Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	$ 139.4	$ 147.8	$ 138.2	$ 153.8
Charge-Offs	(0.6)	(2.2)	(1.6)	(6.5)
Recoveries	1.8	2.1	4.0	3.4
Net (Charge-Offs) Recoveries	1.2	(0.1)	2.4	(3.1)
Provision for Credit Losses	(6.5)	1.5	(6.5)	(1.5)
Balance at end of period	134.1	149.2	134.1	149.2
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	78.6	77.1	78.7	80.8
Charge-Offs	0.0	(0.1)	(0.1)	(0.9)
Recoveries	0.3	0.5	0.7	1.1
Net (Charge-Offs) Recoveries	0.3	0.4	0.6	0.2
Provision for Credit Losses	(0.5)	4.3	(0.9)	0.8
Balance at end of period	78.4	81.8	78.4	81.8
Personal
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	60.8	70.7	59.5	73.0
Charge-Offs	(0.6)	(2.1)	(1.5)	(5.6)
Recoveries	1.5	1.6	3.3	2.3
Net (Charge-Offs) Recoveries	0.9	(0.5)	1.8	(3.3)
Provision for Credit Losses	(6.0)	(2.8)	(5.6)	(2.3)
Balance at end of period	$ 55.7	$ 67.4	$ 55.7	$ 67.4