Document And Entity Information - shares	6 Months Ended
	Jun. 30, 2018	Aug. 06, 2018
Document Information [Line Items]
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-Q
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Common Stock, Shares Outstanding		12,255,045
Trading Symbol	THFF