Document and Entity Information - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Mar. 02, 2020	Jun. 30, 2019
Document And Entity Information [Abstract]
Entity Registrant Name		FIRST FINANCIAL CORP /IN/
Entity Central Index Key		0000714562
Current Fiscal Year End Date		--12-31
Entity Filer Category		Accelerated Filer
Entity Small Business		false
Entity Emerging Growth Company		false
Document Type		10-K
Entity Interactive Data Current		Yes
Document Period End Date		Dec. 31, 2019
Document Fiscal Year Focus		2019
Document Fiscal Period Focus		FY
Amendment Flag		false
Entity Common Stock, Shares Outstanding (shares)			13,753,790
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity Public Float				$ 457,533,380