Portfolio loans - Narrative (Details)	3 Months Ended
Mar. 31, 2020 USD ($) loan	Mar. 31, 2019 USD ($) loan
Distributions of portfolio loans
Gross interest income that would have been recorded if impaired loans had been current	$ 500,000	$ 700,000
Loans classified as a TDR		$ 0
TDRs subsequently classified as non-performing and had payment defaults \| loan	0	0
Commercial Loans
Distributions of portfolio loans
Loans classified as a TDR	$ 500,000
Commercial Real Estate Loans
Distributions of portfolio loans
Loans classified as a TDR	700,000
Retail Real Estate
Distributions of portfolio loans
Loans classified as a TDR		$ 3,100,000
Performing loans classified as TDRs \| loan		1
Loans in the process of foreclosure	1,300,000
Maximum \| Commercial Real Estate Loans
Distributions of portfolio loans
Limit where loans are processed through an expedited underwriting process	1,000,000.0
Watch \| Commercial Loans
Distributions of portfolio loans
Limit above which loans are annually reviewed	1,000,000.0
Special Mention
Distributions of portfolio loans
Limit above which loans are annually reviewed	$ 350,000

Represents the limit of loan above which loans are reviewed annually.

No definition available.

Name:	buse_LoansAndLeasesReceivableLimitAboveWhichLoansAreAnnuallyReviewed
Namespace Prefix:	buse_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the limit of loan where loans are processed through an expedited underwriting process.

+ References

No definition available.

+ Details

Name:	buse_LoansAndLeasesReceivableLimitAboveWhichLoansAreProcessedThroughExpeditedUnderwritingProcess
Namespace Prefix:	buse_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents residential real estate loans that are in the process of foreclosure.

+ References

No definition available.

+ Details

Name:	buse_LoansInProcessOfForeclosure
Namespace Prefix:	buse_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

No definition available.

+ Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of financing receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118939409&loc=d3e18794-111554

+ Details

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

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=buse_RetailRealEstateMember
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:	us-gaap_InternalCreditAssessmentAxis=buse_WatchListMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: