Document And Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Nov. 01, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2021
Entity File Number	0-16759
Entity Registrant Name	FIRST FINANCIAL CORPORATION
Entity Incorporation, State or Country Code	IN
Entity Tax Identification Number	35-1546989
Entity Address, Address Line One	One First Financial Plaza
Entity Address, City or Town	Terre Haute
Entity Address, State or Province	IN
Entity Address, Postal Zip Code	47807
City Area Code	(812)
Local Phone Number	238-6000
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		12,871,936
Entity Central Index Key	0000714562
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Amendment Flag	false
Trading Symbol	THFF
Document Transition Report	false
Title of 12(b) Security	Common Stock, par value $0.125 per share
Security Exchange Name	NASDAQ