Other Borrowings (Narrative) (Details)			3 Months Ended			12 Months Ended
	Jul. 20, 2016 USD ($)	May 10, 2016 USD ($)	Sep. 30, 2017 USD ($) shares	Mar. 31, 2017 USD ($) shares	Sep. 30, 2016 USD ($) shares	Dec. 31, 2017 USD ($) subsidiary	Dec. 31, 2016 USD ($)	Feb. 05, 2016 USD ($)	Dec. 15, 2015 USD ($)	Dec. 17, 2014 USD ($)
Debt Instrument [Line Items]
FHLB stock, required collateral						$ 11,300,000	$ 9,900,000
Additional collateral, aggregate fair value						2,910,000,000	2,740,000,000
FHLB borrowing capacity						1,330,000,000
Long-term debt						$ 285,011,000	288,534,000
Number of wholly-owned trust subsidiaries that issue preferred securities | subsidiary						11
Senior notes						$ 11,000,000	16,000,000
Borrowing capacity							20,000,000
CIC Bancshares, Inc.
Debt Instrument [Line Items]
Deferred issuance costs						28,000	44,000
CIC Bancshares, Inc. | Common Stock
Debt Instrument [Line Items]
Number of shares issued upon conversion (in shares) | shares			14,353	6,128	52,913
Nonrevolving Credit Facility
Debt Instrument [Line Items]
Borrowing capacity	$ 75,000,000					75,000,000			$ 50,000,000
Increase in available capacity	$ 25,000,000.0
Available borrowings						39,300,000
Line of credit outstanding						0
Interest Rate Swap
Debt Instrument [Line Items]
Amount of hedged debt		$ 40,000,000
Interest Rate Swap | Cash Flow Hedging
Debt Instrument [Line Items]
Amount of hedged debt						85,000,000
Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedging | Nonrevolving Credit Facility
Debt Instrument [Line Items]
Amount of hedged debt		$ 40,000,000				33,667,000	37,667,000
Fixed interest rate on derivative (as a percent)		2.50%
Term of derivative instrument		5 years
Tier 1
Debt Instrument [Line Items]
Subordinated notes qualified as Tier 2 capital						74,000,000
Wholesale repurchase agreements
Debt Instrument [Line Items]
Long-term debt						30,000,000	30,000,000
Trust preferred securities
Debt Instrument [Line Items]
Long-term debt						121,886,000	115,232,000
Deferred issuance costs						127,000
Trust preferred securities | Tier 1
Debt Instrument [Line Items]
Long-term debt						121,900,000	115,200,000
Senior notes
Debt Instrument [Line Items]
Long-term debt						11,000,000	16,000,000
Senior notes | February 1, 2018
Debt Instrument [Line Items]
Debt instrument, maturity amount						6,000,000
Senior notes | February 1, 2019
Debt Instrument [Line Items]
Debt instrument, maturity amount						5,000,000
Contracts payable for purchase of real estate and other assets
Debt Instrument [Line Items]
Long-term debt						1,881,000	2,339,000
Stated interest rate (as a percent)										5.75%
Debt issued										$ 75,000,000
Debt discount										$ 1,100,000
Contracts payable for purchase of real estate and other assets | CIC Bancshares, Inc.
Debt Instrument [Line Items]
Stated interest rate (as a percent)								8.00%
Debt discount								$ 168,000
Debt assumed								$ 6,000,000
Contracts payable for purchase of real estate and other assets | Other Assets
Debt Instrument [Line Items]
Unamortized debt issuance expense						265,000	$ 303,000
Subordinated convertible notes | CIC Bancshares, Inc.
Debt Instrument [Line Items]
Stated interest rate (as a percent)								6.50%
Debt discount								$ 16,000
Debt assumed								$ 2,000,000
Amount of debt			$ 391,000	$ 167,000	$ 1,400,000
Heartland Financial Statutory Trust IV | Interest Rate Swap | Cash Flow Hedging
Debt Instrument [Line Items]
Amount of hedged debt						25,000,000
Heartland Financial Statutory Trust IV | Trust preferred securities
Debt Instrument [Line Items]
Long-term debt						10,000,000
Repurchased debt amount						$ 15,000,000