Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022
Financial Assets
Gross Amounts Recognized	$ 43.1		$ 45.2
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Net Amounts in the Balance Sheet	43.1		45.2
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	42.8		45.1
Net Amount	0.3		0.1
Financial Liabilities
Gross Amounts Recognized	126.7		159.6
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Net Amounts in the Balance Sheet	126.7		159.6
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0		0.0
Net Amount	126.7		159.6
Repurchase agreements
Gross Amounts Recognized	970.8		1,052.9
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Net Amounts in the Balance Sheet	970.8		1,052.9
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	970.8		1,052.9
Net Amount	0.0		0.0
Total assets and liabilities
Gross Amounts Recognized	1,097.5		1,212.5
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Net Amounts in the Balance Sheet	1,097.5		1,212.5
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	970.8		1,052.9
Net Amount	126.7		159.6
Derivative Liability, Notional Amount	2,111.6		2,449.0
Not Designated as Hedging Instrument
Total assets and liabilities
Amount of net gains recognized in mortgage banking revenues	0.1	$ (0.5)
Interest Rate Swap
Financial Assets
Gross Amounts Recognized	42.8		45.1
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Net Amounts in the Balance Sheet	42.8		45.1
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	42.8		45.1
Net Amount	0.0		0.0
Financial Liabilities
Gross Amounts Recognized	126.6		159.6
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Net Amounts in the Balance Sheet	126.6		159.6
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0		0.0
Net Amount	126.6		159.6
Interest Rate Swap | Not Designated as Hedging Instrument
Total assets and liabilities
Derivative Liability, Notional Amount	1,674.7		1,728.1
Mortgage Related Derivatives
Financial Assets
Gross Amounts Recognized	0.3		0.1
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Net Amounts in the Balance Sheet	0.3		0.1
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0		0.0
Net Amount	0.3		0.1
Financial Liabilities
Gross Amounts Recognized	0.1
Gross Amounts Offset in the Balance Sheet	0.0
Net Amounts in the Balance Sheet	0.1
Financial Instruments	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0
Net Amount	0.1
Risk Participation Agreements | Not Designated as Hedging Instrument
Total assets and liabilities
Derivative Liability, Notional Amount	$ 104.7		$ 106.1