Regulatory Capital and Dividends - Schedule of Actual and Required Capital (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Total Risk-Based Capital
Actual	$ 851,521	$ 783,776
Prompt Corrective Action Thresholds, Adequate Capital	479,470	419,809
Tier 1 Capital to Risk-weighted Assets, Amount
Actual	720,484	656,323
Prompt Corrective Action Thresholds, Adequate Capital	359,603	314,857
Common Equity Tier One Capital to Risk-weighted Assets, Amount
Actual	665,445	603,063
Prompt Corrective Action Thresholds, Adequate Capital	269,702	236,143
Tier I Capital to Average Assets, Amount
Actual	720,484	656,323
Prompt Corrective Action Thresholds, Adequate Capital	$ 273,456	$ 242,001
Risk Based Ratios
Total Capital to Risk-weighted Assets, Actual Ratio	14.21%	14.94%
Total Capital to Risk-weighted Assets, Prompt Corrective Action Thresholds, Adequate Capital Ratio	8.00%	8.00%
Tier 1 Capital to Risk-Weighted Assets	12.02%	12.51%
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)	11.10%	11.49%
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%	10.85%
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	$ 800,598	$ 739,793
Prompt Corrective Action Thresholds, Adequate Capital	481,490	423,242
Prompt Corrective Action Thresholds, Well Capitalized	601,862	529,052
Tier 1 Capital to Risk-weighted Assets, Amount
Actual	734,561	677,340
Prompt Corrective Action Thresholds, Adequate Capital	361,117	317,431
Prompt Corrective Action Thresholds, Well Capitalized	481,490	423,242
Common Equity Tier One Capital to Risk-weighted Assets, Amount
Actual	734,561	677,340
Prompt Corrective Action Thresholds, Adequate Capital	270,838	238,074
Prompt Corrective Action Thresholds, Well Capitalized	391,210	343,884
Tier I Capital to Average Assets, Amount
Actual	734,561	677,340
Prompt Corrective Action Thresholds, Adequate Capital	272,461	241,423
Prompt Corrective Action Thresholds, Well Capitalized	$ 340,576	$ 301,779
Risk Based Ratios
Total Capital to Risk-weighted Assets, Actual Ratio	13.30%	13.98%
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%	12.80%
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%	12.80%
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	10.78%	11.22%
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%