Derivative Financial Instruments (Executed Interest Rate Swap) (Details) $ in Thousands	Dec. 31, 2018 USD ($) transaction	May 18, 2018 USD ($)	Dec. 31, 2017 USD ($)	May 10, 2016 USD ($)
Derivative [Line Items]
Long-term debt	$ 274,905		$ 285,011
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				$ 40,000
Interest Rate Swap | Cash Flow Hedging
Derivative [Line Items]
Number of interest rate swap transactions | transaction	6
Derivative, notional amount	$ 105,000	$ 9,000
Interest Rate Swap | Cash Flow Hedging | Heartland Financial Statutory Trust IV, V and VII
Derivative [Line Items]
Derivative, notional amount	85,000
Interest Rate Swap | Cash Flow Hedging | Morrill Statutory Trust I and II
Derivative [Line Items]
Derivative, notional amount	20,000
Interest Rate Swap | Cash Flow Hedging | Heartland Financial Statutory Trust IV
Derivative [Line Items]
Derivative, notional amount			25,000
Interest Rate Swap | Cash Flow Hedging | OCGI Statutory Trust III
Derivative [Line Items]
Derivative, notional amount		3,000
Interest Rate Swap | Cash Flow Hedging | OCGI Capital Trust IV
Derivative [Line Items]
Derivative, notional amount		$ 6,000
Trust preferred securities
Derivative [Line Items]
Long-term debt	130,913		121,886
Trust preferred securities | Heartland Financial Statutory Trust IV
Derivative [Line Items]
Repurchased debt amount	$ 15,000		15,000
Long-term debt			10,000
Trust preferred securities | Citywide Statutory Trust V
Derivative [Line Items]
Long-term debt			$ 15,000