Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Financial Assets
Gross Amounts Recognized	$ 42.7		$ 42.7		$ 28.1
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	42.7		42.7		28.1
Financial Instruments	0.0		0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	42.1		42.1		8.0
Net Amount	0.6		0.6		20.1
Financial Liabilities
Gross Amounts Recognized	169.9		169.9		18.2
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	169.9		169.9		18.2
Financial Instruments	0.0		0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	5.6		5.6		0.0
Net Amount	164.3		164.3		18.2
Repurchase agreements
Gross Amounts Recognized					1,051.1
Gross Amounts Offset in the Balance Sheet					0.0
Net Amounts in the Balance Sheet	1,075.6		1,075.6		1,051.1
Financial Instruments					0.0
Fair Value of Financial Collateral in the Balance Sheet					1,051.1
Net Amount					0.0
Total assets and liabilities
Gross Amounts Recognized	1,245.5		1,245.5		1,069.3
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	1,245.5		1,245.5		1,069.3
Financial Instruments	0.0		0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	1,081.2		1,081.2		1,051.1
Net Amount	164.3		164.3		18.2
Derivative Liability, Notional Amount	2,176.4		2,176.4		1,103.9
Not Designated as Hedging Instrument
Total assets and liabilities
Amount of net gains recognized in mortgage banking revenues	(0.2)	$ 1.3	(1.7)	$ 1.7
Interest Rate Swap
Financial Assets
Gross Amounts Recognized	42.1		42.1		26.3
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	42.1		42.1		26.3
Financial Instruments	0.0		0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	42.1		42.1		8.0
Net Amount	0.0		0.0		18.3
Financial Liabilities
Gross Amounts Recognized	169.3		169.3		18.2
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	169.3		169.3		18.2
Financial Instruments	0.0		0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	5.6		5.6		0.0
Net Amount	163.7		163.7		18.2
Interest Rate Swap | Not Designated as Hedging Instrument
Total assets and liabilities
Derivative Liability, Notional Amount	1,743.3		1,743.3		913.9
Mortgage Related Derivatives
Financial Assets
Gross Amounts Recognized	0.6		0.6		1.8
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	0.6		0.6		1.8
Financial Instruments	0.0		0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0		0.0		0.0
Net Amount	0.6		0.6		1.8
Financial Liabilities
Gross Amounts Recognized	0.5		0.5
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Net Amounts in the Balance Sheet	0.5		0.5
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0		0.0
Net Amount	0.5		0.5
Risk Participation Agreements
Financial Liabilities
Gross Amounts Recognized	0.1		0.1
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Net Amounts in the Balance Sheet	0.1		0.1
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0		0.0
Net Amount	0.1		0.1
Risk Participation Agreements | Not Designated as Hedging Instrument
Total assets and liabilities
Derivative Liability, Notional Amount	107.2		107.2		$ 0.0
Repurchase Agreements
Repurchase agreements
Gross Amounts Recognized	1,075.6		1,075.6
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Net Amounts in the Balance Sheet	1,075.6		1,075.6
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	1,075.6		1,075.6
Net Amount	$ 0.0		$ 0.0