Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	$ 190.7	$ 104.5	$ 112.6
Charge-Offs	(0.7)	(9.7)	(6.5)
Recoveries	7.0	6.5	7.2
Net Recoveries (Charge-Offs)	6.3	(3.2)	0.7
Provision for Credit Losses	(58.6)	91.6	(8.8)
Balance at end of period	138.4	190.7	104.5
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	61.1	19.9	25.6
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	(27.0)	32.3	(5.7)
Balance at End of Period	34.1	61.1	19.9
HELD TO MATURITY DEBT SECURITIES
Balance at beginning of period	7.3	0.0
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	3.9	0.7
Balance at end of period	11.2	7.3	0.0
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	0.8	0.0
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	0.2	0.4
Balance at End of Period	1.0	0.8	0.0
TOTAL
Balance at Beginning of Period	259.9	124.4	138.2
Charge-Offs	(0.7)	(9.7)	(6.5)
Recoveries	7.0	6.5	7.2
Net Recoveries (Charge-Offs)	6.3	(3.2)	0.7
Provision for Credit Losses	(81.5)	125.0	(14.5)
Balance at End of Period	$ 184.7	259.9	124.4
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
HELD TO MATURITY DEBT SECURITIES
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
HELD TO MATURITY DEBT SECURITIES
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