FAIR VALUE MEASUREMENTS - Schedule of quantitative information about Level 3 fair value measurements (Details) $ in Thousands	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Fair Value
Loans evaluated individually, net of related allowance	$ 15,647	$ 15,380
OREO with subsequent impairment	70	3,074
Fair Value, Nonrecurring \| Loans evaluated individually, net of related allowance
Fair Value
Loans evaluated individually, net of related allowance	5,345	2,926
Fair Value, Nonrecurring \| OREO with subsequent impairment
Fair Value
OREO with subsequent impairment		51
Fair Value, Nonrecurring \| Bank property held for sale with impairment
Fair Value
Bank property held for sale with impairment	7,923	10,103
Fair Value, Nonrecurring \| Level 3 Inputs \| Loans evaluated individually, net of related allowance
Fair Value
Loans evaluated individually, net of related allowance	5,345	2,926
Fair Value, Nonrecurring \| Level 3 Inputs \| OREO with subsequent impairment
Fair Value
OREO with subsequent impairment		51
Fair Value, Nonrecurring \| Level 3 Inputs \| Bank property held for sale with impairment
Fair Value
Bank property held for sale with impairment	7,923	10,103
Fair Value, Nonrecurring \| Level 3 Inputs \| Appraisal of collateral \| Loans evaluated individually, net of related allowance
Fair Value
Loans evaluated individually, net of related allowance	5,345	2,926
Fair Value, Nonrecurring \| Level 3 Inputs \| Appraisal of collateral \| OREO with subsequent impairment
Fair Value
OREO with subsequent impairment		51
Fair Value, Nonrecurring \| Level 3 Inputs \| Appraisal of collateral or real estate listing price \| Bank property held for sale with impairment
Fair Value
Bank property held for sale with impairment	$ 7,923	$ 10,103
Fair Value, Nonrecurring \| Level 3 Inputs \| Measurement Input, Comparability Adjustment \| Appraisal of collateral \| Loans evaluated individually, net of related allowance \| Minimum
Range (Weighted Average)
Loans evaluated individually, net of related allowance	(0.227)	(0.500)
Fair Value, Nonrecurring \| Level 3 Inputs \| Measurement Input, Comparability Adjustment \| Appraisal of collateral \| Loans evaluated individually, net of related allowance \| Maximum
Range (Weighted Average)
Loans evaluated individually, net of related allowance	(1.000)	(1.000)
Fair Value, Nonrecurring \| Level 3 Inputs \| Measurement Input, Comparability Adjustment \| Appraisal of collateral \| Loans evaluated individually, net of related allowance \| Weighted Average
Range (Weighted Average)
Loans evaluated individually, net of related allowance	(0.457)	(0.551)
Fair Value, Nonrecurring \| Level 3 Inputs \| Measurement Input, Comparability Adjustment \| Appraisal of collateral \| OREO with subsequent impairment \| Minimum
Range (Weighted Average)
OREO with subsequent impairment		(0.330)
Fair Value, Nonrecurring \| Level 3 Inputs \| Measurement Input, Comparability Adjustment \| Appraisal of collateral \| OREO with subsequent impairment \| Maximum
Range (Weighted Average)
OREO with subsequent impairment		(1.000)
Fair Value, Nonrecurring \| Level 3 Inputs \| Measurement Input, Comparability Adjustment \| Appraisal of collateral \| OREO with subsequent impairment \| Weighted Average
Range (Weighted Average)
OREO with subsequent impairment		(0.679)
Fair Value, Nonrecurring \| Level 3 Inputs \| Measurement Input, Comparability Adjustment \| Appraisal of collateral or real estate listing price \| Bank property held for sale with impairment \| Minimum
Range (Weighted Average)
Bank property held for sale with impairment	(0.007)	(0.007)
Fair Value, Nonrecurring \| Level 3 Inputs \| Measurement Input, Comparability Adjustment \| Appraisal of collateral or real estate listing price \| Bank property held for sale with impairment \| Maximum
Range (Weighted Average)
Bank property held for sale with impairment	(0.701)	(0.701)
Fair Value, Nonrecurring \| Level 3 Inputs \| Measurement Input, Comparability Adjustment \| Appraisal of collateral or real estate listing price \| Bank property held for sale with impairment \| Weighted Average
Range (Weighted Average)
Bank property held for sale with impairment	(0.351)	(0.413)

The fair value of bank property held for sale.

No definition available.

Name:	buse_BankPropertyHeldForSaleFairValueDisclosure
Namespace Prefix:	buse_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Value of input used to measure alternative investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258

+ Details

Name:	us-gaap_AlternativeInvestmentMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=123577603&loc=SL6953550-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Value of input used to measure long-term debt.

+ References

+ Details

Name:	us-gaap_LongTermDebtMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Value of input used to measure real estate owned by financial institution by financial institution which is not directly related to its business.

+ References

+ Details

Name:	us-gaap_OtherRealEstateOwnedMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11
-URI https://asc.fasb.org/extlink&oid=123577603&loc=d3e5162-111524

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 3
-URI https://asc.fasb.org/extlink&oid=124259787&loc=d3e4437-111522

+ Details

Name:	us-gaap_RealEstateAcquiredThroughForeclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=buse_CollateralDependentLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_RealEstateAcquiredInSatisfactionOfDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=buse_BankPropertyHeldForSaleMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_MarketApproachValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=buse_AppraisalOfCollateralOrRealEstateListingPriceValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputComparabilityAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: