Portfolio Loans - Schedule of activity on the allowance for loan losses (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Activity in the allowance for loan losses
ACL beginning balance	$ 91,727	$ 88,213	$ 91,608	$ 87,887
Provision for credit losses	627	1,653	1,580	1,400
Charged-off	(1,347)	(1,679)	(2,528)	(1,804)
Recoveries	632	570	979	1,274
ACL ending balance	91,639	88,757	91,639	88,757
Commercial
Activity in the allowance for loan losses
ACL beginning balance	24,276	24,173	23,860	23,855
Provision for credit losses	690	(743)	1,385	(492)
Charged-off	(575)	(208)	(975)	(208)
Recoveries	119	137	240	204
ACL ending balance	24,510	23,359	24,510	23,359
Commercial real estate
Activity in the allowance for loan losses
ACL beginning balance	34,421	37,339	38,299	38,249
Provision for credit losses	(392)	1,028	(3,751)	(190)
Charged-off	(534)	(1,372)	(1,073)	(1,372)
Recoveries	161	187	181	495
ACL ending balance	33,656	37,182	33,656	37,182
Real estate construction
Activity in the allowance for loan losses
ACL beginning balance	5,159	5,705	6,457	5,102
Provision for credit losses	(179)	(63)	(1,508)	447
Charged-off	0	0	0	0
Recoveries	91	27	122	120
ACL ending balance	5,071	5,669	5,071	5,669
Retail real estate
Activity in the allowance for loan losses
ACL beginning balance	24,255	17,555	18,193	17,589
Provision for credit losses	353	312	6,301	142
Charged-off	(103)	(17)	(108)	(33)
Recoveries	170	134	289	286
ACL ending balance	24,675	17,984	24,675	17,984
Retail other
Activity in the allowance for loan losses
ACL beginning balance	3,616	3,441	4,799	3,092
Provision for credit losses	155	1,119	(847)	1,493
Charged-off	(135)	(82)	(372)	(191)
Recoveries	91	85	147	169
ACL ending balance	$ 3,727	$ 4,563	$ 3,727	$ 4,563

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

No definition available.

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 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-6

Reference 2: 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

+ 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

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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