PENSION AND OTHER POST RETIREMENT BENEFIT PLANS - Schedule of Plan Assets by Level (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 83,342	$ 79,535	$ 82,258
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	69,690	69,200
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Cash & Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,768	2,609
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Corporate Bonds and Notes
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	30,932	29,206
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Government Agency and Municipal Bonds and Notes
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Certificates of Deposit
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Common Stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,268	2,413
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Taxable Bond
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15,872	15,648
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Large Cap Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9,120	11,047
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Mid Cap Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,892	3,403
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Small Cap Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,245	1,275
Quoted Prices in Active Markets for Identical Assets (Level 1) \| International Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,593	3,599
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13,651	10,335
Significant Other Observable Inputs (Level 2) \| Cash & Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) \| Corporate Bonds and Notes
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) \| Government Agency and Municipal Bonds and Notes
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13,651	10,089
Significant Other Observable Inputs (Level 2) \| Certificates of Deposit
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		246
Significant Other Observable Inputs (Level 2) \| Common Stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) \| Taxable Bond
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) \| Large Cap Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) \| Mid Cap Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) \| Small Cap Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) \| International Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) \| Cash & Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) \| Corporate Bonds and Notes
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) \| Government Agency and Municipal Bonds and Notes
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) \| Certificates of Deposit
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0
Significant Unobservable Inputs (Level 3) \| Common Stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) \| Taxable Bond
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) \| Large Cap Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) \| Mid Cap Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) \| Small Cap Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) \| International Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fair Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	83,341	79,535
Fair Value \| Cash & Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,768	2,609
Fair Value \| Corporate Bonds and Notes
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	30,932	29,206
Fair Value \| Government Agency and Municipal Bonds and Notes
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13,651	10,089
Fair Value \| Certificates of Deposit
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		246
Fair Value \| Common Stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,268	2,413
Fair Value \| Taxable Bond
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15,872	15,648
Fair Value \| Large Cap Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9,120	11,047
Fair Value \| Mid Cap Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,892	3,403
Fair Value \| Small Cap Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,245	1,275
Fair Value \| International Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 4,593	$ 3,599

FIRST MERCHANTS CORP 10-K