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v3.20.2

Significant Accounting Policies - Subsequent Events (Details) - Center | 3 Months Ended
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Dec. 31, 2020 | Sep. 30, 2020
Subsequent event |  |
Subsequent Event [Line Items] |  |
Number of banking centers consolidated | 12 | 12

X
- Definition Represents the number of banking centers consolidated. + References No definition available. + Details
Name: | buse_NumberOfBankingCentersConsolidated
Namespace Prefix: | buse_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |