Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Financial Assets
Gross Amounts Recognized	$ 24.8		$ 24.8		$ 28.1
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	24.8		24.8		28.1
Financial Instruments	0.0		0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	23.7		23.7		8.0
Net Amount	1.1		1.1		20.1
Derivative Assets (Liabilities), at Fair Value, Net	24.8		24.8
Financial Liabilities
Gross Amounts Recognized	99.4		99.4		18.2
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	99.4		99.4		18.2
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	99.4		99.4		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	1,234.7		1,234.7		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	1,334.1		1,334.1		1,069.3
Gross Amounts Offset in the Balance Sheet					0.0
Net Amounts in the Balance Sheet	1,334.1		1,334.1		1,069.3
Financial Instruments	0.0		0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	1,234.7		1,234.7		1,051.1
Net Amount	99.4		99.4		18.2
Derivative Liability, Notional Amount	1,867.2		1,867.2		1,103.9
Not Designated as Hedging Instrument
Total assets and liabilities
Amount of net gains recognized in mortgage banking revenues	1.0	$ 0.2	1.5	$ 0.5
Interest Rate Swap
Financial Assets
Gross Amounts Recognized	24.6		24.6		26.3
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	24.6		24.6		26.3
Financial Instruments	0.0		0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	23.7		23.7		8.0
Net Amount	0.9		0.9		18.3
Financial Liabilities
Gross Amounts Recognized	99.2		99.2		18.2
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	99.2		99.2		18.2
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	99.2		99.2		18.2
Interest Rate Swap | Not Designated as Hedging Instrument
Total assets and liabilities
Derivative Liability, Notional Amount	1,758.7		1,758.7		913.9
Mortgage Related Derivatives
Financial Assets
Gross Amounts Recognized	0.2		0.2		1.8
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	0.2		0.2		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	0.2		0.2		1.8
Risk Participation Agreements
Financial Liabilities
Gross Amounts Recognized	0.2		0.2
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Net Amounts in the Balance Sheet	0.2		0.2
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0		0.0
Net Amount	0.2		0.2
Risk Participation Agreements | Not Designated as Hedging Instrument
Total assets and liabilities
Derivative Liability, Notional Amount	108.5		108.5		$ 0.0
Repurchase Agreements
Financial Liabilities
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Repurchase agreements
Gross Amounts Recognized	1,234.7		1,234.7
Net Amounts in the Balance Sheet	1,234.7		1,234.7
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	1,234.7		1,234.7
Net Amount	$ 0.0		$ 0.0