Treasury Stock Transactions (Details) - USD ($) shares in Millions, $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity, Class of Treasury Stock
Expiration Date of Share Repurchase Program Authorized in May 2019		Nov. 23, 2020
Number of Treasury Shares Repurchased		12.3	8.7	8.2
Commission Paid on Stock Repurchases		$ 0.4	$ 0.7	$ 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		0.0
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
Remaining Amount of Treasury Share Repurchases Authorized Under Stock Repurchase Program		516.2
Open Market Repurchase
Equity, Class of Treasury Stock
Cost of Shares Repurchased	[1]	$ 400.4	$ 350.7	$ 400.4

[1] Includes commissions of $0.4 million, $0.7 million, and $0.3 million for the years ended December 31, 2019, 2018, and 2017, respectively.