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Borrowings - Narrative (Details)					3 Months Ended		12 Months Ended
	Mar. 16, 2020 USD ($)	Jul. 14, 2017 USD ($)	Aug. 10, 2015 USD ($)	Nov. 01, 2013 USD ($) investor	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Apr. 23, 2020 USD ($)	Sep. 15, 2012	Jul. 02, 2007 USD ($)	Mar. 31, 2006 USD ($)
Debt Instrument [Line Items]
Total available remaining borrowing capacity from FHLB					$ 723,300,000		$ 723,300,000
Putable advances with the FHLB					389,430,000	$ 351,072,000	389,430,000	$ 351,072,000
Debt face amount				$ 70,000,000
Repayment of borrowings							$ 621,548,000	643,169,000	$ 1,677,860,000
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]
Putable advances with the FHLB					$ 125,000,000.0		$ 125,000,000.0
Minimum
Debt Instrument [Line Items]
Required value of assets pledged as collateral as a percentage to outstanding advances							145.00%
Interest rate, minimum (as a percent)					0.35%		0.35%
Maximum
Debt Instrument [Line Items]
Interest rate, minimum (as a percent)					2.62%		2.62%
Securities Sold Under Repurchase Agreements
Debt Instrument [Line Items]
Maximum amount of outstanding agreements							$ 197,900,000	191,600,000
Total of average agreements							180,700,000	136,300,000
Debt outstanding					$ 177,102,000		177,102,000
Subordinated Debentures and Term Loans
Debt Instrument [Line Items]
Debt outstanding					118,380,000	138,700,000	$ 118,380,000	138,700,000
Subordinated Debentures and Term Loans | Grabill Capital Trust I
Debt Instrument [Line Items]
Debt face amount										$ 10,000,000.0
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					$ 41,700,000	$ 51,700,000	$ 41,700,000	51,700,000
Debt face amount	$ 10,000,000.0											$ 56,700,000
Cancellation of debt, amount			$ 5,000,000
Gain from cancellation of debt			$ 1,300,000
Interest rate on notes (as a percent)	3.45%										6.495%
Interest rate during the period					1.78%	3.45%
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													$ 10,300,000
Basis spread on variable rate (as a percent)							1.50%
Interest rate during the period					1.72%	3.39%
Subordinated Debenture | Grabill Capital Trust I
Debt Instrument [Line Items]
Debt outstanding						$ 10,300,000		$ 10,300,000
Debt face amount		$ 10,300,000								$ 10,000,000.0
Interest rate on notes (as a percent)						4.53%		4.53%		4.41%
Basis spread on variable rate (as a percent)		2.60%
Capital Securities | First Merchant Capital Trust II
Debt Instrument [Line Items]
Repayment of borrowings	$ 10,000,000.0
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%

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