Capital (Details) - USD ($) $ in Thousands	Oct. 12, 2017	Jan. 01, 2019	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Jul. 31, 2013
Total Capital (to Risk Weighted Assets), Amount
Actual			$ 865,366		$ 837,183
Minimum Capital Requirement with Capital Buffer			592,648		547,265
Minimum To Be Well Capitalized			$ 600,150		$ 591,638
Total Capital (to Risk Weighted Assets), Ratio
Actual (as a percent)			14.42%		14.15%
Minimum Capital Requirement with Capital Buffer (as a percent)			9.875%		9.25%
Minimum To Be Well Capitalized (as a percent)			10.00%		10.00%
Tier 1 Capital (to Risk Weighted Assets), Amount
Actual			$ 746,561		$ 718,101
Minimum Capital Requirement with Capital Buffer			472,618		428,937
Minimum To Be Well Capitalized			$ 480,120		$ 473,310
Tier 1 Capital (to Risk Weighted Assets), Ratio
Actual (as a percent)			12.44%		12.14%
Minimum Capital Requirement with Capital Buffer (as a percent)			7.875%		7.25%
Minimum To Be Well Capitalized (as a percent)			8.00%		8.00%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Amount
Actual			$ 672,561		$ 644,633
Minimum Capital Requirement with Capital Buffer			382,596		340,192
Minimum To Be Well Capitalized			$ 390,098		$ 384,565
Common Equity Tier 1 Capital (to Risk Weighted Assets), Ratio
Actual (as a percent)			11.21%		10.90%
Minimum Capital Requirement with Capital Buffer (as a percent)			6.375%		5.75%
Minimum To Be Well Capitalized (as a percent)			6.50%		6.50%
Tier 1 Capital (to Average Assets), Amount
Actual			$ 746,561		$ 718,101
Minimum Capital Requirement with Capital Buffer			$ 294,897		$ 293,588
Tier 1 Capital (to Average Assets), Ratio
Actual (as a percent)			10.13%		9.78%
Minimum Capital Requirement with Capital Buffer (as a percent)			4.00%		4.00%
Busey Bank
Total Capital (to Risk Weighted Assets), Amount
Actual			$ 837,823		$ 704,807
Minimum Capital Requirement with Capital Buffer			589,537		509,978
Minimum To Be Well Capitalized			$ 596,999		$ 551,327
Total Capital (to Risk Weighted Assets), Ratio
Actual (as a percent)			14.03%		12.78%
Minimum Capital Requirement with Capital Buffer (as a percent)			9.875%		9.25%
Minimum To Be Well Capitalized (as a percent)			10.00%		10.00%
Tier 1 Capital (to Risk Weighted Assets), Amount
Actual			$ 784,518		$ 651,432
Minimum Capital Requirement with Capital Buffer			470,137		399,713
Minimum To Be Well Capitalized			$ 477,599		$ 441,062
Tier 1 Capital (to Risk Weighted Assets), Ratio
Actual (as a percent)			13.14%		11.82%
Minimum Capital Requirement with Capital Buffer (as a percent)			7.875%		7.25%
Minimum To Be Well Capitalized (as a percent)			8.00%		8.00%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Amount
Actual			$ 784,518		$ 651,432
Minimum Capital Requirement with Capital Buffer			380,587		317,013
Minimum To Be Well Capitalized			$ 388,050		$ 358,363
Common Equity Tier 1 Capital (to Risk Weighted Assets), Ratio
Actual (as a percent)			13.14%		11.82%
Minimum Capital Requirement with Capital Buffer (as a percent)			6.375%		5.75%
Minimum To Be Well Capitalized (as a percent)			6.50%		6.50%
Tier 1 Capital (to Average Assets), Amount
Actual			$ 784,518		$ 651,432
Minimum Capital Requirement with Capital Buffer			293,941		265,847
Minimum To Be Well Capitalized			$ 367,427		$ 332,309
Tier 1 Capital (to Average Assets), Ratio
Actual (as a percent)			10.68%		9.80%
Minimum Capital Requirement with Capital Buffer (as a percent)			4.00%		4.00%
Minimum To Be Well Capitalized (as a percent)			5.00%		5.00%
South Side Bank
Total Capital (to Risk Weighted Assets), Amount
Actual					$ 84,914
Minimum Capital Requirement with Capital Buffer					34,744
Minimum To Be Well Capitalized					$ 37,561
Total Capital (to Risk Weighted Assets), Ratio
Actual (as a percent)					22.61%
Minimum Capital Requirement with Capital Buffer (as a percent)					9.25%
Minimum To Be Well Capitalized (as a percent)					10.00%
Tier 1 Capital (to Risk Weighted Assets), Amount
Actual					$ 84,707
Minimum Capital Requirement with Capital Buffer					27,232
Minimum To Be Well Capitalized					$ 30,049
Tier 1 Capital (to Risk Weighted Assets), Ratio
Actual (as a percent)					22.55%
Minimum Capital Requirement with Capital Buffer (as a percent)					7.25%
Minimum To Be Well Capitalized (as a percent)					8.00%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Amount
Actual					$ 84,707
Minimum Capital Requirement with Capital Buffer					21,598
Minimum To Be Well Capitalized					$ 24,415
Common Equity Tier 1 Capital (to Risk Weighted Assets), Ratio
Actual (as a percent)					22.55%
Minimum Capital Requirement with Capital Buffer (as a percent)					5.75%
Minimum To Be Well Capitalized (as a percent)					6.50%
Tier 1 Capital (to Average Assets), Amount
Actual					$ 84,707
Minimum Capital Requirement with Capital Buffer					26,571
Minimum To Be Well Capitalized					$ 33,214
Tier 1 Capital (to Average Assets), Ratio
Actual (as a percent)					12.75%
Minimum Capital Requirement with Capital Buffer (as a percent)					4.00%
Minimum To Be Well Capitalized (as a percent)					5.00%
Dodd-Frank Act ("Basel III Rules")
Regulatory Capital
Maximum consolidated assets of bank holding companies						$ 1,000,000
Maximum consolidated assets size of bank holding companies potentially unaffected by certain Tier 1 capital restrictions			$ 15,000,000
Common Equity Tier 1 capital conservation buffer of risk weighted assets			2.50%
Tier 1 Capital (to Average Assets), Ratio
Increase in minimum capital requirement ratios				1.875%
Forecast
Total Capital (to Risk Weighted Assets), Ratio
Minimum Capital Requirement with Capital Buffer (as a percent)		10.50%
Tier 1 Capital (to Risk Weighted Assets), Ratio
Minimum Capital Requirement with Capital Buffer (as a percent)		8.50%
Common Equity Tier 1 Capital (to Risk Weighted Assets), Ratio
Minimum Capital Requirement with Capital Buffer (as a percent)		7.00%
Busey Bank
Regulatory Capital
Distribution received through chartered amendment	$ 40,000