SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income attributable to HTLF	$ (70,363)	$ 48,091	$ 49,416	$ 52,776	$ 60,654	$ 56,564	$ 51,873	$ 43,089	$ 79,920	$ 212,180	$ 219,923
Preferred dividends	(2,012)	(2,013)	(2,012)	(2,013)	(2,012)	(2,013)	(2,012)	(2,013)	(8,050)	(8,050)	(8,050)
NET INCOME AVAILABLE TO COMMON STOCKHOLDERS	$ (72,375)	$ 46,078	$ 47,404	$ 50,763	$ 58,642	$ 54,551	$ 49,861	$ 41,076	$ 71,870	$ 204,130	$ 211,873
Weighted average common shares outstanding for basic earnings per share (in shares)	42,770,347	42,760,406	42,695,522	42,614,806	42,578,977	42,574,557	42,474,835	42,359,582	42,701,000	42,496,000	42,260,000
Assumed incremental common shares issued upon exercise of stock options and non-vested restricted stock units (in shares)									91,000	135,000	151,000
Weighted average common shares for diluted earnings per share (in shares)	42,838,405	42,812,563	42,757,603	42,742,878	42,699,752	42,643,940	42,565,391	42,540,953	42,792,000	42,631,000	42,411,000
Earnings per common share — basic (in dollars per share)	$ (1.69)	$ 1.08	$ 1.11	$ 1.19	$ 1.38	$ 1.28	$ 1.17	$ 0.97	$ 1.68	$ 4.80	$ 5.01
Earnings per common share — diluted (in dollars per share)	$ (1.69)	$ 1.08	$ 1.11	$ 1.19	$ 1.37	$ 1.28	$ 1.17	$ 0.97	$ 1.68	$ 4.79	$ 5.00
Stock Units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of antidilutive stock units excluded from diluted earnings per share computation									112,000	5,000	1,000
Employee Stock Option
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of antidilutive stock units excluded from diluted earnings per share computation									60,000	5,000	0