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RESTRICTIONS ON CASH AND DUE FROM BANKS: (Details Textual) - USD ($) | Dec. 31, 2021 | Dec. 31, 2020
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Restrictions On Cash and Due From Banks Disclosure [Abstract] |  |
Cash Reserve Balance | $ 0 | $ 0

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- Definition -- None. No documentation exists for this element. -- + References No definition available. + Details
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Namespace Prefix: | thff_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Federal Reserve Bank stock represents an equity interest in the Federal Reserve Bank. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 325 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=123599499&loc=d3e63345-112809 + Details
Name: | us-gaap_FederalReserveBankStock
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant