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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, 2020 USD ($) | Dec. 31, 2019 USD ($)
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Estimated Fair Value | $ 4,008.7 | $ 2,960.0
Derivative assets | 55.3 | 23.2
Derivative liabilities | 53.3 | 22.2
Deferred compensation plan assets | 19.1 | 18.2
Deferred compensation plan liabilities | 19.1 | 18.2
Fair Value, Nonrecurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other real estate owned |  | 1.2
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 | 74.0 | 100.9
Deferred compensation plan assets | 19.1 | 18.2
Deferred compensation plan liabilities | 19.1 | 18.2
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
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
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
US Treasury Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Estimated Fair Value |  | 9.0
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
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
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
State, county and municipal securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Estimated Fair Value | 465.9 | 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 | 465.9 |
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 | 331.9 | 366.8
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 | 331.9 | 366.8
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 | 2,897.6 | 2,317.2
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 | 2,897.6 | 2,317.2
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 | 10.9 | 47.2
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 | 10.9 | 47.2
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 | 302.2 | 135.7
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 | 302.2 | 135.7
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 | 4,008.7 | 2,960.0
Derivative assets | 55.3 | 23.2
Derivative liabilities | 53.3 | 22.2
Deferred compensation plan assets | 19.1 | 18.2
Deferred compensation plan liabilities | 19.1 | 18.2
Estimated Fair Value | Fair Value, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Loans held for sale | 74.0 | 100.9
Deferred compensation plan assets | 19.1 | 18.2
Deferred compensation plan liabilities | 19.1 | 18.2
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 | 4,008.7 | 2,960.0
Derivative assets | 55.3 | 23.2
Derivative liabilities | 53.3 | 22.2
Deferred compensation plan assets | 19.1 | 18.2
Deferred compensation plan liabilities | 19.1 | 18.2
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
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
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 | 465.9 |
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 | 331.9 | 366.8
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 | 2,897.6 | 2,317.2
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 | 10.9 | 47.2
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 | 302.2 | 135.7
Interest Rate Swap |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 52.0 | 21.9
Derivative liabilities | 52.2 | 21.9
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 | 52.0 | 21.9
Derivative liabilities | 52.2 | 21.9
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 | 52.0 | 21.9
Derivative liabilities | 52.2 | 21.9
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 | 3.3 | 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 | 3.3 | 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 | 1.1 | 0.3
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 | $ 1.1 | $ 0.3
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 | 0.18 |
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 | 0.09 |

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