Regulatory Capital and Dividends - Schedule of Actual and Required Capital (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Total Risk-Based Capital
Actual	$ 1,048,757	$ 851,521
Prompt Corrective Action Thresholds, Adequate Capital	612,848	479,470
Tier 1 Capital to Risk-weighted Assets, Amount
Actual	908,725	720,484
Prompt Corrective Action Thresholds, Adequate Capital	459,636	359,603
Common Equity Tier One Capital to Risk-weighted Assets, Amount
Actual	842,806	665,445
Prompt Corrective Action Thresholds, Adequate Capital	344,727	269,702
Tier I Capital to Average Assets, Amount
Actual	908,725	720,484
Prompt Corrective Action Thresholds, Adequate Capital	$ 348,407	$ 273,456
Risk Based Ratios
Total Capital to Risk-weighted Assets, Actual Ratio	13.69%	14.21%
Total Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	8.00%	8.00%
Tier 1 Capital to Risk-Weighted Assets	11.86%	12.02%
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)	11.00%	11.10%
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	10.43%	10.54%
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	$ 1,016,355	$ 800,598
Prompt Corrective Action Thresholds, Adequate Capital	617,477	481,490
Prompt Corrective Action Thresholds, Well Capitalized	771,847	601,862
Tier 1 Capital to Risk-weighted Assets, Amount
Actual	941,323	734,561
Prompt Corrective Action Thresholds, Adequate Capital	463,108	361,117
Prompt Corrective Action Thresholds, Well Capitalized	617,477	481,490
Common Equity Tier One Capital to Risk-weighted Assets, Amount
Actual	941,323	734,561
Prompt Corrective Action Thresholds, Adequate Capital	347,331	270,838
Prompt Corrective Action Thresholds, Well Capitalized	501,700	391,210
Tier I Capital to Average Assets, Amount
Actual	941,323	734,561
Prompt Corrective Action Thresholds, Adequate Capital	347,794	272,461
Prompt Corrective Action Thresholds, Well Capitalized	$ 434,742	$ 340,576
Risk Based Ratios
Total Capital to Risk-weighted Assets, Actual Ratio	13.17%	13.30%
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	12.20%	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)	12.20%	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	10.83%	10.78%
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%