DERIVATIVE FINANCIAL INSTRUMENTS (Cash flow hedges narrative) (Details) $ in Thousands	6 Months Ended
	Jun. 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	$ 600
Estimated amount to be reclassified from accumulated other comprehensive income to interest expense	$ 1,700
Interest Rate Swaps | Cash Flow Hedges
Derivative [Line Items]
Number of swap transactions | transaction	6
Derivative, notional amount	$ 105,000	$ 9,000
Interest Rate Swaps | Cash Flow Hedges | Heartland Financial Statutory Trust IV, V and VII
Derivative [Line Items]
Derivative, notional amount	85,000
Interest Rate Swaps | Cash Flow Hedges | Morrill Statutory Trust I and II
Derivative [Line Items]
Derivative, notional amount	20,000
Interest Rate Swaps | Cash Flow Hedges | Heartland Financial Statutory Trust VI
Derivative [Line Items]
Derivative, notional amount	$ 20,000
Interest Rate Swaps | Cash Flow Hedges | OCGI Statutory Trust III
Derivative [Line Items]
Derivative, notional amount		3,000
Interest Rate Swaps | Cash Flow Hedges | OCGI Capital Trust IV
Derivative [Line Items]
Derivative, notional amount		$ 6,000