Loans and allowance for loan losses - Additional Information (Details)
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3 Months Ended |
12 Months Ended |
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Mar. 31, 2021
USD ($)
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Mar. 31, 2020
USD ($)
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Mar. 31, 2021
USD ($)
loan
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Mar. 31, 2020
USD ($)
loan
|
Dec. 31, 2020
USD ($)
|
Dec. 31, 2019
USD ($)
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| Accounts, Notes, Loans and Financing Receivable [Line Items] |
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| Loans Pledged as Collateral |
$ 44,419,000
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$ 44,419,000
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$ 65,587,000
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| Defaults on TDRs that were modified as TDRs | loan |
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0
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0
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| Amount of loans under modified terms |
19,000,000
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$ 19,000,000
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38,000,000
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| Provision for loan losses |
0
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$ 400,000
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| Loans and Leases Receivable, Allowance |
$ 3,992,000
|
3,444,000
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$ 3,992,000
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$ 3,444,000
|
3,970,000
|
$ 3,186,000
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| Guaranteed Student loans [Member] |
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| Accounts, Notes, Loans and Financing Receivable [Line Items] |
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| Percentage Of Recovery Loss On Portfolio From Historical Experience |
98.00%
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98.00%
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| Unallocated Financing Receivable [Member] |
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| Accounts, Notes, Loans and Financing Receivable [Line Items] |
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| Loans and Leases Receivable, Allowance |
$ 748,000
|
$ 142,000
|
$ 748,000
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$ 142,000
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$ 154,000
|
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