STOCK-BASED COMPENSATION - Schedule of Unamortized Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2025	Dec. 31, 2024
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total unamortized stock-based compensation	$ 19,090	$ 10,317
SSARs
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total unamortized stock-based compensation	114	0
RSU Awards
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total unamortized stock-based compensation	11,402	7,093
PSU Awards
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total unamortized stock-based compensation	7,146	3,043
DSU Awards
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total unamortized stock-based compensation	$ 428	$ 181
Busey Awards \| 2020 Equity Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Weighted average period over which expense is to be recognized on awards issued under Busey's 2020 Equity Plan	1 year 10 months 24 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 awards issued under Busey's 2020 Equity Plan	1 year 3 months 18 days

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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