Securities - Securities Gross (Details) $ in Thousands	Mar. 31, 2026 USD ($) segment	Dec. 31, 2025 USD ($)
Less Than 12 Months
Fair Value	$ 252,146	$ 47,368
Unrealized Losses	(3,178)	(189)
More Than 12 Months
Fair Value	781,909	852,798
Unrealized Losses	(119,988)	(113,315)
Total
Fair Value	1,034,055	900,166
Unrealized Losses	$ (123,166)	(113,504)
Number of investment securities segregation for impairment evaluation \| segment	2
U.S. Government agencies
Less Than 12 Months
Fair Value	$ 21,026	7,137
Unrealized Losses	(150)	(31)
More Than 12 Months
Fair Value	55,768	59,562
Unrealized Losses	(7,688)	(7,718)
Total
Fair Value	76,794	66,699
Unrealized Losses	(7,838)	(7,749)
Mortgage Backed Securities - residential
Less Than 12 Months
Fair Value	74,202	11,961
Unrealized Losses	(913)	(29)
More Than 12 Months
Fair Value	414,240	427,877
Unrealized Losses	(59,577)	(58,427)
Total
Fair Value	488,442	439,838
Unrealized Losses	(60,490)	(58,456)
Mortgage Backed Securities - commercial
More Than 12 Months
Fair Value	10,971	11,114
Unrealized Losses	(429)	(328)
Total
Fair Value	10,971	11,114
Unrealized Losses	(429)	(328)
Collateralized Mortgage Obligations
Less Than 12 Months
Fair Value	30,378	4,381
Unrealized Losses	(431)	(43)
More Than 12 Months
Fair Value	123,671	135,393
Unrealized Losses	(22,707)	(21,579)
Total
Fair Value	154,049	139,774
Unrealized Losses	(23,138)	(21,622)
State and municipal obligations
Less Than 12 Months
Fair Value	125,649	23,889
Unrealized Losses	(1,680)	(86)
More Than 12 Months
Fair Value	163,503	204,976
Unrealized Losses	(27,577)	(23,370)
Total
Fair Value	289,152	228,865
Unrealized Losses	(29,257)	(23,456)
Municipal taxable
Less Than 12 Months
Fair Value	891
Unrealized Losses	(4)
More Than 12 Months
Fair Value	13,756	13,876
Unrealized Losses	(2,010)	(1,893)
Total
Fair Value	14,647	13,876
Unrealized Losses	$ (2,014)	$ (1,893)

-- None. No documentation exists for this element. --

No definition available.

Name:	thff_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAbstract
Namespace Prefix:	thff_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

-- None. No documentation exists for this element. --

+ References

No definition available.

+ Details

Name:	thff_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAbstract
Namespace Prefix:	thff_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

-- None. No documentation exists for this element. --

+ References

No definition available.

+ Details

Name:	thff_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTotalAbstract
Namespace Prefix:	thff_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of segment investment securities are segregated for impairment evaluation related to credit losses.

+ References

No definition available.

+ Details

Name:	thff_InvestmentSecuritiesNumberOfSegmentForImpairmentEvaluation
Namespace Prefix:	thff_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479081/326-30-55-8

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-7

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-5

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-5

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479081/326-30-55-8

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-6

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-4

+ Details

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

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-4

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_TaxableMunicipalBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: