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Investments - Additional Information (Details) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2019 USD ($) | Jun. 30, 2018 USD ($) security | Dec. 31, 2019 USD ($) security | Dec. 31, 2018 USD ($) security | Dec. 31, 2017 USD ($)
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost |  |  | $ 485,756 | $ 182,325 |
Par value of mortgage-backed securities pledged to FRB |  |  | 252,000 | 147,000 |
Gain on equity securities investment | $ 8,000 |  |  |  |
Non-marketable securities investment |  |  | 26,000 | 8,000 |
Low income housing tax credit investments |  |  | 6,000 | 4,000 | $ 1,000
Amount of amortization reported in income tax expense |  |  | 4,000 | 4,000 | $ 1,000
Total carrying value of LIHTC investments |  |  | 58,000 | 48,000 |
Liability for unfunded commitments |  |  | 29,000 | 37,000 |
Mortgage-backed securities |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost |  |  | $ 215,888 | $ 159,937 |
Number of mortgage-backed securities sold | security |  | 9 |  |  |
Proceeds from sale of securities |  | $ 41,000 |  |  |
Recognized loss from sale of securities |  | $ 2,000 |  |  |
Number of mortgage-backed securities with unrealized losses | security |  |  | 33 | 74 |
Number of mortgage-backed securities | security |  |  | 107 | 86 |
Utah Housing Corporation bonds |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost |  |  | $ 19,474 | $ 22,388 |
Utah Housing Corporation bonds | Aa3 Rating |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost |  |  | 19,000 | $ 22,000 |
U.S. government-sponsored enterprises |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost |  |  | $ 250,394 |  |
Number of government sponsored securities | security |  |  | 14 |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions | security |  |  | 1 |  |
Government National Mortgage Association Certificates and Obligations (GNMA) | Mortgage-backed securities |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost |  |  | $ 54,000 |  |
Number of mortgage-backed securities with unrealized losses | security |  |  | 18 | 34 |
Federal National Mortgage Association Certificates and Obligations (FNMA) | Mortgage-backed securities |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost |  |  | $ 106,000 |  |
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) | Mortgage-backed securities |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost |  |  | $ 56,000 |  |

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