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Investment Securities - Narrative (Details) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 USD ($) security | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) security | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) security
Schedule of Available-for-sale Securities and Held-to-maturity Securities [Line Items] |  |  |  |  |
Securities transferred from available-for-sale to held-to-maturity | $ |  |  | $ 27,422 | $ 128,965 | $ 129,000
Unrealized pre-tax loss from transfer of available-for-sale securities to held-to-maturity | $ |  |  | 278 |  | 3,000
Amortization of net unrealized losses on securities transferred from available-for-sale to held-to-maturity | $ | $ 128 | $ 135 | $ 396 | $ 299 | $ 299
Number of investment securities in unrealized loss position | security | 253 |  | 253 |  | 198
Number of investment securities in unrealized loss position longer than 12 months | security | 83 |  | 83 |  |
Number of investment securities in unrealized loss position less than 12 months | security | 170 |  | 170 |  |

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