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Investment Securities - Other narratives (Details) - USD ($) | 3 Months Ended
--- | ---
Mar. 31, 2024 | Mar. 31, 2023
Investment Securities |  |
Debt securities, available-for-sale | $ 18,685,000 | $ 0
Realized loss | $ 1,144,000 |

X
- Definition The amount of available for sale securities sold during the period. + References No definition available. + Details
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Namespace Prefix: | fdsb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 9 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-9 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration