Document and Entity Information - USD ($)	6 Months Ended
	Jun. 30, 2018	Aug. 06, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	FIRST MID ILLINOIS BANCSHARES INC
Entity Central Index Key	0000700565
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float	$ 513,588,874
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q2
Document Type	10-Q
Amendment Flag	false
Entity Common Stock, Shares Outstanding		15,290,146
Document Period End Date	Jun. 30, 2018