Share-Based Compensation Plans (Narrative) (Details) - USD ($) $ / shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Restricted Stock Unit Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, equity instruments other than options, grants in period (in shares)	793,995	839,354
Share-based compensation arrangement by share-based payment award, equity instruments other than options, grants in period, weighted average grant date fair value	$ 82.6	$ 73.9
Allocated share-based compensation expense, employees expensed in entirety	$ 26.9	$ 21.6
Allocated stock compensation expense for fully vested equity options	$ 2.7	2.5
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense, employees expensed in entirety		$ 5.5
Share-based compensation arrangement by share-based payment award, options, grants in period, gross	0	468,381
Share-based compensation arrangement by share-based payment award, options, grants in period, weighted average grant date fair value		$ 9.0
Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, equity instruments other than options, grants in period (in shares)	242,232	206,091
Share-based compensation arrangement by share-based payment award, equity instruments other than options, grants in period, weighted average grant date fair value	$ 25.4	$ 18.1
Allocated share-based compensation expense, employees expensed in entirety	$ 4.7	$ 3.3