Repurchase Agreements and Other Borrowings (Details) - USD ($)	3 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Repurchase Agreements and Other Borrowings [Abstract]
Securities Sold under Agreements to Repurchase	$ 231,649,000	$ 208,109,000
Securities Sold under Agreements to Repurchase Increase (Decrease)	23,500,000
Disclosure of Repurchase Agreements [Abstract]
Securities pledged to Repurchase Agreements	$ 231,649,000	208,109,000
Securities Sold Under Agreements to Repurchase Weighted Average Rate	0.12%
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term		114,000,000
FHLB Advance 1.5-Year Original Maturity [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 10,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P1Y6M
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	2.95%
FHLB Advance, 2-Year Original Maturity [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 5,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P2Y
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	2.75%
FHLB advance, 3-Year Original Maturity [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 5,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P3Y
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	1.75%
FHLB Advance 3.5 Year Original Maturity [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 5,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P3Y6M
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	1.83%
FHLB Advance 3.5-Year Original Maturity 2 [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 5,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P3Y6M
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	1.51%
FHLB Advance 3.5-Year Original Maturity 3 [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 5,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P3Y6M
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	0.77%
FHLB Advance 4-Year Original Maturity [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 5,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P4Y
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	1.79%
FHLB Advance 5-Year Original Maturity [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 5,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P5Y
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	1.85%
FHLB Advance 5-Year Original Maturity 2 [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 5,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P5Y
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	2.71%
FHLB Advance 5-Year Original Maturity 3 [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 10,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P5Y
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	1.45%
FHLB Advance 5-Year Original Maturity 4 [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 5,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P5Y
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	0.91%
FHLB advance, 6-Year Maturity [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 5,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P6Y
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	2.30%
FHLB Advance, 7 Year Maturity [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 5,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P7Y
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	2.55%
FHLB advance, 8-Year Maturity [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 5,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P8Y
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	2.40%
FHLB Advance 10-Year Original Maturity [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 5,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P10Y
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	1.14%
FHLB Advance 10-Year Original Maturity 2 [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 5,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P10Y
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	1.15%
FHLB Advance 10-Year Original Maturity 3 [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 5,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P10Y
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	1.12%
FHLB Advance 10-Year Original Maturity 4 [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 10,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P10Y
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	1.39%
FHLB Advance 10-Year Original Maturity 5 [Member]
FHLB Advances [Abstract]
Federal Home Loan Bank Advances Short and Long Term	$ 15,000,000
Federal Home Loan Bank Advances, Original Maturity Term	P10Y
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	1.41%
U.S. Treasury securities and obligations of U.S. government corporations & agencies
Disclosure of Repurchase Agreements [Abstract]
Securities pledged to Repurchase Agreements	$ 36,437,000	77,333,000
Residential Mortgage-backed Securities [Member]
Disclosure of Repurchase Agreements [Abstract]
Securities pledged to Repurchase Agreements	194,144,000	128,401,000
Obligations of states and political subdivisions
Disclosure of Repurchase Agreements [Abstract]
Securities pledged to Repurchase Agreements	0	2,375,000
Other Security Investments [Member]
Disclosure of Repurchase Agreements [Abstract]
Securities pledged to Repurchase Agreements	$ 1,068,000	$ 0

FHLB Advances Terms

No definition available.

Name:	fmbh_FHLBAdvancesAbstract
Namespace Prefix:	fmbh_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Federal Home Loan Bank Advances, Original Term

+ References

No definition available.

+ Details

Name:	fmbh_FederalHomeLoanBankAdvancesOriginalMaturityTerm
Namespace Prefix:	fmbh_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Short-term and long-term Federal Home Loan Bank borrowings.

+ References

No definition available.

+ Details

Name:	fmbh_FederalHomeLoanBankAdvancesShortAndLongTerm
Namespace Prefix:	fmbh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

Name:	fmbh_RepurchaseAgreementsAndOtherBorrowingsAbstract
Namespace Prefix:	fmbh_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The increase (decrease) in the carrying value as of the balance sheet date of securities that an institution sells and agrees to repurchase (the identical or substantially the same securities) as a seller-borrower at a specified date for a specified price, also known as a repurchase agreement, or repo. Most repos involve obligations of the federal government or its agencies, but other financial instruments, such as commercial paper, banker's acceptances, and negotiable certificates of deposit, are sometimes used in repos.

+ References

No definition available.

+ Details

Name:	fmbh_SecuritiesSoldUnderAgreementsToRepurchaseIncreaseDecrease
Namespace Prefix:	fmbh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Securities Sold Under Agreements to Repurchase Weighted Average Rate

+ References

No definition available.

+ Details

Name:	fmbh_SecuritiesSoldUnderAgreementsToRepurchaseWeightedAverageRate
Namespace Prefix:	fmbh_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_DisclosureOfRepurchaseAgreementsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Discloses the single interest rate [if] applicable to the advances from the Federal Home Loan Bank, by branch, as of the Balance Sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The carrying amount as of the date of the latest financial statement presented of securities which are owned but transferred to serve as collateral for repurchase agreements, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

+ Details

Name:	us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesForRepurchaseAgreements
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=49176635&loc=d3e9760-107771

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.13(1))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1)(a)(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant