| **Borrowings - Narrative (Details)**                                                   |             |              |              | 12 Months Ended |               |        |              |
|----------------------------------------------------------------------------------------|-------------|--------------|--------------|-----------------|---------------|--------|--------------|
||
| **Debt Instrument [Line Items]**                                                       |        |         |         |            |          |   |         |   |   |
| Interest rate, minimum (as a percent)                                                  |        |         |         | 0.00%           |          |   |         |   |   |
| Interest rate, maximum (as a percent)                                                  |        |         |         | 6.81%           |          |   |         |   |   |
| Total available remaining borrowing capacity from FHLB                                 |        |         |         | $ 388,176,000   |          |   |         |   |   |
| Putable advances with the FHLB                                                         |        |         |         | 235,652,000     | $ 145,264,000 |   |         |   |   |
| Gain from cancellation of debt                                                         |        |         |         | 1,250,000       |               |        |         |   |   |
| Private debt issuance, number of institutional investors | investor                    |        |         | 4            |            |          |   |         |   |   |
| US Bank, NA                                                                            |        |         |         |            |          |   |         |   |   |
| **Debt Instrument [Line Items]**                                                       |        |         |         |            |          |   |         |   |   |
| Line of credit                                                                         |        |         |         | 0               | $ 0           |   |         |   |   |
| Line of credit maximum borrowing capacity                                              |        | $ 20,000,000 |         |            |          |   |         |   |   |
| Commitment fee percentage                                                              |        | 0.25%        |         |            |          |   |         |   |   |
| 5.00% Senior Notes Due 2028                                                            |        |         |         |            |          |   |         |   |   |
| **Debt Instrument [Line Items]**                                                       |        |         |         |            |          |   |         |   |   |
| Debt face amount                                                                       |        |         | $ 5,000,000  |            |          |   |         |   |   |
| Interest rate on notes (as a percent)                                                  |        |         | 5.00%        |            |          |   |         |   |   |
| 6.75% Subordinated Notes Due 2028                                                      |        |         |         |            |          |   |         |   |   |
| **Debt Instrument [Line Items]**                                                       |        |         |         |            |          |   |         |   |   |
| Debt face amount                                                                       |        |         | $ 65,000,000 |            |          |   |         |   |   |
| Interest rate on notes (as a percent)                                                  |        |         | 6.75%        |            |          |   |         |   |   |
| Debt term with fixed interest rate                                                     |        |         | 10 years     |            |          |   |         |   |   |
| One-Month LIBOR | US Bank, NA                                                          |        |         |         |            |          |   |         |   |   |
| **Debt Instrument [Line Items]**                                                       |        |         |         |            |          |   |         |   |   |
| Basis spread on variable rate (as a percent)                                           |        | 2.00%        |         |            |          |   |         |   |   |
| Ameriana                                                                               |        |         |         |            |          |   |         |   |   |
| **Debt Instrument [Line Items]**                                                       |        |         |         |            |          |   |         |   |   |
| Federal Home Loan Bank advances assumed at acquisition                                 |        |         |         | 24,938,000      |          |   |         |   |   |
| C Financial                                                                            |        |         |         |            |          |   |         |   |   |
| **Debt Instrument [Line Items]**                                                       |        |         |         |            |          |   |         |   |   |
| Federal Home Loan Bank advances assumed at acquisition                                 |        |         |         | 11,597,000      |          |   | $ 18,958,000 |   |   |
| Federal Home Loan Bank advances matured during the period                              |        |         |         | 7,361,000       |          |   |         |   |   |
| Putable Advances                                                                       |        |         |         |            |          |   |         |   |   |
| **Debt Instrument [Line Items]**                                                       |        |         |         |            |          |   |         |   |   |
| Putable advances with the FHLB                                                         |        |         |         | $ 25,000,000    |          |   |         |   |   |
| Minimum                                                                                |        |         |         |            |          |   |         |   |   |
| **Debt Instrument [Line Items]**                                                       |        |         |         |            |          |   |         |   |   |
| Required value of assets pledged as collateral as a percentage to outstanding advances |        |         |         | 146.00%         |          |   |         |   |   |
| Securities Sold Under Repurchase Agreements                                            |        |         |         |            |          |   |         |   |   |
| **Debt Instrument [Line Items]**                                                       |        |         |         |            |          |   |         |   |   |
