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Loans - Troubled Debt Restructuring Loans (Details) | 12 Months Ended
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Dec. 31, 2021 USD ($) borrower loan | Dec. 31, 2020 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2020 USD ($) loan | Dec. 31, 2020 USD ($) contract | Dec. 31, 2020 USD ($) borrower
Financing Receivable, Modifications [Line Items] |  |  |  |  |  |
Number of Relationships | 2 |  |  | 4 | 3 |
Pre-Modification Outstanding Recorded Investment | $ 96,000 |  | $ 731,000 |  |  |
Post-Modification Outstanding Recorded Investment | $ 96,000 |  | 731,000 |  |  |
Number of borrowers | borrower | 2 |  |  |  |  | 3
Net loans held-for-investment | $ 3,767,644,000 | $ 3,785,631,000 | 3,785,631,000 | $ 3,785,631,000 | $ 3,785,631,000 | $ 3,785,631,000
Troubled debt restructurings | $ 9,000,000.0 | $ 12,100,000 | 12,100,000 | $ 12,100,000 | $ 12,100,000 | $ 12,100,000
Number of loans modified as TDR that subsequently defaulted | loan | 0 |  |  |  |  |
Delinquent status classification term | 90 days | 90 days |  |  |  |
COVID-19 |  |  |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |  |  |
Number of Relationships | loan |  |  |  | 4 |  |
Number of borrowers | borrower | 2 |  |  |  |  |
Loan modification program, payment deferral period | 90 days |  |  |  |  |
Loan modification program, extension for payment deferral period | 90 days |  |  |  |  |
Net loans held-for-investment | $ 774,000 |  |  |  |  |
Residential |  |  |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |  |  |
Number of Relationships | loan | 2 |  |  |  |  |
Pre-Modification Outstanding Recorded Investment | $ 96,000 |  |  |  |  |
Post-Modification Outstanding Recorded Investment | $ 96,000 |  |  |  |  |
One-to-four family residential mortgage |  |  |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |  |  |
Number of Relationships | contract |  |  |  |  | 1 |
Pre-Modification Outstanding Recorded Investment |  |  | 187,000 |  |  |
Post-Modification Outstanding Recorded Investment |  |  | 187,000 |  |  |
Commercial Mortgage Backed Securities |  |  |  |  |  |
Financing Receivable, Modifications [Line Items] |  |  |  |  |  |
Number of Relationships | 4 |  |  |  | 2 |
Pre-Modification Outstanding Recorded Investment |  |  | 544,000 |  |  |
Post-Modification Outstanding Recorded Investment |  |  | 544,000 |  |  |
Number of loans modified as TDR that subsequently defaulted | loan |  |  |  | 2 |  |
Defaulted troubled debt restructuring loans |  |  | $ 462,500 |  |  |
Delinquent status classification term |  | 90 days |  |  |  |

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