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Investments - Additional Information (Details) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2019 USD ($) security | Jun. 30, 2018 USD ($) security | Sep. 30, 2019 USD ($) security | Dec. 31, 2018 USD ($) security
Investment Holdings [Line Items] |  |  |  |
Available-for-sale investments, cost | $ 454,603 |  | $ 454,603 | $ 182,325
Par value of mortgage-backed securities pledged to FRB | 214,000 |  | 214,000 | 147,000
Gain on equity securities investment | 8,000 |  |  |
Non-marketable securities investment | 26,000 |  | 26,000 | 8,000
Low income housing tax credit investments | 59,000 |  | 59,000 | 48,000
Low income housing tax credit investments, liability for unfunded commitments | 33,000 |  | 33,000 | 37,000
Low income housing tax credit investments, tax credits and benefits |  |  | $ 800 | 900
Low income housing tax credit investments, expected tax benefits recognized |  |  | 25.00% |
Mortgage-backed securities |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale investments, cost | $ 219,531 |  | $ 219,531 | $ 159,937
Number of securities in net loss position | security | 35 |  | 35 | 74
Number of mortgage-backed securities | security | 103 |  | 103 | 86
Number of mortgage backed securities sold | security |  | 9 |  |
Proceeds from sale of securities |  | $ 41,000 |  |
Recognized loss from sale of securities |  | $ 2,000 |  |
Utah Housing Corporation bonds |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale investments, cost | $ 19,474 |  | $ 19,474 | $ 22,388
Number of securities in net loss position | security | 1 |  | 1 |
Number of mortgage-backed securities | security | 3 |  | 3 |
U.S. government-sponsored enterprises |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale investments, cost | $ 215,598 |  | $ 215,598 |
Number of securities in net loss position | security | 2 |  | 2 |
Number of government sponsored securities | security | 11 |  | 11 |
Ginnie Mae | Mortgage-backed securities |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale investments, cost | $ 56,000 |  | $ 56,000 |
Number of securities in net loss position | security | 16 |  | 16 | 34
Number of mortgage-backed securities | security | 35 |  | 35 | 74
Fannie Mae | Mortgage-backed securities |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale investments, cost | $ 103,000 |  | $ 103,000 |
Freddie Mac | Mortgage-backed securities |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale investments, cost | $ 60,000 |  | $ 60,000 |

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