Fair Value (Details 3) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Loans identified as impaired by class of loans
Carrying Value	$ 1,691	$ 2,376
Allowance for Loan Losses Allocated	658	737
Fair value	8,308	10,516
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,033	1,639
Commercial & Industrial
Loans identified as impaired by class of loans
Carrying Value	1,691	1,819
Allowance for Loan Losses Allocated	658	593
Fair value	1,033	1,226
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,691	1,819
Farmland
Loans identified as impaired by class of loans
Carrying Value	0	211
Allowance for Loan Losses Allocated	0	44
Fair value	0	167
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0	211
Non Farm, Non Residential
Loans identified as impaired by class of loans
Carrying Value	0	0
Allowance for Loan Losses Allocated	0	0
Fair value	0	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0	0
Agriculture
Loans identified as impaired by class of loans
Carrying Value	0	346
Allowance for Loan Losses Allocated	0	100
Fair value	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0	346
All Other Commercial
Loans identified as impaired by class of loans
Carrying Value	0	0
Allowance for Loan Losses Allocated	0	0
Fair value	0	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0	0
First Liens
Loans identified as impaired by class of loans
Carrying Value	0	0
Allowance for Loan Losses Allocated	0	0
Fair value	0	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0	0
Home Equity
Loans identified as impaired by class of loans
Carrying Value	0	0
Allowance for Loan Losses Allocated	0	0
Fair value	0	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0	0
Junior Liens
Loans identified as impaired by class of loans
Carrying Value	0	0
Allowance for Loan Losses Allocated	0	0
Fair value	0	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0	0
Multifamily
Loans identified as impaired by class of loans
Carrying Value	0	0
Allowance for Loan Losses Allocated	0	0
Fair value	0	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0	0
All Other Residential
Loans identified as impaired by class of loans
Carrying Value	0	0
Allowance for Loan Losses Allocated	0	0
Fair value	0	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0	0
Motor Vehicle
Loans identified as impaired by class of loans
Carrying Value	0	0
Allowance for Loan Losses Allocated	0	0
Fair value	0	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0	0
All Other Consumer
Loans identified as impaired by class of loans
Carrying Value	0	0
Allowance for Loan Losses Allocated	0	0
Fair value	0	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	$ 0	$ 0

Impaired Financing Receivable, Unpaid Principal Balance, Fair Value Disclosure

No definition available.

Name:	thff_ImpairedFinancingReceivableUnpaidPrincipalBalanceFairValueDisclosure
Namespace Prefix:	thff_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableImpairedLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953550-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5212-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRelatedAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=thff_CommercialAndIndustrialReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=thff_FarmlandMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=thff_NonFarmNonResidentialReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=thff_AgricultureReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=thff_OtherCommercialReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_FirstMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_SecondMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=srt_MultifamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=thff_OtherResidentialReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_AutomobileLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=thff_OtherConsumerReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: