PORTFOLIO LOANS - Schedule of activity on the allowance for loan losses (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Activity in the allowance for loan losses
ACL beginning balance	$ 87,887	$ 101,048	$ 53,748
Provision for credit losses	4,623	(15,101)	38,797
Charged-off	(3,179)	(4,783)	(11,088)
Recoveries	2,277	2,545	2,758
ACL ending balance	91,608	87,887	101,048
Adoption of ASC 326-30
Activity in the allowance for loan losses
ACL beginning balance		16,833
ACL ending balance			16,833
Financial Asset Acquired with Credit Deterioration
Activity in the allowance for loan losses
ACL beginning balance	4,178
ACL ending balance		4,178
Commercial
Activity in the allowance for loan losses
ACL beginning balance	23,855	23,866	18,291
Provision for credit losses	497	(2,160)	10,832
Charged-off	(1,069)	(2,026)	(6,376)
Recoveries	577	629	404
ACL ending balance	23,860	23,855	23,866
Commercial \| Adoption of ASC 326-30
Activity in the allowance for loan losses
ACL beginning balance		715
ACL ending balance			715
Commercial \| Financial Asset Acquired with Credit Deterioration
Activity in the allowance for loan losses
ACL beginning balance	3,546
ACL ending balance		3,546
Commercial real estate
Activity in the allowance for loan losses
ACL beginning balance	38,249	46,230	21,190
Provision for credit losses	892	(7,651)	17,511
Charged-off	(1,375)	(925)	(1,972)
Recoveries	533	259	195
ACL ending balance	38,299	38,249	46,230
Commercial real estate \| Adoption of ASC 326-30
Activity in the allowance for loan losses
ACL beginning balance		9,306
ACL ending balance			9,306
Commercial real estate \| Financial Asset Acquired with Credit Deterioration
Activity in the allowance for loan losses
ACL beginning balance	336
ACL ending balance		336
Real estate construction
Activity in the allowance for loan losses
ACL beginning balance	5,102	8,193	3,204
Provision for credit losses	1,142	(3,180)	1,452
Charged-off	(23)	(209)	(18)
Recoveries	236	298	601
ACL ending balance	6,457	5,102	8,193
Real estate construction \| Adoption of ASC 326-30
Activity in the allowance for loan losses
ACL beginning balance		2,954
ACL ending balance			2,954
Real estate construction \| Financial Asset Acquired with Credit Deterioration
Activity in the allowance for loan losses
ACL beginning balance	0
ACL ending balance		0
Retail real estate
Activity in the allowance for loan losses
ACL beginning balance	17,589	21,992	10,495
Provision for credit losses	219	(4,456)	9,050
Charged-off	(251)	(1,145)	(2,057)
Recoveries	636	1,069	1,212
ACL ending balance	18,193	17,589	21,992
Retail real estate \| Adoption of ASC 326-30
Activity in the allowance for loan losses
ACL beginning balance		3,292
ACL ending balance			3,292
Retail real estate \| Financial Asset Acquired with Credit Deterioration
Activity in the allowance for loan losses
ACL beginning balance	129
ACL ending balance		129
Retail other
Activity in the allowance for loan losses
ACL beginning balance	3,092	767	568
Provision for credit losses	1,873	2,346	(48)
Charged-off	(461)	(478)	(665)
Recoveries	295	290	346
ACL ending balance	4,799	3,092	767
Retail other \| Adoption of ASC 326-30
Activity in the allowance for loan losses
ACL beginning balance		566
ACL ending balance			$ 566
Retail other \| Financial Asset Acquired with Credit Deterioration
Activity in the allowance for loan losses
ACL beginning balance	$ 167
ACL ending balance		$ 167

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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

+ 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (e)
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

+ 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (d)
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

+ 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

+ Details

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

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditLossStatusAxis=us-gaap_FinancialAssetAcquiredWithCreditDeteriorationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialLoanMember
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: