Document and Entity Information - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Mar. 01, 2018	Jun. 30, 2017
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
Document Type		10-K
Document Period End Date		Dec. 31, 2017
Document Fiscal Year Focus		2017
Document Fiscal Period Focus		FY
Amendment Flag		false
Entity Common Stock, Shares Outstanding (shares)			12,255,045
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float				$ 515,437,610