| **Pension and Other Post Retirement Benefit Plans**   | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          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| **Compensation and Retirement Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   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| Pension and Other Post Retirement Benefit Plans       | PENSION AND OTHER POST RETIREMENT BENEFIT PLANSThe Corporation’s defined-benefit pension plans cover approximately 19 percent of the Corporation’s employees. In 2005, the Board of Directors of the Corporation approved the curtailment of the accumulation of defined benefits for future services provided by certain participants in the First Merchants Corporation Retirement Plan.  No additional pension benefits have been earned by any employees who had not attained both the age of 55 and accrued at least 10 years of vesting service as of March 1, 2005.  The benefits are based primarily on years of service and employees’ pay near retirement. Contributions are intended to provide not only for benefits attributed to service-to-date, but also for those expected to be earned in the future. The Corporation also maintains post retirement benefit plans that provide health insurance benefits to retirees. The plans allow retirees to be carried under the Corporation’s health insurance plan, generally from ages 55 to 65. The retirees pay 100 percent of the premiums due for their coverage.  The table below sets forth the plans’ funded status and amounts recognized in the consolidated balance sheets at December 31, using measurement dates of December 31, 2016 and 2015. 2016 2015Change in Benefit Obligation:   Benefit obligation at beginning of year$71,747 $80,650Service cost9 55Interest cost3,273 3,087Actuarial loss (gain)2,679 (5,617)Benefits paid(5,263) (6,428)Net transfers in from Ameriana acquisition11,645 Benefit obligation at end of year$84,090 $71,747    Change in Plan Assets:   Fair value of plan assets at beginning of year$71,396 $77,139Actual return on plan assets6,959 153Employer contributions739 532Benefits paid(5,263) (6,428)Ameriana acquisition9,726 End of year83,557 71,396Funded status at end of year$(533) $(351)    Assets and Liabilities Recognized in the Balance Sheets:   Deferred tax asset$7,041 $7,461Assets$5,028 $4,006Liabilities$5,561 $4,357    Amounts Recognized in Accumulated Other Comprehensive Income Not Yet Recognized as Components of Net Periodic (Benefit) Cost, net of tax, consist of:   Accumulated loss$(12,624) $(13,857)Prior service credit(451) (305) $(13,075) $(14,162) The actuarial (gain) loss recognized in 2016 and 2015 was primarily a result of discount rate assumption fluctuations.Ameriana participated in the Pentegra Defined Benefit Plan for Financial Institutions (the "Pentegra Plan”), an industry-wide, tax-qualified defined-benefit pension plan. The Pentegra Plan operated as a multi-employer plan for accounting purposes and as a multiple employer plan under ERISA and the Internal Revenue Code. On November 20, 2015, Ameriana provided Pentegra a 60-day written notice of withdrawal as a Participating Employer in the Pentegra Plan. Upon completion of the withdrawal from the Pentegra Plan, the assets and liabilities were merged into the First Merchants Corporation Retirement Plan in 2016.The accumulated benefit obligation for all defined benefit plans was $84,090,000 and $71,747,000 at December 31, 2016 and 2015, respectively.Information for pension plans with an accumulated benefit obligation in excess of plan assets is included in the table below.  December 31, 2016 December 31, 2015Projected benefit obligation$5,561 $4,357Accumulated benefit obligation$5,561 $4,357Fair value of plan assets The following table shows the components of net periodic pension costs: December 31, 2016 December 31, 2015 December 31, 2014Service cost$9 $55 $73Interest cost3,273 3,087 3,235Expected return on plan assets(4,508) (4,471) (4,467)Amortization of prior service costs79 64 81Amortization of net loss1,656 1,722 478Net periodic pension (benefit) cost$509 $457 $(600)Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss): December 31, 2016 December 31, 2015 December 31, 2014Net periodic pension (benefit) cost$509 $457 $(600)Net gain (loss)(228) 1,299 (12,203)Amortization of loss1,656 1,722 478Amortization of prior service cost79 64 81Total recognized in other comprehensive income (loss)1,507 3,085 (11,644)Total recognized in net periodic pension cost and other comprehensive income (loss)$998 $2,628 $(11,044)The estimated net loss and transition obligation for the defined benefit pension plans that will be amortized from accumulated other comprehensive income into net periodic pension cost over the next fiscal year are:December 31, 2016December 31, 2015 December 31, 2014Amortization of net loss$(1,208)$(1,505) $(1,770)Amortization of prior service cost(90)(61) (64)Total$(1,298)$(1,566) $(1,834)Significant assumptions include: December 31, 2016 December 31, 2015 December 31, 2014Weighted-average Assumptions Used to Determine Benefit Obligation:     Discount rate4.20% 4.50% 4.00%Rate of compensation increase for accruing active participants3.00% 3.00% 3.00%Weighted-average Assumptions Used to Determine Cost:    Discount rate4.50% 4.00% 4.80%Expected return on plan assets6.00% 6.00% 6.00%Rate of compensation increase for accruing active participants3.00% 3.00% 3.00% At December 31, 2016 and 2015, the Corporation based its estimate of the expected long-term rate of return on analysis of the historical returns of the plans and current market information available. The plans’ investment strategies are to provide for preservation of capital with an emphasis on long-term growth without undue exposure to risk. The assets of the plans’ are invested in accordance with the plans’ Investment Policy Statement, subject to strict compliance with ERISA and any other applicable statutes.The plans’ risk management practices include semi-annual evaluations of investment managers, including reviews of compliance with investment manager guidelines and restrictions; ability to exceed performance objectives; adherence to the investment