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Investments - Additional Information (Details) $ in Thousands | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Mar. 31, 2020 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) security numberOfInvestments | Dec. 31, 2019 USD ($) security
Investment Holdings [Line Items] |  |  |  |
Vertical risk retention interest | 5.00% |  |  |
Trading investments |  |  | $ 14,651 | $ 0
Available-for-sale investments, cost |  |  | 2,081,236 | 485,756
Par value of mortgage-backed securities pledged to FRB |  |  | 748,000 | 252,000
Gain on equity securities investment |  | $ 8,000 |  |
Non-marketable securities investment |  |  | 26,000 | 26,000
Low income housing tax credit investments |  |  | 55,000 | 58,000
Low income housing tax credit investments, liability for unfunded commitments |  |  | 22,000 | 29,000
Low income housing tax credit investments, tax credits and benefits |  |  | $ 1,000 | $ 6,000
Low income housing tax credit investments, expected tax benefits recognized |  |  | 25.00% |
Minimum |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Number of securities in net loss position | numberOfInvestments |  |  | 8 |
Maximum |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Number of securities in net loss position | numberOfInvestments |  |  | 10 |
Mortgage-backed securities |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Number of securities in net loss position | security |  |  | 3 | 33
Number of mortgage-backed securities | security |  |  |  | 107
Utah Housing Corporation bonds |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Number of securities in net loss position | security |  |  | 0 |
Number of mortgage-backed securities | security |  |  | 3 |
U.S. government-sponsored enterprises and Treasuries |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Number of securities in net loss position | security |  |  | 2 |
Number of government sponsored securities | security |  |  | 32 |
Ginnie Mae | Mortgage-backed securities |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale investments, cost |  |  | $ 38,000 |
Number of securities in net loss position | security |  |  |  | 18
Number of mortgage-backed securities | security |  |  |  | 33
Fannie Mae | Mortgage-backed securities |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale investments, cost |  |  | 93,000 |
Freddie Mac | Mortgage-backed securities |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale investments, cost |  |  | $ 178,000 |
Student Loan |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Private education loans sold | $ 1,700,000 |  |  |

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