Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	$ 138.2	$ 153.8
Charge-Offs	(1.0)	(4.3)
Recoveries	2.2	1.3
Net (Charge-Offs) Recoveries	1.2	(3.0)
Provision for Credit Losses	0.0	(3.0)
Balance at end of period	139.4	147.8
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	78.7	80.8
Charge-Offs	(0.1)	(0.8)
Recoveries	0.4	0.6
Net (Charge-Offs) Recoveries	0.3	(0.2)
Provision for Credit Losses	(0.4)	(3.5)
Balance at end of period	78.6	77.1
Personal
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	59.5	73.0
Charge-Offs	(0.9)	(3.5)
Recoveries	1.8	0.7
Net (Charge-Offs) Recoveries	0.9	(2.8)
Provision for Credit Losses	0.4	0.5
Balance at end of period	$ 60.8	$ 70.7