DERIVATIVE FINANCIAL INSTRUMENTS (Executed Interest Rate Swap) (Details) $ in Millions	Dec. 31, 2020 USD ($) transaction	Mar. 31, 2019 USD ($)	May 18, 2018 USD ($)	May 10, 2016 USD ($)
Forward-starting Interest Rate Swap | Cash Flow Hedging
Derivative [Line Items]
Number of interest rate swap transactions | transaction	6
Interest Rate Swap
Derivative [Line Items]
Derivative, notional amount				$ 40.0
Interest Rate Swap | Cash Flow Hedging
Derivative [Line Items]
Number of interest rate swap transactions | transaction	6
Derivative, notional amount	$ 105.0		$ 9.0
Interest Rate Swap | Cash Flow Hedging | Heartland Financial Statutory Trust IV, V, VI and VII
Derivative [Line Items]
Derivative, notional amount	85.0
Interest Rate Swap | Cash Flow Hedging | Morrill Statutory Trust I and II
Derivative [Line Items]
Derivative, notional amount	$ 20.0	$ 20.0
Interest Rate Swap | Cash Flow Hedging | OCGI Statutory Trust III
Derivative [Line Items]
Derivative, notional amount			3.0
Interest Rate Swap | Cash Flow Hedging | OCGI Capital Trust IV
Derivative [Line Items]
Derivative, notional amount			$ 6.0