Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Financial Assets
Gross Amounts Recognized	$ 57.4		$ 57.4		$ 45.2
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	57.4		57.4		45.2
Financial Instruments	0.0		0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	38.6		38.6		45.1
Net Amount	18.8		18.8		0.1
Financial Liabilities
Gross Amounts Recognized	210.0		210.0		159.6
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	210.0		210.0		159.6
Financial Instruments	0.0		0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	1.0		1.0		0.0
Net Amount	209.0		209.0		159.6
Repurchase agreements
Gross Amounts Recognized	889.5		889.5		1,052.9
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	889.5		889.5		1,052.9
Financial Instruments	0.0		0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	889.5		889.5		1,052.9
Net Amount	0.0		0.0		0.0
Total assets and liabilities
Gross Amounts Recognized	1,099.5		1,099.5		1,212.5
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	1,099.5		1,099.5		1,212.5
Financial Instruments	0.0		0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	890.5		890.5		1,052.9
Net Amount	209.0		209.0		159.6
Derivative Liability, Notional Amount	2,898.0		2,898.0		2,449.0
Not Designated as Hedging Instrument
Total assets and liabilities
Amount of net gains recognized in mortgage banking revenues	0.1	$ (0.2)	(0.1)	$ (1.7)
Interest Rate Swap
Financial Assets
Gross Amounts Recognized	57.4		57.4		45.1
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	57.4		57.4		45.1
Financial Instruments	0.0		0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	38.6		38.6		45.1
Net Amount	18.8		18.8		0.0
Financial Liabilities
Gross Amounts Recognized	210.0		210.0		159.6
Gross Amounts Offset in the Balance Sheet	0.0		0.0		0.0
Net Amounts in the Balance Sheet	210.0		210.0		159.6
Financial Instruments	0.0		0.0		0.0
Fair Value of Financial Collateral in the Balance Sheet	1.0		1.0		0.0
Net Amount	209.0		209.0		159.6
Interest Rate Swap | Not Designated as Hedging Instrument
Total assets and liabilities
Derivative Liability, Notional Amount	1,638.8		1,638.8		1,728.1
Mortgage Related Derivatives
Financial Assets
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	102.2		102.2		106.1
Interest Rate Lock Commitments | Not Designated as Hedging Instrument
Total assets and liabilities
Derivative Liability, Notional Amount	$ 7.0		$ 7.0		$ 14.8