May 2019
Equity, Class of Treasury Stock
Authorized Stock Repurchases		750.0		750.0
Remaining Authorized Share Repurchases		716.2		716.2
Repurchase Program Authorized in May 2018
Equity, Class of Treasury Stock
Authorized Stock Repurchases		$ 750.0		$ 750.0

[1] ¹ Includes commissions of $0.2 million for the three months ended June 30, 2019 and a de minimis amount of commissions for the three months ended June 30, 2018. For the six months ended June 30, 2019 and 2018, these amounts included commissions of $0.2 million, respectively.