DERIVATIVE FINANCIAL INSTRUMENTS (Cash flow hedges narrative) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($) transaction	May 18, 2018 USD ($)
Derivative [Line Items]
Change in net unrealized losses and reclassification from AOCI to interest expense on cash flow hedges	$ 1.2
Estimated amount to be reclassified from accumulated other comprehensive income to interest expense	$ 2.4
Interest Rate Swaps | Cash Flow Hedges
Derivative [Line Items]
Number of swap transactions | transaction	4
Derivative, notional amount	$ 85.0	$ 9.0
Interest Rate Swaps | Cash Flow Hedges | Heartland Financial Statutory Trust IV, V, VI, And VII
Derivative [Line Items]
Derivative, notional amount	85.0
Interest Rate Swaps | Cash Flow Hedges | Heartland Financial Statutory Trust VI
Derivative [Line Items]
Derivative, notional amount	$ 20.0
Interest Rate Swaps | Cash Flow Hedges | OCGI Statutory Trust III
Derivative [Line Items]
Derivative, notional amount		3.0
Interest Rate Swaps | Cash Flow Hedges | OCGI Capital Trust IV
Derivative [Line Items]
Derivative, notional amount		$ 6.0