Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	$ 149.2	$ 178.8	$ 153.8	$ 192.0
Charge-Offs	(2.8)	(3.5)	(9.3)	(13.2)
Recoveries	3.1	5.1	6.5	9.6
Net (Charge-Offs) Recoveries	0.3	1.6	(2.8)	(3.6)
Provision for Credit Losses	(9.0)	(7.0)	(10.5)	(15.0)
Balance at end of period	140.5	173.4	140.5	173.4
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	81.8	97.3	80.8	104.9
Charge-Offs	0.0	0.0	(0.9)	(4.6)
Recoveries	0.1	2.9	1.2	5.4
Net (Charge-Offs) Recoveries	0.1	2.9	0.3	0.8
Provision for Credit Losses	(3.1)	(7.8)	(2.3)	(13.3)
Balance at end of period	78.8	92.4	78.8	92.4
Personal
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	67.4	81.5	73.0	87.1
Charge-Offs	(2.8)	(3.5)	(8.4)	(8.6)
Recoveries	3.0	2.2	5.3	4.2
Net (Charge-Offs) Recoveries	0.2	(1.3)	(3.1)	(4.4)
Provision for Credit Losses	(5.9)	0.8	(8.2)	(1.7)
Balance at end of period	$ 61.7	$ 81.0	$ 61.7	$ 81.0