Fair Value Measurements - Schedule of Financial Assets and Financial Liabilities Measured at Fair Value on a Recurring and Non-Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Investment debt securities available-for-sale	4,403.6		4,403.6		$ 4,008.7
Derivative assets	34.1		34.1		55.3
Derivative liabilities	21.1		21.1		17.9
Write downs	0.0	$ (0.1)	0.0	$ (0.1)
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale	0.0		0.0		0.0
Deferred compensation plan assets	0.0		0.0		0.0
Deferred compensation plan liabilities	0.0		0.0		0.0
Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale	42.5		42.5		74.0
Deferred compensation plan assets	21.6		21.6		19.1
Deferred compensation plan liabilities	21.6		21.6		19.1
Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale	0.0		0.0		0.0
Deferred compensation plan assets	0.0		0.0		0.0
Deferred compensation plan liabilities	0.0		0.0		0.0
Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Related Allowance	0.7		0.7		2.8
U.S. Treasury notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	691.8		691.8
U.S. Treasury notes | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	0.0		0.0
U.S. Treasury notes | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	691.8		691.8
U.S. Treasury notes | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	0.0		0.0
State, county and municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	439.9		439.9		465.9
State, county and municipal securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	0.0		0.0		0.0
State, county and municipal securities | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	439.9		439.9		465.9
State, county and municipal securities | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	0.0		0.0		0.0
Obligations of U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	325.9		325.9		331.9
Obligations of U.S. government agencies | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	0.0		0.0		0.0
Obligations of U.S. government agencies | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	325.9		325.9		331.9
Obligations of U.S. government agencies | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	0.0		0.0		0.0
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	2,031.3		2,031.3		2,897.6
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	0.0		0.0		0.0
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	2,031.3		2,031.3		2,897.6
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	0.0		0.0		0.0
Private mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	83.5		83.5		10.9
Private mortgage-backed securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	0.0		0.0		0.0
Private mortgage-backed securities | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	83.5		83.5		10.9
Private mortgage-backed securities | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	0.0		0.0		0.0
Corporate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	378.2		378.2		302.2
Corporate securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	0.0		0.0		0.0
Corporate securities | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	378.2		378.2		302.2
Corporate securities | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	0.0		0.0		0.0
Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale					0.2
Other investments | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale					0.0
Other investments | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale					0.2
Other investments | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale					0.0
Collateral dependent loans | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans	0.0		0.0		0.0
Collateral dependent loans | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans	0.0		0.0		0.0
Collateral dependent loans | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans					14.7
Collateralized Loan Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	453.0		453.0
Collateralized Loan Obligations | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	0.0		0.0
Collateralized Loan Obligations | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	453.0		453.0
Collateralized Loan Obligations | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	0.0		0.0
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	4,403.6		4,403.6		4,008.7
Derivative assets	34.1		34.1		55.3
Derivative liabilities	21.1		21.1		17.9
Deferred compensation plan assets	21.6		21.6		19.1
Deferred compensation plan liabilities	21.6		21.6		19.1
Estimated Fair Value | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale	42.5		42.5		74.0
Deferred compensation plan assets	21.6		21.6		19.1
Deferred compensation plan liabilities	21.6		21.6		19.1
Estimated Fair Value | Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets to be disposed of by sale					5.3
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	0.0		0.0		0.0
Derivative assets	0.0		0.0		0.0
Derivative liabilities	0.0		0.0		0.0
Deferred compensation plan assets	0.0		0.0		0.0
Deferred compensation plan liabilities	0.0		0.0		0.0
Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	4,403.6		4,403.6		4,008.7
Derivative assets	34.1		34.1		55.3
Derivative liabilities	21.1		21.1		17.9
Deferred compensation plan assets	21.6		21.6		19.1
Deferred compensation plan liabilities	21.6		21.6		19.1
Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	0.0		0.0		0.0
Derivative assets	0.0		0.0		0.0
Derivative liabilities	0.0		0.0		0.0
Deferred compensation plan assets	0.0		0.0		0.0
Deferred compensation plan liabilities	0.0		0.0		0.0
Estimated Fair Value | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans	12.5		12.5		14.7
Estimated Fair Value | U.S. Treasury notes | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	691.8		691.8
Estimated Fair Value | State, county and municipal securities | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	439.9		439.9		465.9
Estimated Fair Value | Obligations of U.S. government agencies | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	325.9		325.9		331.9
Estimated Fair Value | U.S. agency residential mortgage-backed securities & collateralized mortgage obligations | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	2,031.3		2,031.3		2,897.6
Estimated Fair Value | Private mortgage-backed securities | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	83.5		83.5		10.9
Estimated Fair Value | Corporate securities | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	378.2		378.2		302.2
Estimated Fair Value | Other investments | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale					0.2
Estimated Fair Value | Collateral dependent loans | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans	12.5		12.5		14.7
Estimated Fair Value | Collateralized Loan Obligations | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	453.0		453.0
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment debt securities available-for-sale	4,403.6		4,403.6		4,008.7
Derivative assets	34.1		34.1		55.3
Derivative liabilities	21.1		21.1		17.9
Deferred compensation plan assets	21.6		21.6		19.1
Deferred compensation plan liabilities	21.6		21.6		19.1
Carrying Amount | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans	13.2		13.2		17.5
Long-lived assets to be disposed of by sale	3.2		3.2		5.5
Change During Period | Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets to be disposed of by sale					0.2
Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	30.1		30.1		52.0
Derivative liabilities	21.1		21.1		16.8
Interest Rate Swap [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.0		0.0		0.0
Derivative liabilities	0.0		0.0		0.0
Interest Rate Swap [Member] | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	30.1		30.1		52.0
Derivative liabilities	21.1		21.1		16.8
Interest Rate Swap [Member] | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.0		0.0		0.0
Derivative liabilities	0.0		0.0		0.0
Interest Rate Swap [Member] | Estimated Fair Value | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	30.1		30.1		52.0
Derivative liabilities	21.1		21.1		16.8
Interest Rate Lock Commitments | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.0		0.0		0.0
Interest Rate Lock Commitments | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	3.1		3.1		3.3
Interest Rate Lock Commitments | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.0		0.0		0.0
Interest Rate Lock Commitments | Estimated Fair Value | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	3.1		3.1		3.3
Forward Contracts | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.0		0.0
Derivative liabilities					0.0
Forward Contracts | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.9		0.9
Derivative liabilities					1.1
Forward Contracts | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.0		0.0
Derivative liabilities					0.0
Forward Contracts | Estimated Fair Value | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.9		0.9
Derivative liabilities					1.1
Long-lived assets to be disposed of by sale | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets to be disposed of by sale	0.0		0.0		0.0
Long-lived assets to be disposed of by sale | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets to be disposed of by sale	0.0		0.0		0.0
Long-lived assets to be disposed of by sale | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets to be disposed of by sale	3.2		3.2		5.3
Long-lived assets to be disposed of by sale | Estimated Fair Value | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets to be disposed of by sale	$ 3.2		$ 3.2		$ 5.3