DERIVATIVE FINANCIAL INSTRUMENTS (Cash flow hedges narrative) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021 USD ($)	Mar. 31, 2021 swap
Derivative [Line Items]
Change in net unrealized losses and reclassification from AOCI to interest expense on cash flow hedges	$ 1,400
Estimated amount to be reclassified from accumulated other comprehensive income to interest expense	733
Unaffiliated Bank
Derivative [Line Items]
Derivative, number of swaps | swap		2
Interest Rate Swaps | Cash Flow Hedges
Derivative [Line Items]
Derivative, notional amount	40,000
Interest Rate Swaps | Cash Flow Hedges | Heartland Financial Statutory Trust VI and VII
Derivative [Line Items]
Derivative, notional amount	$ 40,000