OTHER BORROWINGS (Trust Preferred Offerings) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Less: deferred issuance costs	$ (74)
Total	457,042	$ 275,773
Trust preferred securities
Debt Instrument [Line Items]
Total trust preferred offerings	146,397
Total	$ 146,323	$ 145,343
Interest Rate Swap
Debt Instrument [Line Items]
Fixed interest rate on derivative (as a percent)	3.40%
Heartland Financial Statutory Trust IV | Trust preferred securities
Debt Instrument [Line Items]
Total trust preferred offerings	$ 10,310
Interest rate (as a percent)	2.98%
Heartland Financial Statutory Trust IV | Interest Rate Swap
Debt Instrument [Line Items]
Fixed interest rate on derivative (as a percent)	5.01%
Heartland Financial Statutory Trust V | Trust preferred securities
Debt Instrument [Line Items]
Total trust preferred offerings	$ 20,619
Interest rate (as a percent)	1.57%
Heartland Financial Statutory Trust VI | Trust preferred securities
Debt Instrument [Line Items]
Total trust preferred offerings	$ 20,619
Interest rate (as a percent)	1.70%
Heartland Financial Statutory Trust VI | Interest Rate Swap
Debt Instrument [Line Items]
Fixed interest rate on derivative (as a percent)	3.87%
Heartland Financial Statutory Trust VII | Trust preferred securities
Debt Instrument [Line Items]
Total trust preferred offerings	$ 18,042
Interest rate (as a percent)	1.71%
Heartland Financial Statutory Trust VII | Interest Rate Swap
Debt Instrument [Line Items]
Fixed interest rate on derivative (as a percent)	3.83%
Morrill Statutory Trust I | Trust preferred securities
Debt Instrument [Line Items]
Total trust preferred offerings	$ 9,182
Interest rate (as a percent)	3.50%
Morrill Statutory Trust II | Trust preferred securities
Debt Instrument [Line Items]
Total trust preferred offerings	$ 8,865
Interest rate (as a percent)	3.08%
Sheboygan Statutory Trust I | Trust preferred securities
Debt Instrument [Line Items]
Total trust preferred offerings	$ 6,615
Interest rate (as a percent)	3.18%
CBNM Capital Trust I | Trust preferred securities
Debt Instrument [Line Items]
Total trust preferred offerings	$ 4,458
Interest rate (as a percent)	3.47%
Citywide Capital Trust III | Trust preferred securities
Debt Instrument [Line Items]
Total trust preferred offerings	$ 6,494
Interest rate (as a percent)	3.01%
Citywide Capital Trust IV | Trust preferred securities
Debt Instrument [Line Items]
Total trust preferred offerings	$ 4,353
Interest rate (as a percent)	2.41%
Citywide Capital Trust V | Trust preferred securities
Debt Instrument [Line Items]
Total trust preferred offerings	$ 11,973
Interest rate (as a percent)	1.76%
OCGI Statutory Trust III | Trust preferred securities
Debt Instrument [Line Items]
Total trust preferred offerings	$ 3,004
Interest rate (as a percent)	3.89%
OCGI Statutory Trust III | Interest Rate Swap
Debt Instrument [Line Items]
Fixed interest rate on derivative (as a percent)	5.53%
OCGI Capital Trust IV | Trust preferred securities
Debt Instrument [Line Items]
Total trust preferred offerings	$ 5,399
Interest rate (as a percent)	2.72%
OCGI Capital Trust IV | Interest Rate Swap
Debt Instrument [Line Items]
Fixed interest rate on derivative (as a percent)	4.37%
BVBC Capital Trust II | Trust preferred securities
Debt Instrument [Line Items]
Total trust preferred offerings	$ 7,238
Interest rate (as a percent)	3.46%
BVBC Capital Trust III | Trust preferred securities
Debt Instrument [Line Items]
Total trust preferred offerings	$ 9,226
Interest rate (as a percent)	1.85%
LIBOR | Heartland Financial Statutory Trust IV | Trust preferred securities
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	2.75%
LIBOR | Heartland Financial Statutory Trust V | Trust preferred securities
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.33%
LIBOR | Heartland Financial Statutory Trust VI | Trust preferred securities
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.48%
LIBOR | Heartland Financial Statutory Trust VII | Trust preferred securities
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.48%
LIBOR | Morrill Statutory Trust I | Trust preferred securities
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	3.25%
LIBOR | Morrill Statutory Trust II | Trust preferred securities
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	2.85%
LIBOR | Sheboygan Statutory Trust I | Trust preferred securities
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	2.95%
LIBOR | CBNM Capital Trust I | Trust preferred securities
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	3.25%
LIBOR | Citywide Capital Trust III | Trust preferred securities
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	2.80%
LIBOR | Citywide Capital Trust IV | Trust preferred securities
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	2.20%
LIBOR | Citywide Capital Trust V | Trust preferred securities
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.54%
LIBOR | OCGI Statutory Trust III | Trust preferred securities
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	3.65%
LIBOR | OCGI Capital Trust IV | Trust preferred securities
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	2.50%
LIBOR | BVBC Capital Trust II | Trust preferred securities
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	3.25%
LIBOR | BVBC Capital Trust III | Trust preferred securities
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.60%