Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	$ 153.8	$ 192.0
Charge-Offs	(4.3)	(4.7)
Recoveries	1.3	2.7
Net (Charge-Offs) Recoveries	(3.0)	(2.0)
Provision for Credit Losses	(3.0)	(1.0)
Balance at end of period	147.8	189.0
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	80.8	104.9
Charge-Offs	(0.8)	(1.1)
Recoveries	0.6	1.6
Net (Charge-Offs) Recoveries	(0.2)	0.5
Provision for Credit Losses	(3.5)	6.4
Balance at end of period	77.1	111.8
Personal
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	73.0	87.1
Charge-Offs	(3.5)	(3.6)
Recoveries	0.7	1.1
Net (Charge-Offs) Recoveries	(2.8)	(2.5)
Provision for Credit Losses	0.5	(7.4)
Balance at end of period	$ 70.7	$ 77.2