STOCK-BASED COMPENSATION - Schedule of Unamortized Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2025	Dec. 31, 2024
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total unamortized stock-based compensation	$ 30,179	$ 10,317
SSARs
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total unamortized stock-based compensation	241	0
RSU Awards
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total unamortized stock-based compensation	18,174	7,093
PSU Awards
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total unamortized stock-based compensation	10,893	3,043
DSU Awards
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total unamortized stock-based compensation	$ 871	$ 181
Busey Awards
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Weighted average period over which expense is to be recognized on Busey awards	1 year 9 months 18 days	2 years 6 months
CrossFirst Awards
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Weighted average period over which expense is to be recognized on Busey awards	1 year 4 months 24 days