Cover Page - shares	9 Months Ended
	Sep. 30, 2020	Nov. 02, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2020
Document Transition Report	false
Entity File Number	0-17071
Entity Registrant Name	FIRST MERCHANTS CORP
Entity Incorporation, State or Country Code	IN
Entity Tax Identification Number	35-1544218
Entity Address, Address Line One	200 East Jackson Street
Entity Address, City or Town	Muncie
Entity Address, State or Province	IN
Entity Address, Postal Zip Code	47305-2814
City Area Code	765
Local Phone Number	747-1500
Title of 12(b) Security	Common Stock, $0.125 stated value per share
Trading Symbol	FRME
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		54,257,996
Entity Central Index Key	0000712534
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Amendment Flag	false