Regulatory Capital and Dividends - Schedule of Actual and Required Capital (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Total Risk-Based Capital
Actual	$ 1,400,617	$ 1,177,725
Prompt Corrective Action Thresholds, Adequate Capital	783,946	644,871
Tier 1 Capital to Risk-weighted Assets, Amount
Actual	1,255,333	1,032,173
Prompt Corrective Action Thresholds, Adequate Capital	587,960	483,653
Common Equity Tier One Capital to Risk-weighted Assets, Amount
Actual	1,188,970	966,032
Prompt Corrective Action Thresholds, Adequate Capital	440,970	362,740
Tier I Capital to Average Assets, Amount
Actual	1,255,333	1,032,173
Prompt Corrective Action Thresholds, Adequate Capital	$ 476,383	$ 378,379
Risk Based Ratios
Total Capital to Risk-weighted Assets, Actual Ratio	14.29%	14.61%
Total Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	8.00%	8.00%
Tier 1 Capital to Risk-Weighted Assets	12.81%	12.80%
Tier I Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	6.00%	6.00%
Common Equity Tier 1 Capital Ratio (Regulatory)	12.13%	11.98%
Common Equity Tier 1 Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequately Capitalized Ratio	4.50%	4.50%
Leverage Ratios
Tier 1 Capital to Average Assets	10.54%	10.91%
Tier I Capital to Average Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	4.00%	4.00%
First Merchants Bank
Total Risk-Based Capital
Actual	$ 1,267,649	$ 1,092,602
Prompt Corrective Action Thresholds, Adequate Capital	787,753	649,531
Prompt Corrective Action Thresholds, Well Capitalized	984,691	811,914
Tier 1 Capital to Risk-weighted Assets, Amount
Actual	1,187,365	1,012,050
Prompt Corrective Action Thresholds, Adequate Capital	590,815	487,148
Prompt Corrective Action Thresholds, Well Capitalized	787,753	649,531
Common Equity Tier One Capital to Risk-weighted Assets, Amount
Actual	1,187,365	1,012,050
Prompt Corrective Action Thresholds, Adequate Capital	443,111	365,361
Prompt Corrective Action Thresholds, Well Capitalized	640,049	527,744
Tier I Capital to Average Assets, Amount
Actual	1,187,365	1,012,050
Prompt Corrective Action Thresholds, Adequate Capital	475,564	379,397
Prompt Corrective Action Thresholds, Well Capitalized	$ 594,455	$ 472,996
Risk Based Ratios
Total Capital to Risk-weighted Assets, Actual Ratio	12.87%	13.46%
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 1 Capital to Risk-Weighted Assets	12.06%	12.47%
Tier I Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	6.00%	6.00%
Tier I Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Well Capitalized Ratio	8.00%	8.00%
Common Equity Tier 1 Capital Ratio (Regulatory)	12.06%	12.47%
Common Equity Tier 1 Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequately Capitalized Ratio	4.50%	4.50%
Common Equity Tier 1 Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Well Capitalized Ratio	6.50%	6.50%
Leverage Ratios
Tier 1 Capital to Average Assets	9.99%	10.70%
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%