Borrowings - Narrative (Details)				12 Months Ended
	Jul. 14, 2017 USD ($)	Aug. 10, 2015 USD ($)	Nov. 01, 2013 USD ($) investor	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 15, 2012	Jul. 02, 2007 USD ($)
Debt Instrument [Line Items]
Total available remaining borrowing capacity from FHLB				$ 664,177,000
Puttable advances with the FHLB				414,377,000	$ 298,923,000
Debt face amount			$ 70,000,000
Gain from cancellation of debt						$ 1,250,000
Private debt issuance, number of institutional investors | investor			4
5.00% Senior Notes Due 2028
Debt Instrument [Line Items]
Redemption price (percent)				100.00%
5% Senior Notes Due 2028 and 6.75% Subordinated Notes due 2028
Debt Instrument [Line Items]
Debt term with fixed interest rate			10 years
Putable Advances
Debt Instrument [Line Items]
Puttable advances with the FHLB				$ 45,000,000
Minimum
Debt Instrument [Line Items]
Required value of assets pledged as collateral as a percentage to outstanding advances				142.00%
Interest rate, minimum (as a percent)				0.88%
Maximum
Debt Instrument [Line Items]
Interest rate, minimum (as a percent)				5.86%
Securities Sold Under Repurchase Agreements
Debt Instrument [Line Items]
Maximum amount of outstanding agreements				$ 145,883,000	158,185,000
Total of average agreements				134,401,000	145,379,000
Debt outstanding				136,623,000
Subordinated Debentures and Term Loans
Debt Instrument [Line Items]
Debt outstanding				$ 139,349,000	$ 128,445,000
Subordinated Debenture | 6.75% Subordinated Notes Due 2028
Debt Instrument [Line Items]
Debt face amount			$ 65,000,000
Interest rate on notes (as a percent)			6.75%
Redemption price (percent)				100.00%
Subordinated Debenture | Three-Month LIBOR | 6.75% Subordinated Notes Due 2028
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)				4.095%
Subordinated Debenture | First Merchant Capital Trust II
Debt Instrument [Line Items]
Debt outstanding				$ 51,702,000
Debt face amount								$ 56,702,000
Cancellation of debt, amount		$ 5,000,000
Gain from cancellation of debt		$ 1,250,000
Interest rate on notes (as a percent)				3.15%	2.52%		6.495%
Subordinated Debenture | First Merchant Capital Trust II | Three-Month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)				1.56%
Subordinated Debenture | Ameriana Capital Trust I
Debt Instrument [Line Items]
Debt face amount						$ 10,310,000
Interest rate on notes (as a percent)				3.09%	2.46%
Basis spread on variable rate (as a percent)						1.50%
Subordinated Debenture | Grabill Capital Trust I
Debt Instrument [Line Items]
Debt face amount	$ 10,310,000
Interest rate on notes (as a percent)				3.96%
Basis spread on variable rate (as a percent)	2.60%
Senior Notes | 5.00% Senior Notes Due 2028
Debt Instrument [Line Items]
Debt face amount			$ 5,000,000
Interest rate on notes (as a percent)			5.00%
Senior Notes | Three-Month LIBOR | 5.00% Senior Notes Due 2028
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)				2.345%