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Derivatives and Hedging Activities - Schedule of Derivative Instruments, Effect on Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Aug. 02, 2021 | Jun. 30, 2021 | Jun. 17, 2021 | Dec. 31, 2020
Derivative [Line Items] |  |  |  |  |  |  |  |
Amount of net fee income recognized in other non-interest income |  |  |  |  |  |  |  |
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net | 0.1 |  | $ 0.1 |  |  |  |  |
Derivative Instruments and Hedges, Assets | 697.7 |  | 697.7 |  |  |  |  | $ 0.0
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease) | (2.3) |  | (2.3) |  |  |  |  | 0.0
Financial Instruments | 0.0 |  | 0.0 |  |  |  |  | 0.0
Derivative Asset, Notional Amount | 1,368.8 |  | 1,368.8 |  |  |  |  | 901.6
Derivative Asset, Estimated Fair Value | 34.1 |  | 34.1 |  |  |  |  | 55.3
Collateral Pledged |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Derivative, Fair Value, Net | 10.7 |  | 10.7 |  |  |  |  |
Financial Instruments | 0.3 |  | 0.3 |  |  |  |  |
Interest Rate Swap [Member] |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Derivative, Fixed Interest Rate |  |  |  |  | 1.22% |  | 1.19% |
Derivative, Notional Amount |  |  |  |  | $ 200.0 |  | $ 500.0 |
Not Designated as Hedging Instrument |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Amount of net fee income recognized in other non-interest income | 0.4 | $ 3.7 | 1.6 | $ 6.4 |  |  |  |
Amount of net gains recognized in mortgage banking revenues | 1.3 | (0.6) | 1.7 | 4.4 |  |  |  |
Interest Rate Swap [Member] |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Financial Instruments | 0.0 |  | 0.0 |  |  |  |  | 0.0
Derivative Asset, Notional Amount | 587.6 |  | 587.6 |  |  |  |  | 87.6
Derivative Asset, Estimated Fair Value | 0.4 |  | 0.4 |  |  |  |  | 0.2
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Derivative Asset, Notional Amount | 200.0 |  | 200.0 |  |  |  |  | 0.0
Derivative Asset, Estimated Fair Value | 2.6 |  | 2.6 |  |  |  |  | 0.0
Interest Rate Swap [Member] | Not Designated as Hedging Instrument |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Derivative Asset, Notional Amount | 854.8 |  | 854.8 |  |  |  |  | 799.7
Derivative Asset, Estimated Fair Value | 27.5 |  | 27.5 |  |  |  |  | 52.0
Forward Contracts | Not Designated as Hedging Instrument |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Derivative, Fair Value, Net |  |  |  |  |  | $ 0.9 |  | 0.0
Derivative Asset, Notional Amount |  |  |  |  |  | $ 164.2 |  | $ 0.0
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification, Tax | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 |  |  |  |

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- Definition Nominal or face amount used to calculate payments on the derivative asset. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5580258-113959 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959 + Details
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