SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Net income attributable to Heartland	$ 39,809	$ 47,958	$ 30,131	$ 20,040	$ 37,851	$ 34,612	$ 45,169	$ 31,497	$ 137,938	$ 149,129	$ 116,998
Preferred dividends	(2,014)	(2,437)	0	0	0	0	0	0	(4,451)	0	(39)
NET INCOME AVAILABLE TO COMMON STOCKHOLDERS	$ 37,795	$ 45,521	$ 30,131	$ 20,040	$ 37,851	$ 34,612	$ 45,169	$ 31,497	$ 133,487	$ 149,129	$ 116,959
Weighted average common shares outstanding for basic earnings per share (in shares)	38,420,063	36,941,110	36,880,325	36,820,972	36,758,025	36,692,381	35,743,986	34,564,378	37,269,000	35,991,000	33,012,000
Assumed incremental common shares issued upon exercise of stock options and non-vested restricted stock units (in shares)									88,000	71,000	201,000
Weighted average common shares for diluted earnings per share (in shares)	38,534,082	36,995,572	36,915,630	36,895,591	36,840,519	36,835,191	35,879,259	34,699,839	37,357,000	36,062,000	33,213,000
Earnings per common share — basic (in dollars per share)	$ 0.98	$ 1.23	$ 0.82	$ 0.54	$ 1.03	$ 0.94	$ 1.26	$ 0.91	$ 3.58	$ 4.14	$ 3.54
Earnings per common share — diluted (in dollars per share)	$ 0.98	$ 1.23	$ 0.82	$ 0.54	$ 1.03	$ 0.94	$ 1.26	$ 0.91	$ 3.57	$ 4.14	$ 3.52