REGULATORY CAPITAL (Details) $ in Thousands	Mar. 31, 2026 USD ($)	Dec. 31, 2025 USD ($)
Common Equity Tier 1 Capital to Risk Weighted Assets, Amount
Actual	$ 1,880,068	$ 1,920,388
Minimum Capital Requirement	687,386	694,987
Minimum To Be Well Capitalized	$ 992,892	$ 1,003,870
Common Equity Tier 1 Capital to Risk Weighted Assets, Ratio
Actual	0.1231	0.1243
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	$ 2,102,818	$ 2,143,138
Minimum Capital Requirement	916,515	926,650
Minimum To Be Well Capitalized	$ 1,222,020	$ 1,235,533
Tier 1 Capital to Risk Weighted Assets, Ratio
Actual	0.1377	0.1388
Minimum Capital Requirement	0.0600	0.0600
Minimum To Be Well Capitalized	0.0800	0.0800
Total Capital to Risk Weighted Assets, Amount
Actual	$ 2,423,843	$ 2,459,847
Minimum Capital Requirement	1,222,020	1,235,533
Minimum To Be Well Capitalized	$ 1,527,525	$ 1,544,416
Total Capital to Risk Weighted Assets, Ratio
Actual	0.1587	0.1593
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	$ 2,102,818	$ 2,143,138
Minimum Capital Requirement	$ 708,101	$ 718,334
Leverage Ratio of Tier 1 Capital to Average Assets, Ratio
Actual	0.1188	0.1193
Minimum Capital Requirement	0.0400	0.0400
Busey Bank
Common Equity Tier 1 Capital to Risk Weighted Assets, Amount
Actual	$ 2,144,335	$ 2,150,048
Minimum Capital Requirement	686,292	692,654
Minimum To Be Well Capitalized	$ 991,311	$ 1,000,500
Common Equity Tier 1 Capital to Risk Weighted Assets, Ratio
Actual	0.1406	0.1397
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	$ 2,144,335	$ 2,150,048
Minimum Capital Requirement	915,057	923,539
Minimum To Be Well Capitalized	$ 1,220,075	$ 1,231,385
Tier 1 Capital to Risk Weighted Assets, Ratio
Actual	0.1406	0.1397
Minimum Capital Requirement	0.0600	0.0600
Minimum To Be Well Capitalized	0.0800	0.0800
Total Capital to Risk Weighted Assets, Amount
Actual	$ 2,285,782	$ 2,287,179
Minimum Capital Requirement	1,220,075	1,231,385
Minimum To Be Well Capitalized	$ 1,525,094	$ 1,539,231
Total Capital to Risk Weighted Assets, Ratio
Actual	0.1499	0.1486
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	$ 2,144,335	$ 2,150,048
Minimum Capital Requirement	706,482	716,476
Minimum To Be Well Capitalized	$ 883,103	$ 895,596
Leverage Ratio of Tier 1 Capital to Average Assets, Ratio
Actual	0.1214	0.1200
Minimum Capital Requirement	0.0400	0.0400
Minimum To Be Well Capitalized	0.0500	0.0500