REGULATORY CAPITAL - Schedule of Regulatory Capital Requirements Applicable to Busey and Busey Bank (Details) $ in Thousands	Sep. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)
Common Equity Tier 1 Capital to Risk Weighted Assets, Amount
Actual	$ 1,907,982	$ 1,237,301
Minimum Capital Requirement	696,419	394,840
Minimum To Be Well Capitalized	$ 1,005,938	$ 570,325
Common Equity Tier 1 Capital to Risk Weighted Assets, Ratio
Actual	0.1233	0.1410
Minimum Capital Requirement	0.0450	0.0450
Minimum To Be Well Capitalized	0.0650	0.0650
Tier 1 Capital to Risk Weighted Assets, Amount
Actual	$ 2,130,732	$ 1,314,301
Minimum Capital Requirement	928,558	526,453
Minimum To Be Well Capitalized	$ 1,238,078	$ 701,938
Tier 1 Capital to Risk Weighted Assets, Ratio
Actual	0.1377	0.1498
Minimum Capital Requirement	0.0600	0.0600
Minimum To Be Well Capitalized	0.0800	0.0800
Total Capital to Risk Weighted Assets, Amount
Actual	$ 2,458,553	$ 1,625,943
Minimum Capital Requirement	1,238,078	701,938
Minimum To Be Well Capitalized	$ 1,547,597	$ 877,422
Total Capital to Risk Weighted Assets, Ratio
Actual	0.1589	0.1853
Minimum Capital Requirement	0.0800	0.0800
Minimum To Be Well Capitalized	0.1000	0.1000
Leverage Ratio of Tier 1 Capital to Average Assets, Amount
Actual	$ 2,130,732	$ 1,314,301
Minimum Capital Requirement	$ 733,327	$ 475,348
Leverage Ratio of Tier 1 Capital to Average Assets, Ratio
Actual	0.1162	0.1106
Minimum Capital Requirement	0.0400	0.0400
Busey Bank
Common Equity Tier 1 Capital to Risk Weighted Assets, Amount
Actual	$ 2,118,059	$ 1,438,296
Minimum Capital Requirement	694,126	393,277
Minimum To Be Well Capitalized	$ 1,002,626	$ 568,067
Common Equity Tier 1 Capital to Risk Weighted Assets, Ratio
Actual	0.1373	0.1646
Minimum Capital Requirement	0.0450	0.0450
Minimum To Be Well Capitalized	0.0650	0.0650
Tier 1 Capital to Risk Weighted Assets, Amount
Actual	$ 2,118,059	$ 1,438,296
Minimum Capital Requirement	925,501	524,369
Minimum To Be Well Capitalized	$ 1,234,002	$ 699,159
Tier 1 Capital to Risk Weighted Assets, Ratio
Actual	0.1373	0.1646
Minimum Capital Requirement	0.0600	0.0600
Minimum To Be Well Capitalized	0.0800	0.0800
Total Capital to Risk Weighted Assets, Amount
Actual	$ 2,262,302	$ 1,520,938
Minimum Capital Requirement	1,234,002	699,159
Minimum To Be Well Capitalized	$ 1,542,502	$ 873,949
Total Capital to Risk Weighted Assets, Ratio
Actual	0.1467	0.1740
Minimum Capital Requirement	0.0800	0.0800
Minimum To Be Well Capitalized	0.1000	0.1000
Leverage Ratio of Tier 1 Capital to Average Assets, Amount
Actual	$ 2,118,059	$ 1,438,296
Minimum Capital Requirement	731,219	473,878
Minimum To Be Well Capitalized	$ 914,024	$ 592,347
Leverage Ratio of Tier 1 Capital to Average Assets, Ratio
Actual	0.1159	0.1214
Minimum Capital Requirement	0.0400	0.0400
Minimum To Be Well Capitalized	0.0500	0.0500