Regulatory Capital (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Total Risk-based Capital to Risk-weighted Assets, Amount
Actual	$ 783,776	$ 685,507
Prompt Corrective Action Thresholds, Adequate Capital	419,809	357,581
Tier 1 Capital to Risk-weighted Assets, Amount
Actual	656,323	564,535
Prompt Corrective Action Thresholds, Adequate Capital	314,857	178,791
Common Equity Tier One Capital to Risk-weighted Assets, Amount
Actual	603,063
Prompt Corrective Action Thresholds, Adequate Capital	236,143
Tier I Capital to Average Assets, Amount
Actual	656,323	564,535
Prompt Corrective Action Thresholds, Adequate Capital	$ 242,001	$ 222,533
Risk Based Ratios
Total Capital to Risk-weighted Assets, Actual Ratio	14.94%	15.34%
Total Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	8.00%	8.00%
Tier I Capital to Risk-weighted Assets, Actual Ratio	12.51%	12.63%
Tier I Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	6.00%	4.00%
Common Equity Tier 1 Capital to Risk-weighted Assets, Actual Ratio	11.49%
Common Equity Tier 1 Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequately Capitalized Ratio	4.50%
Leverage Ratios
Tier I Capital to Average Assets, Actual Ratio	10.85%	10.15%
Tier I Capital to Average Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	4.00%	4.00%
First Merchants Bank
Total Risk-based Capital to Risk-weighted Assets, Amount
Actual	$ 739,793	$ 653,169
Prompt Corrective Action Thresholds, Adequate Capital	423,242	356,884
Prompt Corrective Action Thresholds, Well Capitalized	529,052	446,105
Tier 1 Capital to Risk-weighted Assets, Amount
Actual	677,340	597,305
Prompt Corrective Action Thresholds, Adequate Capital	317,431	178,442
Prompt Corrective Action Thresholds, Well Capitalized	423,242	267,663
Common Equity Tier One Capital to Risk-weighted Assets, Amount
Actual	677,340
Prompt Corrective Action Thresholds, Adequate Capital	238,074
Prompt Corrective Action Thresholds, Well Capitalized	343,884
Tier I Capital to Average Assets, Amount
Actual	677,340	597,305
Prompt Corrective Action Thresholds, Adequate Capital	241,423	226,339
Prompt Corrective Action Thresholds, Well Capitalized	$ 301,779	$ 282,953
Risk Based Ratios
Total Capital to Risk-weighted Assets, Actual Ratio	13.98%	14.64%
Total Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	8.00%	8.00%
Total Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Well Capitalized Ratio	10.00%	10.00%
Tier I Capital to Risk-weighted Assets, Actual Ratio	12.80%	13.39%
Tier I Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	6.00%	4.00%
Tier I Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Well Capitalized Ratio	8.00%	6.00%
Common Equity Tier 1 Capital to Risk-weighted Assets, Actual Ratio	12.80%
Common Equity Tier 1 Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequately Capitalized Ratio	4.50%
Common Equity Tier 1 Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Well Capitalized Ratio	6.50%
Leverage Ratios
Tier I Capital to Average Assets, Actual Ratio	11.22%	10.56%
Tier I Capital to Average Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	4.00%	4.00%
Tier I Capital to Average Assets, Prompt Corrective Action Thresholds, Well Capitalized Ratio	5.00%	5.00%