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Goodwill and Core Deposit Intangibles - Schedule of Finite-Lived Intangible Assets (Details) - Core Deposits - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2019 | Dec. 31, 2018 | Apr. 08, 2019 | Dec. 31, 2017
Finite-lived Intangible Assets [Roll Forward] |  |  |  |
Gross CDI, at beginning of the period |  | $ 89.7 |  | $ 74.0
Established through acquisitions or provisional adjustments | $ 16.6 | 15.7 |  |
Reductions due to sale of accounts | (0.3) | 0.0 |  |
Accumulated amortization | (41.0) | (32.8) |  |
Net CDI, end of period | $ 65.0 | $ 56.9 | $ 3.0 |

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- Definition Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 + Details
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