BASIS OF PRESENTATION (Narrative) (Details) $ / shares in Units, $ in Thousands			3 Months Ended	9 Months Ended
	Nov. 08, 2024 USD ($)	Apr. 27, 2024 $ / shares	Sep. 30, 2024 USD ($) $ / shares	Sep. 30, 2024 USD ($) segment $ / shares	Oct. 31, 2024 USD ($)	Apr. 28, 2024 $ / shares shares	Dec. 31, 2023 USD ($) $ / shares
Business Acquisition [Line Items]
Common stock, par value (in dollars per share) | $ / shares			$ 1	$ 1			$ 1
Pledged securities to support borrowings			$ 4,057,335	$ 4,057,335			$ 4,646,891
Number of operating segments | segment				1
Qualified Affordable Housing Investments
Affordable housing project investments			5,200	$ 5,200			$ 5,900
Affordable housing tax credits, amount			257	770
Affordable housing, other tax benefits, amount			33	98
Affordable housing tax credits and other tax benefits, amount			290	868
Affordable housing project investments, amortization			235	704
Income (loss) from affordable housing projects			55	164
Asset Pledged as Collateral
Business Acquisition [Line Items]
Pledged securities to support borrowings			$ 501,800	$ 501,800
Subsequent Event
Business Acquisition [Line Items]
Repayment of bank term funding program advances	$ 500,000
Interest Rate Swap, Protection For Unrealized Securities Losses | Fair value hedges | Subsequent Event
Business Acquisition [Line Items]
Notional amount					$ 838,100
Interest Rate Swap, Conversion From Fixed To Floating Rates | Fair value hedges | Subsequent Event
Business Acquisition [Line Items]
Notional amount					2,500,000
Fair value underlying asset					$ 1,800
Bank Merger
Business Acquisition [Line Items]
Common stock, par value (in dollars per share) | $ / shares						$ 1.00
Business combination, equity interest conversion per common share (in shares) | shares						0.55
Business combination, equity interest conversion, per preferred share (in shares) | shares						1
Bank Merger | HTLF
Business Acquisition [Line Items]
Common stock, par value (in dollars per share) | $ / shares		$ 1.00
Preferred stock, percentage		7.00%
Preferred stock, par value (in dollars per share) | $ / shares		$ 1.00