Shareholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 31, 2013
Statement [Line Items]
Purchase for treasury		$ 299.8	$ 600.1	$ 749.5
Payments for Repurchase of Common Stock	$ 60.0	$ 299.8	$ 600.1	749.5
Preferred stock, shares authorized (in shares)		5,000,000
Shares issued (in shares)	0	0	0
Shares held in employee trust (in shares)	50,000,000	50,000,000	50,000,000
Employee benefit trust	$ 3,013.2	$ 3,013.2	$ 3,013.2
2013 Share Repurchase Program
Statement [Line Items]
Purchase for treasury		359.8	$ 540.1	$ 749.5
Stock repurchase program, authorized amount (in shares)					$ 5,000.0
Remaining authorized repurchase amount (in shares)		$ 2,050.0