| Maximum amount of outstanding agreements                                               |        |         |         | $ 163,128,000   | 155,941,000   |   |         |   |   |
| Total of average agreements                                                            |        |         |         | 143,491,000     | 130,910,000   |   |         |   |   |
| Debt outstanding                                                                       |        |         |         | 155,325,000     |          |   |         |   |   |
| Subordinated Debentures and Term Loans                                                 |        |         |         |            |          |   |         |   |   |
| **Debt Instrument [Line Items]**                                                       |        |         |         |            |          |   |         |   |   |
| Debt outstanding                                                                       |        |         |         | 127,846,000     | $ 126,810,000 |   |         |   |   |
| Subordinated Debenture | First Merchant Capital Trust II                               |        |         |         |            |          |   |         |   |   |
| **Debt Instrument [Line Items]**                                                       |        |         |         |            |          |   |         |   |   |
| Debt outstanding                                                                       |        |         |         | $ 51,702,000    |          |   |         |   |   |
| Debt face amount                                                                       |        |         | $ 70,000,000 |            |          |   |         |   | $ 56,702,000 |
| Cancellation of debt, amount                                                           | $ 5,000,000 |         |         |            |          |   |         |   |   |
| Gain from cancellation of debt                                                         | $ 1,250,000 |         |         |            |          |   |         |   |   |
| Interest rate on notes (as a percent)                                                  |        |         |         | 2.10%           | 1.80%         |   |         | 6.495% |   |
| Basis spread on variable rate (as a percent)                                           |        |         |         |            | 1.56%         |   |         |   |   |
| Subordinated Debenture | First Merchant Capital Trust II | Three-Month LIBOR           |        |         |         |            |          |   |         |   |   |
| **Debt Instrument [Line Items]**                                                       |        |         |         |            |          |   |         |   |   |
| Basis spread on variable rate (as a percent)                                           |        |         |         | 1.56%           |          |   |         |   |   |
| Subordinated Debenture | Ameriana Capital Trust I                                      |        |         |         |            |          |   |         |   |   |
| **Debt Instrument [Line Items]**                                                       |        |         |         |            |          |   |         |   |   |
| Debt face amount                                                                       |        |         |         | $ 10,310,000    |          |   |         |   |   |
| Interest rate on notes (as a percent)                                                  |        |         |         | 2.00%           |          |   |         |   |   |
| Basis spread on variable rate (as a percent)                                           |        |         |         | 1.50%           |          |   |         |   |   |
| Line of Credit | Bank of America, N.A. | Revolving Credit Facility                     |        |         |         |            |          |   |         |   |   |
| **Debt Instrument [Line Items]**                                                       |        |         |         |            |          |   |         |   |   |
| Line of credit maximum borrowing capacity                                              |        |         | $ 55,000,000 |            |          |   |         |   |   |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Current Liabilities, Federal Home Loan Bank Advances
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No definition available.
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| X                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Debt Instrument, Fixed Interest Rate, Term
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| X                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Federal Home Loan Bank, Advances, Maturities Summary During the Period
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| X                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Federal Home Loan Bank, Advances, Required Collateral, Percent
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| X                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Long-term Debt, Average Outstanding Amount
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| X                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Long-term Debt, Maximum Amount Outstanding During Period
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| X                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Private Debt Issuance Sale, Number of Institutional Investors
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of borrowings as of the balance sheet date from the Federal Home Loan Bank, which are primarily used to cover shortages in the required reserve balance and liquidity shortages.
+ References
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| X                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage points added to the reference rate to compute the variable rate on the debt instrument.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Face (par) amount of debt instrument at time of issuance.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Contractual interest rate for funds borrowed, under the debt agreement.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
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No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.
+ References
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| X                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Gross amount of debt extinguished.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Discloses the lowest interest rate applicable to any of the advances from the Federal Home Loan Bank, by branch, as of the Balance Sheet date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Discloses the highest interest rate applicable to any of the advances from the Federal Home Loan Bank, by branch, as of the Balance Sheet date.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.
+ References
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