Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Captive/Trust) (Details)	12 Months Ended
Dec. 31, 2019 USD ($) trust_accounts	Dec. 31, 2018 USD ($) trust_accounts
Organization, Consolidation and Presentation of Financial Statements [Abstract]
CaptiveMaximumGrossPremiums	$ 2,300,000
Number of Trust Accounts managed or administered \| trust_accounts	1,635	1,141
Value of Trust Accounts	$ 1,487,500,000	$ 1,129,600,000

Amount pursuant to Section 831(b), if gross premiums do not exceed then the Captive Insurance Company is taxable solely on its investment income

No definition available.

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Namespace Prefix:	fmbh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration