Equity, Stock Repurchase (Details) - USD ($) shares in Millions, $ in Millions		1 Months Ended		12 Months Ended
	Jan. 24, 2017	Jan. 31, 2018	Jan. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
2017 Stock Repurchase Program
Shareholders' Equity
Value of shares authorized to be repurchased	$ 1,000.0
Repurchase of stock program, period	2 years
Number of shares repurchased and retired				8.4
Payments for shares repurchased and retired (in dollars)				$ 618.0
Number of shares repurchased		1.1
Payments for shares repurchased (in dollars)		$ 105.5
Value of shares remaining that may be repurchased under the stock repurchase program		$ 276.5
2015 Stock Repurchase Program
Shareholders' Equity
Number of shares authorized to be repurchased under the current open-market stock repurchase program			10.0
Repurchase of stock program, period			2 years
Number of shares repurchased and retired					5.5	4.5
Payments for shares repurchased and retired (in dollars)					$ 325.8	$ 248.9