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Goodwill and Core Deposit Intangibles - Schedule of Finite-Lived Intangible Assets (Details) - Core Deposits - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended |
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Jun. 30, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items] |  |  |
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 | (38.1) | (32.8) |
Net CDI, end of period | $ 67.9 | $ 56.9 |

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