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Goodwill and Core Deposit Intangibles - Schedule of Core Deposit Intangibles Activity (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Finite-lived Intangible Assets [Roll Forward] |  |  |
Accumulated amortization | $ 10.9 | $ 11.2 | $ 7.9
Finite-Lived Intangible Asset, Useful Life | 10 years |  |
Core Deposits Intangibles |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |
Finite-Lived Intangible Assets, Gross |  | 106.0 | $ 89.7
Finite-lived Intangible Assets [Roll Forward] |  |  |
Gross CDI, beginning of period | $ 62.1 |  |
Established through acquisitions | 0.0 | 16.6 |
Reductions due to sale of accounts | 0.0 | (0.3) |
Finite-Lived Intangible Assets, Accumulated Amortization | 54.8 | 43.9 |
Net CDI, end of period | $ 51.2 | $ 62.1 |

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