Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Financial Assets
Gross Amounts Recognized	$ 26.9		$ 28.1
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Net Amounts in the Balance Sheet	26.9		28.1
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	6.4		8.0
Net Amount	20.5		20.1
Derivative Assets (Liabilities), at Fair Value, Net	26.9
Financial Liabilities
Gross Amounts Recognized	62.7		18.2
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Net Amounts in the Balance Sheet	62.7		18.2
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	1.0		0.0
Net Amount	61.7		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	1,071.0		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	1,133.7		1,069.3
Gross Amounts Offset in the Balance Sheet			0.0
Net Amounts in the Balance Sheet	1,133.7		1,069.3
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	1,072.0		1,051.1
Net Amount	61.7		18.2
Derivative Liability, Notional Amount	1,983.1		1,103.9
Not Designated as Hedging Instrument
Total assets and liabilities
Amount of net gains recognized in mortgage banking revenues	0.5	$ 0.3
Interest Rate Swap
Financial Assets
Gross Amounts Recognized	25.6		26.3
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Net Amounts in the Balance Sheet	25.6		26.3
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	6.4		8.0
Net Amount	19.2		18.3
Financial Liabilities
Gross Amounts Recognized	62.3		18.2
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Net Amounts in the Balance Sheet	62.3		18.2
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	1.0		0.0
Net Amount	61.3		18.2
Interest Rate Swap | Not Designated as Hedging Instrument
Total assets and liabilities
Derivative Liability, Notional Amount	1,827.1		913.9
Mortgage Related Derivatives
Financial Assets
Gross Amounts Recognized	1.3		1.8
Gross Amounts Offset in the Balance Sheet	0.0		0.0
Net Amounts in the Balance Sheet	1.3		1.8
Financial Instruments	0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0		0.0
Net Amount	1.3		1.8
Risk Participation Agreements
Financial Liabilities
Gross Amounts Recognized	0.4
Gross Amounts Offset in the Balance Sheet	0.0
Net Amounts in the Balance Sheet	0.4
Financial Instruments	0.0
Fair Value of Financial Collateral in the Balance Sheet	0.0
Net Amount	0.4
Risk Participation Agreements | Not Designated as Hedging Instrument
Total assets and liabilities
Derivative Liability, Notional Amount	109.6		$ 0.0
Repurchase Agreements
Financial Liabilities
Gross Amounts Offset in the Balance Sheet	0.0
Repurchase agreements
Gross Amounts Recognized	1,071.0
Net Amounts in the Balance Sheet	1,071.0
Financial Instruments	0.0
Fair Value of Financial Collateral in the Balance Sheet	1,071.0
Net Amount	$ 0.0