Regulatory Capital (Details) $ in Thousands		3 Months Ended
	Jun. 01, 2020 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Total Capital (to Risk Weighted Assets), Amount
Actual, Amount			$ 1,225,719	$ 1,036,143
Minimum Capital Requirement, Amount			589,691	590,826
Minimum To Be Well Capitalized, Amount			$ 737,114	$ 738,532
Total Capital (to Risk Weighted Assets), Ratio
Actual, Ratio (as a percent)			16.63	14.03
Minimum Capital Requirement, Ratio (as a percent)			8.00	8.00
Minimum To Be Well Capitalized, Ratio (as a percent)			10.00	10.00
Tier 1 Capital (to Risk Weighted Assets), Amount
Actual, Amount			$ 964,399	$ 922,395
Minimum Capital Requirement, Amount			442,268	443,120
Minimum To Be Well Capitalized, Amount			$ 589,691	$ 590,826
Tier 1 Capital (to Risk Weighted Assets), Ratio
Actual, Ratio (as a percent)			13.08	12.49
Minimum Capital Requirement, Ratio (as a percent)			6.00	6.00
Minimum To Be Well Capitalized, Ratio (as a percent)			8.00	8.00
Common Equity Tier 1 Capital (to Risk Weighted Assets), Amount
Actual, Amount			$ 890,399	$ 848,395
Minimum Capital Requirement, Amount			331,701	332,340
Minimum To Be Well Capitalized, Amount			$ 479,124	$ 480,046
Common Equity Tier 1 Capital (to Risk Weighted Assets), Ratio
Actual, Ratio (as a percent)			12.08%	11.49%
Minimum Capital Requirement, Ratio (as a percent)			4.50%	4.50%
Minimum To Be Well Capitalized, Ratio (as a percent)			6.50%	6.50%
Tier 1 Capital (to Average Assets), Amount
Actual, Amount			$ 964,399	$ 922,395
Minimum Capital Requirement, Amount			$ 410,850	$ 373,360
Tier 1 Capital (to Average Assets), Ratio
Actual, Ratio (as a percent)			9.39	9.88
Minimum Capital Requirement, Ratio (as a percent)			4.00	4.00
Busey Bank
Total Capital (to Risk Weighted Assets), Amount
Actual, Amount			$ 1,095,863	$ 1,099,449
Minimum Capital Requirement, Amount			588,690	589,681
Minimum To Be Well Capitalized, Amount			$ 735,862	$ 737,101
Total Capital (to Risk Weighted Assets), Ratio
Actual, Ratio (as a percent)			14.89	14.92
Minimum Capital Requirement, Ratio (as a percent)			8.00	8.00
Minimum To Be Well Capitalized, Ratio (as a percent)			10.00	10.00
Tier 1 Capital (to Risk Weighted Assets), Amount
Actual, Amount			$ 1,019,543	$ 1,045,701
Minimum Capital Requirement, Amount			441,517	442,261
Minimum To Be Well Capitalized, Amount			$ 588,690	$ 589,681
Tier 1 Capital (to Risk Weighted Assets), Ratio
Actual, Ratio (as a percent)			13.86	14.19
Minimum Capital Requirement, Ratio (as a percent)			6.00	6.00
Minimum To Be Well Capitalized, Ratio (as a percent)			8.00	8.00
Common Equity Tier 1 Capital (to Risk Weighted Assets), Amount
Actual, Amount			$ 1,019,543	$ 1,045,701
Minimum Capital Requirement, Amount			331,138	331,696
Minimum To Be Well Capitalized, Amount			$ 478,310	$ 479,116
Common Equity Tier 1 Capital (to Risk Weighted Assets), Ratio
Actual, Ratio (as a percent)			13.86%	14.19%
Minimum Capital Requirement, Ratio (as a percent)			4.50%	4.50%
Minimum To Be Well Capitalized, Ratio (as a percent)			6.50%	6.50%
Tier 1 Capital (to Average Assets), Amount
Actual, Amount			$ 1,019,543	$ 1,045,701
Minimum Capital Requirement, Amount			410,669	373,639
Minimum To Be Well Capitalized, Amount			$ 513,336	$ 467,049
Tier 1 Capital (to Average Assets), Ratio
Actual, Ratio (as a percent)			9.93	11.19
Minimum Capital Requirement, Ratio (as a percent)			4.00	4.00
Minimum To Be Well Capitalized, Ratio (as a percent)			5.00	5.00
Subordinated debt | 5.25% fixed-to-floating rate notes due 2030
Regulatory Capital
Proceeds from public offering	$ 125,000	$ 125,000