PORTFOLIO LOANS - Schedule of Loan Modifications for Borrowers Experiencing Financial Difficulty (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023
Payment Deferral
Financing Receivable, Modified [Line Items]
Amortized cost basis	$ 325
% of total class of financing receivable	0.00%
Payment Deferral \| Commercial loans \| C&I and other commercial
Financing Receivable, Modified [Line Items]
Amortized cost basis	$ 325
% of total class of financing receivable	0.00%	0.00%
Payment Deferral \| Commercial loans \| CRE
Financing Receivable, Modified [Line Items]
Amortized cost basis	$ 0
% of total class of financing receivable	0.00%
Payment Deferral \| Commercial loans \| Real estate construction
Financing Receivable, Modified [Line Items]
Amortized cost basis	$ 0
% of total class of financing receivable	0.00%
Term Extension
Financing Receivable, Modified [Line Items]
Amortized cost basis	$ 49,546	$ 22,836
% of total class of financing receivable	0.60%	0.30%
Term Extension \| Commercial loans \| C&I and other commercial
Financing Receivable, Modified [Line Items]
Amortized cost basis	$ 26,175	$ 16,586
% of total class of financing receivable	1.40%	0.90%
Term Extension \| Commercial loans \| CRE
Financing Receivable, Modified [Line Items]
Amortized cost basis	$ 18,147	$ 923
% of total class of financing receivable	0.60%	0.00%
Term Extension \| Commercial loans \| Real estate construction
Financing Receivable, Modified [Line Items]
Amortized cost basis	$ 5,224	$ 5,327
% of total class of financing receivable	1.40%	1.20%
Interest Rate Reduction
Financing Receivable, Modified [Line Items]
Amortized cost basis		$ 872
% of total class of financing receivable		0.00%
Interest Rate Reduction \| Commercial loans \| C&I and other commercial
Financing Receivable, Modified [Line Items]
Amortized cost basis		$ 0
% of total class of financing receivable		0.00%
Interest Rate Reduction \| Commercial loans \| CRE
Financing Receivable, Modified [Line Items]
Amortized cost basis		$ 872
% of total class of financing receivable		0.00%
Interest Rate Reduction \| Commercial loans \| Real estate construction
Financing Receivable, Modified [Line Items]
Amortized cost basis		$ 0
% of total class of financing receivable		0.00%

Amortized cost excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestModifiedPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage, excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty to total financing receivables.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesExcludingAccruedInterestPercent
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-34

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-33

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

Name:	us-gaap_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_PaymentDeferralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=buse_CommercialAndIndustrialAndOtherCommercialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_RealEstateLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ContractualInterestRateReductionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: