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Nature of Operations and Summary of Significant Accounting Policies - Allowance for Loan Losses (Details) | 12 Months Ended
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Dec. 31, 2018 USD ($) key_element
Accounting Policies [Abstract] |
Number of key element used in assessing the appropriateness of the allowance for loan losses | key_element | 3
Commercial relationships threshold that exhibit credit weaknesses | $ | $ 500,000

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- Definition Allowance for Loan Losses, Number Of Key Elements Applied to Determination Of Valuation Allowance + References No definition available. + Details
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Namespace Prefix: | frme_
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Balance Type: | na
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- Definition Commercial Banking Relationship, Credit Quality Weakness, Threshold for Individual Review + References No definition available. + Details
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Namespace Prefix: | frme_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_AccountingPoliciesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration