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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, 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 |

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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313 + Details
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