Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	$ 104.5	$ 112.6	$ 131.2
Charge-Offs	(9.7)	(6.5)	(10.1)
Recoveries	6.5	7.2	9.0
Net Recoveries (Charge-Offs)	(3.2)	0.7	(1.1)
Provision for Credit Losses	91.6	(8.8)	(17.5)
Balance at End of Period	190.7	104.5	112.6
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	19.9	25.6	22.6
Charge-Offs	0.0	0.0	0.0
Recoveries	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0	0.0
Provision for Credit Losses	32.3	(5.7)	3.0
Balance at End of Period	61.1	19.9	25.6
DEBT SECURITIES HELD TO MATURITY
Balance at Beginning of Period	0.0
Charge-Offs	0.0
Recoveries	0.0
Net Recoveries (Charge-Offs)	0.0
Provision for Credit Losses	0.7
Balance at End of Period	7.3	0.0
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	0.0
Charge-Offs	0.0
Recoveries	0.0
Net Recoveries (Charge-Offs)	0.0
Provision for Credit Losses	0.4
Balance at End of Period	0.8	0.0
TOTAL
Balance at Beginning of Period	124.4	138.2	153.8
Charge-Offs	(9.7)	(6.5)	(10.1)
Recoveries	6.5	7.2	9.0
Net Recoveries (Charge-Offs)	(3.2)
Provision for Credit Losses	125.0	(14.5)	(14.5)
Balance at End of Period	259.9	124.4	$ 138.2
Cumulative Effect Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	(2.2)
Balance at End of Period		(2.2)
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	8.9
Balance at End of Period		8.9
DEBT SECURITIES HELD TO MATURITY
Balance at Beginning of Period	6.6
Balance at End of Period		6.6
Cumulative Effect Adjustment | Commercial and Consumer Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	(2.2)
Balance at End of Period		(2.2)
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	8.9
Balance at End of Period		8.9
DEBT SECURITIES HELD TO MATURITY
Balance at Beginning of Period	6.6
Balance at End of Period		6.6
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	0.4
Balance at End of Period		0.4
TOTAL
Balance at Beginning of Period	13.7
Balance at End of Period		13.7
Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Period	102.3
Balance at End of Period		102.3
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	28.8
Balance at End of Period		28.8
DEBT SECURITIES HELD TO MATURITY
Balance at Beginning of Period	6.6
Balance at End of Period		6.6
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	0.4
Balance at End of Period		0.4
TOTAL
Balance at Beginning of Period	$ 138.1
Balance at End of Period		$ 138.1