Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	$ 178.8	$ 227.1	$ 192.0	$ 233.3
Charge-Offs	(3.5)	(3.0)	(13.2)	(13.2)
Recoveries	5.1	3.8	9.6	8.9
Net (Charge-Offs) Recoveries	1.6	0.8	(3.6)	(4.3)
Provision for Credit Losses	(7.0)	(3.0)	(15.0)	(4.0)
Effect of Foreign Exchange Rates			0.0	(0.1)
Balance at end of period	173.4	224.9	173.4	224.9
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	97.3	112.3	104.9	114.8
Charge-Offs	0.0	(0.3)	(4.6)	(4.7)
Recoveries	2.9	1.0	5.4	3.5
Net (Charge-Offs) Recoveries	2.9	0.7	0.8	(1.2)
Provision for Credit Losses	(7.8)	4.0	(13.3)	3.5
Effect of Foreign Exchange Rates			0.0	(0.1)
Balance at end of period	92.4	117.0	92.4	117.0
Personal
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	81.5	114.8	87.1	118.5
Charge-Offs	(3.5)	(2.7)	(8.6)	(8.5)
Recoveries	2.2	2.8	4.2	5.4
Net (Charge-Offs) Recoveries	(1.3)	0.1	(4.4)	(3.1)
Provision for Credit Losses	0.8	(7.0)	(1.7)	(7.5)
Effect of Foreign Exchange Rates			0.0	0.0
Balance at end of period	$ 81.0	$ 107.9	$ 81.0	$ 107.9