Treasury Stock Transactions (Details) - USD ($) shares in Millions, $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity, Class of Treasury Stock
Expiration Date of Share Repurchase Program Authorized in May 2019		Nov. 23, 2020
Number of Treasury Shares Repurchased		0.0	12.3	8.7
Commission Paid on Stock Repurchases			$ 0.4	$ 0.7
Repurchase Program Authorized in May 2018
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 2019
Equity, Class of Treasury Stock
Authorized Amount of Treasury Share Repurchases Under Stock Repurchase Program		750.0
Open Market Repurchase
Equity, Class of Treasury Stock
Cost of Shares Repurchased	[1]	$ 0.0	$ 400.4	$ 350.7

[1] Includes commissions of $0.4 million and $0.7 million for the years ended December 31, 2019 and 2018, respectively.