Fair Value Measurements - Schedule of Financial Assets and Financial Liabilities Measured at Fair Value on a Recurring and Non-Recurring Basis (Details) $ in Millions	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	$ 2,960.0	$ 2,270.7
Derivative assets	23.2	10.1
Derivative liabilities	22.2	9.4
Deferred compensation plan assets	18.2	12.1
Deferred compensation plan liabilities	18.2	12.1
Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned	1.2	0.6
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Deferred compensation plan liabilities	0.0	0.0
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale	100.9	33.3
Deferred compensation plan assets	18.2	12.1
Deferred compensation plan liabilities	18.2	12.1
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Deferred compensation plan liabilities	0.0	0.0
Loans And Leases Receivable, Other Real Estate Owned [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned	0.0	0.0
Loans And Leases Receivable, Other Real Estate Owned [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned	0.0	0.0
Loans And Leases Receivable, Other Real Estate Owned [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned	2.2	0.6
US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	9.0	2.6
US Treasury Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0.0	0.0
US Treasury Securities [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	9.0	2.6
US Treasury Securities [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0.0	0.0
State, county and municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	80.9
State, county and municipal securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0.0
State, county and municipal securities | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	80.9
State, county and municipal securities | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0.0
US Government Agencies Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	366.8	559.2
US Government Agencies Debt Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0.0	0.0
US Government Agencies Debt Securities [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	366.8	559.2
US Government Agencies Debt Securities [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0.0	0.0
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	2,317.2	1,544.8
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0.0	0.0
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	2,317.2	1,544.8
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0.0	0.0
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	47.2	70.2
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0.0	0.0
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	47.2	70.2
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0.0	0.0
Corporate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	135.7	91.9
Corporate securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0.0	0.0
Corporate securities | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	135.7	91.9
Corporate securities | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0.0	0.0
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	2,960.0	2,270.7
Derivative assets	23.2	10.1
Derivative liabilities	22.2	9.4
Deferred compensation plan assets	18.2	12.1
Deferred compensation plan liabilities	18.2	12.1
Estimated Fair Value | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale	100.9	33.3
Deferred compensation plan assets	18.2	12.1
Deferred compensation plan liabilities	18.2	12.1
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]
Estimated Fair Value	0.0	0.0
Derivative assets	0.0	0.0
Derivative liabilities	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Deferred compensation plan liabilities	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]
Estimated Fair Value	2,960.0	2,270.7
Derivative assets	23.2	10.1
Derivative liabilities	22.2	9.4
Deferred compensation plan assets	18.2	12.1
Deferred compensation plan liabilities	18.2	12.1
Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0.0	0.0
Derivative assets	0.0	0.0
Derivative liabilities	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Deferred compensation plan liabilities	0.0	0.0
Estimated Fair Value | Loans And Leases Receivable, Other Real Estate Owned [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned	2.2	0.6
Estimated Fair Value | US Treasury Securities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	9.0	2.6
Estimated Fair Value | State, county and municipal securities | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	80.9
Estimated Fair Value | US Government Agencies Debt Securities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	366.8	559.2
Estimated Fair Value | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	2,317.2	1,544.8
Estimated Fair Value | Mortgage-backed Securities, Issued by Private Enterprises [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	47.2	70.2
Estimated Fair Value | Corporate securities | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	135.7	91.9
Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	21.9	8.8
Derivative liabilities	21.9	8.8
Interest Rate Swap | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.0	0.0
Derivative liabilities	0.0	0.0
Interest Rate Swap | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	21.9	8.8
Derivative liabilities	21.9	8.8
Interest Rate Swap | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.0	0.0
Derivative liabilities	0.0	0.0
Interest Rate Swap | Estimated Fair Value | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	21.9	8.8
Derivative liabilities	21.9	8.8
Interest rate lock commitments | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.0	0.0
Interest rate lock commitments | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	1.3	1.3
Interest rate lock commitments | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.0	0.0
Interest rate lock commitments | Estimated Fair Value | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	1.3	1.3
Forward loan sales contracts | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	0.0	0.0
Forward loan sales contracts | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	0.3	0.6
Forward loan sales contracts | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	0.0	0.0
Forward loan sales contracts | Estimated Fair Value | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	$ 0.3	$ 0.6
Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] | Loans And Leases Receivable, Impaired Loans [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing Asset, Measurement Input	0.56
Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] | Loans And Leases Receivable, Other Real Estate Owned [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing Asset, Measurement Input	0.65
Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Weighted Average [Member] | Loans And Leases Receivable, Impaired Loans [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing Asset, Measurement Input	0.22
Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Weighted Average [Member] | Loans And Leases Receivable, Other Real Estate Owned [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing Asset, Measurement Input	0.27