Regulatory Capital and Dividends - Schedule of Actual and Required Capital (Details) $ in Thousands	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Total Risk-Based Capital
Actual	$ 1,582,481	$ 1,475,551
Prompt Corrective Action Thresholds, Adequate Capital	1,193,840	1,079,015
Tier 1 Capital to Risk-weighted Assets, Amount
Actual	1,374,240	1,282,070
Prompt Corrective Action Thresholds, Adequate Capital	966,442	873,488
Common Equity Tier One Capital to Risk-weighted Assets, Amount
Actual	1,327,634	1,235,702
Prompt Corrective Action Thresholds, Adequate Capital	795,893	719,343
Tier I Capital to Average Assets, Amount
Actual	1,374,240	1,282,070
Prompt Corrective Action Thresholds, Adequate Capital	$ 590,758	$ 536,123
Risk Based Ratios
Total Capital to Risk-weighted Assets, Actual Ratio	0.1392	0.1436
Total Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	0.1050	0.1050
Tier 1 Capital to Risk-weighted Assets, Actual Ratio	0.1209	0.1248
Tier I Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	0.0850	0.0850
Common Equity Tier 1 Capital to Risk-weighted Assets, Actual Ratio	0.1168	0.1202
Common Equity Tier 1 to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	0.0700	0.0700
Leverage Ratios
Tier 1 Capital to Average Assets	0.0930	0.0957
Tier I Capital to Average Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	0.0400	0.0400
First Merchants Bank
Total Risk-Based Capital
Actual	$ 1,453,358	$ 1,412,805
Prompt Corrective Action Thresholds, Adequate Capital	1,197,515	1,082,430
Prompt Corrective Action Thresholds, Well Capitalized	1,140,490	1,030,886
Tier 1 Capital to Risk-weighted Assets, Amount
Actual	1,309,685	1,283,922
Prompt Corrective Action Thresholds, Adequate Capital	969,417	876,253
Prompt Corrective Action Thresholds, Well Capitalized	912,392	824,708
Common Equity Tier One Capital to Risk-weighted Assets, Amount
Actual	1,309,685	1,283,922
Prompt Corrective Action Thresholds, Adequate Capital	798,343	721,620
Prompt Corrective Action Thresholds, Well Capitalized	741,319	670,076
Tier I Capital to Average Assets, Amount
Actual	1,309,685	1,283,922
Prompt Corrective Action Thresholds, Adequate Capital	589,994	535,279
Prompt Corrective Action Thresholds, Well Capitalized	$ 737,493	$ 669,098
Risk Based Ratios
Total Capital to Risk-weighted Assets, Actual Ratio	0.1274	0.1370
Total Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	0.1050	0.1050
Total Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Well Capitalized Ratio	0.1000	0.1000
Tier 1 Capital to Risk-weighted Assets, Actual Ratio	0.1148	0.1245
Tier I Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	0.0850	0.0850
Tier I Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Well Capitalized Ratio	0.0800	0.0800
Common Equity Tier 1 Capital to Risk-weighted Assets, Actual Ratio	0.1148	0.1245
Common Equity Tier 1 to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	0.0700	0.0700
Common equity Tier I Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Well Capitalized Ratio	0.0650	0.0650
Leverage Ratios
Tier 1 Capital to Average Assets	0.0888	0.0959
Tier I Capital to Average Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	0.0400	0.0400
Tier I Capital to Average Assets, Prompt Corrective Action Thresholds, Well Capitalized Ratio	0.0500	0.0500