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Significant Accounting Policies - Subsequent Events (Details) - Subsequent event $ in Millions | 3 Months Ended
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Dec. 31, 2020 USD ($) Center
Subsequent Event [Line Items] |
Number of banking centers consolidated | Center | 12
Minimum |
Subsequent Event [Line Items] |
Non-operating pre-tax expenses | $ 7.0
Maximum |
Subsequent Event [Line Items] |
Non-operating pre-tax expenses | $ 7.5

X
- Definition It represents the non operating expenses before tax. + References No definition available. + Details
Name: | buse_NonOperatingExpensesBeforeTax
Namespace Prefix: | buse_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |