Regulatory Capital and Dividends - Schedule of Actual and Required Capital (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Total Risk-Based Capital
Actual	$ 1,177,725	$ 1,048,757
Prompt Corrective Action Thresholds, Adequate Capital	644,871	612,848
Tier 1 Capital to Risk-weighted Assets, Amount
Actual	1,032,173	908,725
Prompt Corrective Action Thresholds, Adequate Capital	483,653	459,636
Common Equity Tier One Capital to Risk-weighted Assets, Amount
Actual	966,032	842,806
Prompt Corrective Action Thresholds, Adequate Capital	362,740	344,727
Tier I Capital to Average Assets, Amount
Actual	1,032,173	908,725
Prompt Corrective Action Thresholds, Adequate Capital	$ 378,379	$ 348,407
Risk Based Ratios
Total Capital to Risk-weighted Assets, Actual Ratio	14.61%	13.69%
Total Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	8.00%	8.00%
Tier 1 Capital to Risk-Weighted Assets	12.80%	11.86%
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)	11.98%	11.00%
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	10.91%	10.43%
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	$ 1,092,602	$ 1,016,355
Prompt Corrective Action Thresholds, Adequate Capital	649,531	617,477
Prompt Corrective Action Thresholds, Well Capitalized	811,914	771,847
Tier 1 Capital to Risk-weighted Assets, Amount
Actual	1,012,050	941,323
Prompt Corrective Action Thresholds, Adequate Capital	487,148	463,108
Prompt Corrective Action Thresholds, Well Capitalized	649,531	617,477
Common Equity Tier One Capital to Risk-weighted Assets, Amount
Actual	1,012,050	941,323
Prompt Corrective Action Thresholds, Adequate Capital	365,361	347,331
Prompt Corrective Action Thresholds, Well Capitalized	527,744	501,700
Tier I Capital to Average Assets, Amount
Actual	1,012,050	941,323
Prompt Corrective Action Thresholds, Adequate Capital	378,397	347,794
Prompt Corrective Action Thresholds, Well Capitalized	$ 472,996	$ 434,742
Risk Based Ratios
Total Capital to Risk-weighted Assets, Actual Ratio	13.46%	13.17%
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	12.47%	12.20%
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)	12.47%	12.20%
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	10.70%	10.83%
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%

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The amount of common equity tier 1 risk based capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action

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The amount of total capital as defined in the regulations.

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