Derivatives and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Jun. 17, 2021	Dec. 31, 2020
Financial Assets
Gross Amounts Recognized	$ 36.7		$ 36.7			$ 55.3
Gross Amounts Offset in the Balance Sheet	0.0		0.0			0.0
Net Amounts in the Balance Sheet	36.7		36.7			55.3
Financial Instruments	0.0		0.0			0.0
Fair Value of Financial Collateral in the Balance Sheet	12.0		12.0			17.2
Net Amount	24.7		24.7			38.1
Derivative Assets (Liabilities), at Fair Value, Net	36.7		36.7
Financial Liabilities
Gross Amounts Recognized	19.1		19.1			17.9
Gross Amounts Offset in the Balance Sheet	0.0		0.0			0.0
Net Amounts in the Balance Sheet	19.1		19.1			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	19.1		19.1			17.9
Repurchase agreements
Gross Amounts Recognized	1,038.7		1,038.7			1,091.4
Gross Amounts Offset in the Balance Sheet						0.0
Net Amounts in the Balance Sheet	1,038.7		1,038.7			1,091.4
Financial Instruments	0.0		0.0			0.0
Fair Value of Financial Collateral in the Balance Sheet	1,038.7		1,038.7			1,091.4
Net Amount	0.0		0.0			0.0
Total assets and liabilities
Gross Amounts Recognized	1,057.8		1,057.8			1,109.3
Gross Amounts Offset in the Balance Sheet						0.0
Net Amounts in the Balance Sheet	1,057.8		1,057.8			1,109.3
Financial Instruments	0.0		0.0			0.0
Fair Value of Financial Collateral in the Balance Sheet	1,038.7		1,038.7			1,091.4
Net Amount	$ 19.1		19.1			17.9
Payments for (Proceeds from) Derivative Instrument, Investing Activities			500.0
Derivatives, Basis and Use of Derivatives	87
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	$ 0.0		0.0			0.0
Derivative Asset, Collateral, Obligation to Return Cash, Offset	0.0		0.0			0.0
Derivative Liability, Notional Amount	1,564.1		1,564.1			1,014.1
Interest Rate Derivative Instruments Not Designated as Hedging Instruments, Liability at Fair Value	24.2		24.2			35.4
Interest Rate Derivative Instruments Not Designated as Hedging Instruments, Asset at Fair Value	8.4		8.4			0.0
Interest Rate Swap [Member]
Total assets and liabilities
Derivative, Notional Amount					$ 500.0
Not Designated as Hedging Instrument
Total assets and liabilities
Amount of net gains recognized in mortgage banking revenues	0.2	$ 0.5	0.5	$ 5.0
Interest Rate Swap [Member]
Financial Assets
Gross Amounts Recognized	33.7		33.7			52.0
Gross Amounts Offset in the Balance Sheet	0.0		0.0			0.0
Net Amounts in the Balance Sheet	33.7		33.7			52.0
Financial Instruments	0.0		0.0			0.0
Fair Value of Financial Collateral in the Balance Sheet	12.0		12.0			17.2
Net Amount	21.7		21.7			34.8
Financial Liabilities
Gross Amounts Recognized	18.9		18.9			16.8
Gross Amounts Offset in the Balance Sheet	0.0		0.0			0.0
Net Amounts in the Balance Sheet	18.9		18.9			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	18.9		18.9			16.8
Interest Rate Swap [Member] | Not Designated as Hedging Instrument
Total assets and liabilities
Derivative Liability, Notional Amount	849.7		849.7			799.7
Mortgage Related Derivatives
Financial Assets
Gross Amounts Recognized	3.0		3.0			3.3
Gross Amounts Offset in the Balance Sheet	0.0		0.0			0.0
Net Amounts in the Balance Sheet	3.0		3.0			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	3.0		3.0			3.3
Financial Liabilities
Gross Amounts Recognized	0.2		0.2			1.1
Gross Amounts Offset in the Balance Sheet	0.0		0.0			0.0
Net Amounts in the Balance Sheet	0.2		0.2			1.1
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			$ 1.1
Repurchase Agreements
Financial Liabilities
Gross Amounts Offset in the Balance Sheet	$ 0.0		$ 0.0