Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	$ 147.8	$ 189.0	$ 153.8	$ 192.0
Charge-Offs	(2.2)	(5.0)	(6.5)	(9.7)
Recoveries	2.1	1.8	3.4	4.5
Net (Charge-Offs) Recoveries	(0.1)	(3.2)	(3.1)	(5.2)
Provision for Credit Losses	1.5	(7.0)	(1.5)	(8.0)
Balance at end of period	149.2	178.8	149.2	178.8
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	77.1	111.8	80.8	104.9
Charge-Offs	(0.1)	(3.5)	(0.9)	(4.6)
Recoveries	0.5	0.9	1.1	2.5
Net (Charge-Offs) Recoveries	0.4	(2.6)	0.2	(2.1)
Provision for Credit Losses	4.3	(11.9)	0.8	(5.5)
Balance at end of period	81.8	97.3	81.8	97.3
Personal
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	70.7	77.2	73.0	87.1
Charge-Offs	(2.1)	(1.5)	(5.6)	(5.1)
Recoveries	1.6	0.9	2.3	2.0
Net (Charge-Offs) Recoveries	(0.5)	(0.6)	(3.3)	(3.1)
Provision for Credit Losses	(2.8)	4.9	(2.3)	(2.5)
Balance at end of period	$ 67.4	$ 81.5	$ 67.4	$ 81.5