Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
LOANS AND LEASES
Balance at Beginning of Period	$ 104.5
Charge-Offs	(1.8)	$ (1.0)
Recoveries	1.1	2.2
Net (Charge-Offs) Recoveries	(0.7)	1.2
Provision for Credit Losses	45.6	0.7
Balance at End of Period	147.2	114.5
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	19.9
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net (Charge-Offs) Recoveries	0.0	0.0
Provision for Credit Losses	11.6	(0.7)
Balance at End of Period	40.4	24.9
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.0
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net (Charge-Offs) Recoveries	0.0	0.0
Provision for Credit Losses	2.9	0.0
Balance at end of period	9.5
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	0.0
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net (Charge-Offs) Recoveries	0.0	0.0
Provision for Credit Losses	0.7	0.0
Balance at End of Period	1.1
TOTAL
Balance at Beginning of Period	124.4
Charge-Offs	(1.8)	(1.0)
Recoveries	1.1	2.2
Net (Charge-Offs) Recoveries	(0.7)	1.2
Provision for Credit Losses	60.8	0.0
Balance at End of Period	198.2
Corporate Debt
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.0
Charge-Offs	0.0
Recoveries	0.0
Net (Charge-Offs) Recoveries	0.0
Provision for Credit Losses	1.9
Balance at end of period	2.7
TOTAL
Provision for Credit Losses	0.2
Cumulative Effect Adjustment
LOANS AND LEASES
Balance at Beginning of Period	(2.2)
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	8.9
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	6.6
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	0.4
TOTAL
Balance at Beginning of Period	13.7
Cumulative Effect Adjustment | Corporate Debt
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.8
Adjusted Balance
LOANS AND LEASES
Balance at Beginning of Period	102.3	112.6
Balance at End of Period		114.5
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	28.8	25.6
Balance at End of Period		24.9
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	6.6	0.0
Balance at end of period		0.0
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	0.4	0.0
Balance at End of Period		0.0
TOTAL
Balance at Beginning of Period	138.1	138.2
Balance at End of Period		$ 139.4
Adjusted Balance | Corporate Debt
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	$ 0.8