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Loans and Allowance for Credit Losses - Troubled Debt Restructuring and CARES Act Narrative (Details) | 9 Months Ended | 10 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2020 USD ($) | Dec. 31, 2020 USD ($) borrowingRelationship loan loanProductionOffice | Dec. 31, 2020 USD ($) loan borrowingRelationship | Dec. 31, 2019 USD ($) loan | Dec. 31, 2018 USD ($) loan | Oct. 01, 2020 USD ($)
Financing Receivable, Troubled Debt Restructuring [Line Items] |  |  |  |  |  |
Number of loans removed from TDR designation | loan |  |  | 0 |  |  |
Loan modifications under CARES Act during period | loan |  | 269 |  |  |  |
Loans modified under CARES Act during period, value |  | $ 402,900,000 |  |  |  |
Loan modifications under CARES Act during period, subsequently resumed payments | loanProductionOffice |  | 222 |  |  |  |
Loans modified under CARES Act during period, subsequently resumed payments, value |  | $ 324,200,000 |  |  |  |
Loan modifications under CARES Act during period, subsequently fully paid | loanProductionOffice |  | 18 |  |  |  |
Loan modifications under CARES Act during period, subsequently fully paid, value |  | $ 7,700,000 |  |  |  |
Loan modifications under CARES Act, additional relief, number of borrowing relationships | borrowingRelationship |  | 21 | 21 |  |  |
Loan modifications under CARES Act, additional relief | loan |  | 29 | 29 |  |  |
Loan modifications under CARES Act, additional relief, value |  | $ 71,000,000.0 | $ 71,000,000.0 |  |  |
Total of TDR loans charged-off within previous 12 months, value |  |  | 0 | $ 0 | $ 0 |
Provision for credit losses on loans | $ 5,450,000 |  | 4,594,000 | 900,000 | $ 0 |
Allowance for Credit Losses | 22,113,000 | 22,874,000 | 22,874,000 | $ 16,677,000 |  |
Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |
Financing Receivable, Troubled Debt Restructuring [Line Items] |  |  |  |  |  |
Allowance for Credit Losses | 1,604,000 |  |  |  |  |
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-13 |  |  |  |  |  |
Financing Receivable, Troubled Debt Restructuring [Line Items] |  |  |  |  |  |
Allowance for Credit Losses |  |  |  |  |  | $ 1,604,000
Commercial |  |  |  |  |  |
Financing Receivable, Troubled Debt Restructuring [Line Items] |  |  |  |  |  |
Number of loans removed from TDR designation | loan |  |  |  | 1 |  |
Recorded investment of loans removed from TDR resignation |  |  |  | $ 3,000 |  |
Residential loans | TIC Loan |  |  |  |  |  |
Financing Receivable, Troubled Debt Restructuring [Line Items] |  |  |  |  |  |
Number of loans removed from TDR designation | loan |  |  |  |  | 1 |
Residential loans | Home equity |  |  |  |  |  |
Financing Receivable, Troubled Debt Restructuring [Line Items] |  |  |  |  |  |
Number of loans removed from TDR designation | loan |  |  |  |  | 1 |
Provision for credit losses on loans | 188,000 |  |  | $ (65,000) | $ (116,000) |
Allowance for Credit Losses | 1,038,000 | $ 738,000 | $ 738,000 |  |  |
Residential loans | Home equity | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |
Financing Receivable, Troubled Debt Restructuring [Line Items] |  |  |  |  |  |
Allowance for Credit Losses | $ (361,000) |  |  |  |  |
Residential loans | Home equity | Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-13 |  |  |  |  |  |
Financing Receivable, Troubled Debt Restructuring [Line Items] |  |  |  |  |  |
Allowance for Credit Losses |  |  |  |  |  | $ (361,000)
Residential loans | TIC and Home Equity |  |  |  |  |  |
Financing Receivable, Troubled Debt Restructuring [Line Items] |  |  |  |  |  |
Recorded investment of loans removed from TDR resignation |  |  |  |  | $ 247,000 |
Consumer loans |  |  |  |  |  |
Financing Receivable, Troubled Debt Restructuring [Line Items] |  |  |  |  |  |
TDR loans charged-off within previous 12 months | loan |  |  | 1 |  |  |
Total of TDR loans charged-off within previous 12 months, value |  |  | $ 7,000 |  |  |

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