Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	$ 134.1	$ 149.2	$ 138.2	$ 153.8
Charge-Offs	(1.1)	(2.8)	(2.7)	(9.3)
Recoveries	1.7	3.1	5.7	6.5
Net (Charge-Offs) Recoveries	0.6	0.3	3.0	(2.8)
Provision for Credit Losses	(7.0)	(9.0)	(13.5)	(10.5)
Balance at end of period	127.7	140.5	127.7	140.5
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	78.4	81.8	78.7	80.8
Charge-Offs	0.0	0.0	(0.1)	(0.9)
Recoveries	0.0	0.1	0.7	1.2
Net (Charge-Offs) Recoveries	0.0	0.1	0.6	0.3
Provision for Credit Losses	(2.0)	(3.1)	(2.9)	(2.3)
Balance at end of period	76.4	78.8	76.4	78.8
Personal
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	55.7	67.4	59.5	73.0
Charge-Offs	(1.1)	(2.8)	(2.6)	(8.4)
Recoveries	1.7	3.0	5.0	5.3
Net (Charge-Offs) Recoveries	0.6	0.2	2.4	(3.1)
Provision for Credit Losses	(5.0)	(5.9)	(10.6)	(8.2)
Balance at end of period	$ 51.3	$ 61.7	$ 51.3	$ 61.7