Regulatory Capital (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to Risk-weighted Assets), Actual Amount	$ 526,792	$ 487,393
Total Capital (to Risk-weighted Assets) Actual Ratio	16.34%	16.54%
Total Capital (to RIsk-weighted Assets) Required for Adequate Capital, Amount	257,927	235,781
Total Capital (to Risk-weighted Assets) Required for Adequate Capital, Ratio	8.00%	8.00%
Tier I Capital (to Risk-weighted Assets), Actual Amount	456,132	410,132
Tier I Capital (to Risk-weighted Assets), Ratio	14.15%	13.92%
Tier I Capital (to Risk-weighted Assets) Required for Adequate Capital, Amount	128,964	117,890
Tier I Capital (to Risk-weighted Assets) Required for Adequate Capital, Ratio	4.00%	4.00%
Tier I Capital (to Average Assets), Actual	456,132	410,132
Tier I Capital (to Average Assets), Ratio	11.03%	10.17%
Tier I Capital (to Average Assets) Required for Adequate Capital, Amount	165,460	161,350
Tier I Capital (to Average Assets) Required for Adequate Capital, Ratio	4.00%	4.00%
First Merchants
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to Risk-weighted Assets) Well Capitalized Ratio	10.00%
Tier I Capital (to Risk-weighted Assets) Well Capitalized Ratio	6.00%
Tier I Capital (to Average Assets) Well Capitalized Ratio	5.00%
Total Capital (to Risk-weighted Assets), Actual Amount	515,337	477,805
Total Capital (to Risk-weighted Assets) Actual Ratio	16.01%	16.26%
Total Capital (to RIsk-weighted Assets) Required for Adequate Capital, Amount	257,446	235,089
Total Capital (to Risk-weighted Assets) Required for Adequate Capital, Ratio	8.00%	8.00%
Tier I Capital (to Risk-weighted Assets), Actual Amount	474,782	440,909
Tier I Capital (to Risk-weighted Assets), Ratio	14.75%	15.00%
Tier I Capital (to Risk-weighted Assets) Required for Adequate Capital, Amount	128,723	117,545
Tier I Capital (to Risk-weighted Assets) Required for Adequate Capital, Ratio	4.00%	4.00%
Tier I Capital (to Average Assets), Actual	474,782	440,909
Tier I Capital (to Average Assets), Ratio	11.50%	10.96%
Tier I Capital (to Average Assets) Required for Adequate Capital, Amount	$ 165,124	$ 160,848
Tier I Capital (to Average Assets) Required for Adequate Capital, Ratio	4.00%	4.00%