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Goodwill and Core Deposit Intangibles - Schedule of Core Deposit Intangibles Activity (Details) - USD ($) $ in Millions |  |  | 12 Months Ended
--- | --- | --- | ---
May 30, 2017 | Aug. 12, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Finite-lived Intangible Assets [Roll Forward] |  |  |  |  |
Accumulated amortization |  |  | $ 7.9 | $ 5.5 | $ 3.4
Core Deposits Intangibles |  |  |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |  |  |
Finite-Lived Intangible Assets, Gross |  |  |  | 74.0 | $ 29.2
Finite-lived Intangible Assets [Roll Forward] |  |  |  |  |
Gross CDI, beginning of period |  |  | 49.1 |  |
Established through acquisitions | $ 15.7 | $ 48.0 |  |  |
Reductions due to sale of accounts |  |  | 0.0 | (3.2) |
Accumulated amortization |  |  | (32.8) | (24.9) |
Net CDI, end of period |  |  | $ 56.9 | $ 49.1 |

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- Definition The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274 + Details
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