PORTFOLIO LOANS - Schedule of Activity on the Allowance for Loan Losses (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Activity in the allowance for loan losses
ACL beginning balance	$ 83,404	$ 91,740	$ 91,608
Day 1 PCD	100,783	1,243
Day 2 Provision for loan losses	52,743	7,495	2,860
Provision for loan losses	3,313	8,590	2,399
Charged-off	(59,642)	(19,566)	(4,418)
Recoveries	3,732	1,397	2,151
ACL ending balance	174,023	83,404	91,740
CrossFirst
Activity in the allowance for loan losses
Day 2 Provision for loan losses	42,433
Charge-offs	36,800
Commercial loans \| C&I and other commercial
Activity in the allowance for loan losses
ACL beginning balance	21,589	21,256	23,860
Day 1 PCD	75,569	824
Provision for loan losses	(16,574)	14,455	(727)
Charged-off	(44,559)	(15,453)	(2,429)
Recoveries	2,697	507	552
ACL ending balance	61,370	21,589	21,256
Commercial loans \| C&I and other commercial \| CrossFirst
Activity in the allowance for loan losses
Day 2 Provision for loan losses	22,648
Commercial loans \| CRE
Activity in the allowance for loan losses
ACL beginning balance	32,301	35,465	38,299
Day 1 PCD	21,588	322
Provision for loan losses	13,677	(318)	(2,455)
Charged-off	(12,607)	(3,314)	(953)
Recoveries	265	146	574
ACL ending balance	70,328	32,301	35,465
Commercial loans \| CRE \| CrossFirst
Activity in the allowance for loan losses
Day 2 Provision for loan losses	15,104
Commercial loans \| Real estate construction
Activity in the allowance for loan losses
ACL beginning balance	3,345	5,163	6,457
Day 1 PCD	2,112	0
Provision for loan losses	3,105	(1,885)	(1,465)
Charged-off	0	0	0
Recoveries	95	67	171
ACL ending balance	11,568	3,345	5,163
Commercial loans \| Real estate construction \| CrossFirst
Activity in the allowance for loan losses
Day 2 Provision for loan losses	2,911
Retail loans \| CRE
Activity in the allowance for loan losses
ACL beginning balance	23,711	26,298	18,193
Day 1 PCD	1,430	96
Provision for loan losses	3,159	(3,031)	7,922
Charged-off	(1,251)	(168)	(407)
Recoveries	501	516	590
ACL ending balance	29,178	23,711	26,298
Retail loans \| CRE \| CrossFirst
Activity in the allowance for loan losses
Day 2 Provision for loan losses	1,628
Retail loans \| Retail other
Activity in the allowance for loan losses
ACL beginning balance	2,458	3,558	4,799
Day 1 PCD	84	1
Provision for loan losses	(54)	(631)	(876)
Charged-off	(1,225)	(631)	(629)
Recoveries	174	161	264
ACL ending balance	1,579	$ 2,458	$ 3,558
Retail loans \| Retail other \| CrossFirst
Activity in the allowance for loan losses
Day 2 Provision for loan losses	$ 142

Financing Receivable, Excluding Accrued Interest, Credit Loss Expense (Reversal), Excluding Business Acquisition

No definition available.

Name:	buse_FinancingReceivableExcludingAccruedInterestCreditLossExpenseReversalExcludingBusinessAcquisition
Namespace Prefix:	buse_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, excluding accrued interest, of increase in allowance for credit loss on financing receivable from recovery. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRecovery
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, excluding accrued interest, of decrease in allowance for credit loss on financing receivable from writeoff. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-6

+ Details

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

- Definition

Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147476148/220-40-50-22

+ Details

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

- Definition

Amount, excluding accrued interest, of allowance for credit loss at acquisition date of financing receivable purchased with credit deterioration. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 19
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-19

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestPurchasedWithCreditDeteriorationAllowanceForCreditLossAtAcquisitionDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of allowance for credit loss at acquisition date of financing receivable purchased with credit deterioration.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 19
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-19

+ Details

Name:	us-gaap_FinancingReceivablePurchasedWithCreditDeteriorationAllowanceForCreditLossAtAcquisitionDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionAxis=buse_CrossFirstBanksharesIncMember
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_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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