Cover - USD ($)	12 Months Ended
	Dec. 31, 2021	Mar. 01, 2022	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
Document Transition Report	false
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
Title of 12(b) Security	Common Stock, par value $0.125 per share
Trading Symbol	THFF
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
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 Public Float			$ 491,919,028
Entity Common Stock, Shares Outstanding		12,470,926
Documents Incorporated by Reference	Portions of the Definitive Proxy Statement for the First Financial Corporation Annual Meeting of Shareholders to be held April 20, 2022 are incorporated by reference into Part III.
Entity Central Index Key	0000714562
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--12-31
Amendment Flag	false
ICFR Auditor Attestation Flag	true