Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Aug. 02, 2021	Jun. 17, 2021	Dec. 31, 2020
Financial Assets
Gross Amounts Recognized	$ 34.1				$ 34.1				$ 55.3
Gross Amounts Offset in the Balance Sheet	0.0				0.0				0.0
Net Amounts in the Balance Sheet	34.1				34.1				55.3
Financial Instruments	0.0				0.0				0.0
Fair Value of Financial Collateral in the Balance Sheet	11.0				11.0				17.2
Net Amount	23.1				23.1				38.1
Derivative Assets (Liabilities), at Fair Value, Net	34.1				34.1
Financial Liabilities
Gross Amounts Recognized	21.1				21.1				17.9
Gross Amounts Offset in the Balance Sheet	0.0				0.0				0.0
Net Amounts in the Balance Sheet	21.1				21.1				17.9
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	21.1				21.1				17.9
Repurchase agreements
Gross Amounts Recognized	1,007.5				1,007.5				1,091.4
Gross Amounts Offset in the Balance Sheet									0.0
Net Amounts in the Balance Sheet	1,007.5				1,007.5				1,091.4
Financial Instruments	0.0				0.0				0.0
Fair Value of Financial Collateral in the Balance Sheet	1,007.5				1,007.5				1,091.4
Net Amount	0.0				0.0				0.0
Total assets and liabilities
Gross Amounts Recognized	1,028.6				1,028.6				1,109.3
Gross Amounts Offset in the Balance Sheet									0.0
Net Amounts in the Balance Sheet	1,028.6				1,028.6				1,109.3
Financial Instruments	0.0				0.0				0.0
Fair Value of Financial Collateral in the Balance Sheet	1,007.5				1,007.5				1,091.4
Net Amount	$ 21.1				21.1				17.9
Payments for (Proceeds from) Derivative Instrument, Investing Activities				$ 500.0	200.0
Derivatives, Basis and Use of Derivatives	99	87
Derivative Liability, Notional Amount	$ 1,442.4				1,442.4				1,014.1
Interest Rate Swap [Member]
Total assets and liabilities
Derivative, Notional Amount							$ 200.0	$ 500.0
Not Designated as Hedging Instrument
Total assets and liabilities
Amount of net gains recognized in mortgage banking revenues	1.3		$ (0.6)		1.7	$ 4.4
Interest Rate Swap [Member]
Financial Assets
Gross Amounts Recognized	30.1				30.1				52.0
Gross Amounts Offset in the Balance Sheet	0.0				0.0				0.0
Net Amounts in the Balance Sheet	30.1				30.1				52.0
Financial Instruments	0.0				0.0				0.0
Fair Value of Financial Collateral in the Balance Sheet	11.0				11.0				17.2
Net Amount	19.1				19.1				34.8
Financial Liabilities
Gross Amounts Recognized	21.1				21.1				16.8
Gross Amounts Offset in the Balance Sheet	0.0				0.0				0.0
Net Amounts in the Balance Sheet	21.1				21.1				16.8
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	21.1				21.1				16.8
Interest Rate Swap [Member] | Not Designated as Hedging Instrument
Total assets and liabilities
Derivative Liability, Notional Amount	854.8				854.8				799.7
Mortgage Related Derivatives
Financial Assets
Gross Amounts Recognized	4.0				4.0				3.3
Gross Amounts Offset in the Balance Sheet	0.0				0.0				0.0
Net Amounts in the Balance Sheet	4.0				4.0				3.3
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	4.0				4.0				3.3
Financial Liabilities
Gross Amounts Recognized									1.1
Gross Amounts Offset in the Balance Sheet									0.0
Net Amounts in the Balance Sheet									1.1
Financial Instruments									0.0
Fair Value of Financial Collateral in the Balance Sheet									0.0
Net Amount									$ 1.1
Repurchase Agreements
Financial Liabilities
Gross Amounts Offset in the Balance Sheet	$ 0.0				$ 0.0