philosophy and style; and ability to exceed the performance of other investment managers. The evaluations are reviewed by management with appropriate follow-up and actions taken, as deemed necessary. The Investment Policy Statement generally allows investments in cash and cash equivalents, real estate, fixed income debt securities and equity securities, and specifically prohibits investments in derivatives, options, futures, private placements, short selling, non-marketable securities and purchases of individual non-investment grade bonds.At December 31, 2016, the maturities of the plans’ debt securities ranged from 15 days to 8.03 years, with a weighted average maturity of 3.97 years. At December 31, 2015, the maturities of the plans’ debt securities ranged from 15 days to 9.04 years, with a weighted average maturity of 4.47 years.The following benefit payments, which reflect expected future service, as appropriate, are expected to be paid as of December 31, 2016. The minimum contribution required in 2017 will likely be zero but the Corporation may decide to make a discretionary contribution during the year.2017$5,75620185,64420195,80220205,68820215,557After 202127,118 $55,565Plan assets are re-balanced quarterly. At December 31, 2016 and 2015, plan assets by category are as follows: December 31, 2016December 31, 2015 ActualTargetActualTargetCash and cash equivalents4.5%3.0%2.2%2.0%Equity securities55.855.058.160.0Debt securities37.740.037.436.0Alternative investments2.02.02.32.0 100.0%100.0%100.0%100.0%The Savings Plan, a Section 401(k) qualified defined contribution plan, was amended on March 1, 2005 to provide enhanced retirement benefits, including employer and matching contributions, for eligible employees of the Corporation and its subsidiaries. The Corporation matches employees’ contributions primarily at the rate of 50 percent for the first 6 percent of base salary contributed by participants.Beginning in 2005, employees who have completed 1000 hours of service and are an active employee on the last day of the year receive an additional retirement contribution after year-end. Employees hired after January 1, 2010 do not participate in the additional retirement contribution.  Effective January 1, 2013, the additional retirement contribution was fixed at 2 percent. Full vesting occurs after five years of service. The Corporation’s expense for the Savings Plan, including the additional retirement contribution, was $3,441,000, $3,526,000 and $3,396,000 for 2016, 2015 and 2014, respectively.The Corporation maintains post retirement benefit plans that provide health insurance benefits to retirees. The plans allow retirees to be carried under the Corporation’s health insurance plan, generally from ages 55 to 65. The retirees pay 100 percent of the premiums due for their coverage.  The Corporation elected to terminate this plan effective December 31, 2017.  As a result of the termination, the Corporation recognized a curtailment gain of $2,157,000 as a component of salaries and benefits expense in the accompanying CONSOLIDATED STATEMENTS OF INCOME.  The obligations payable under the plans totaled $54,000 and $1,403,000 at December 31, 2016 and 2015, respectively. Post retirement plan expense totaled $(2,437,000), $(127,000) and $97,000 for the years ending December 31, 2016, 2015 and 2014, respectively.Pension Plan AssetsFollowing is a description of the valuation methodologies used for pension plan assets measured at fair value on a recurring basis, as well as the general classification of pension plan assets pursuant to the valuation hierarchy.Where quoted market prices are available in an active market, plan assets are classified within Level 1 of the valuation hierarchy.  Level 1 plan assets total $64,064,000 and include cash and cash equivalents, common stocks, mutual funds and corporate bonds and notes.  If quoted market prices are not available, then fair values are estimated by using pricing models, quoted prices of plan assets with similar characteristics or discounted cash flows.  Level 2 plan assets total $19,493,000 and include governmental agencies, taxable municipals, common collective trust investments (which are classified below as Party-in-Interest investments -- common bond fund and common equity fund) and certificates of deposit.  In certain cases where Level 1 or Level 2 inputs are not available, plan assets are classified within Level 3 of the hierarchy.  There are no assets classified within Level 3 of the hierarchy at December 31, 2016 and 2015.  Fair Value Measurements Using   Quoted Prices inActive Markets forIdentical Assets Significant Other Observable Inputs SignificantUnobservableInputsDecember 31, 2016Fair Value(Level 1)(Level 2)(Level 3)Cash & Cash Equivalents$3,778$3,778Corporate Bonds and Notes10,64610,646Government Agency and Municipal Bonds and Notes6,855$6,855Certificates of Deposit1,2681,268Party-in-Interest InvestmentsCommon Stock2,2782,278Common Bond Fund4,7444,744Common Equity Fund6,6266,626Mutual FundsTaxable Bond7,9747,974Large Cap Equity19,61119,611Mid Cap Equity10,30410,304Small Cap Equity4,6804,680International Equity3,0933,093Specialty Alternative Equity1,7001,700 $83,557 $64,064 $19,493    Fair Value Measurements Using   Quoted Prices inActive Markets forIdentical Assets Significant Other Observable Inputs SignificantUnobservableInputsDecember 31, 2015Fair Value (Level 1) (Level 2) (Level 3)Cash & Cash Equivalents$1,608 $1,608    Corporate Bonds and Notes9,113 9,113    Government Agency and Municipal Bonds and Notes6,993   $6,993  Certificates of Deposit1,511   1,511  Party-in-Interest Investments       Common Stock1,538 1,538    Common Bond Fund4,647   4,647  Common Equity Fund5,812   5,812  Mutual Funds       Taxable Bond4,536 4,536    Large Cap Equity18,528 18,528    Mid Cap Equity8,537 8,537    Small Cap Equity3,578 3,578    International Equity3,353 3,353    Specialty Alternative Equity1,642 1,642     $71,396 $52,433 $18,963  |

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| - Definition
The entire disclosure for pension and other postretirement benefits.
+ References
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