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Other Borrowings - Summary of Short-term Credit Lines Available for Use (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Federal Reserve Bank of Chicago Discount Window Line |  |
Debt Instrument [Line Items] |  |
Short-term credit lines available for use | $ 144,248 | $ 110,600
Available Federal Funds Line |  |
Debt Instrument [Line Items] |  |
Short-term credit lines available for use | $ 55,000 | $ 20,000

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- Definition Short-term line of credit available for use. + References No definition available. + Details
Name: | by_ShortTermLinesOfCreditAvailableForUse
Namespace Prefix: | by_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_LineOfCreditFacilityAxis=by_FederalReserveBankOfChicagoDiscountWindowLineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_LineOfCreditFacilityAxis=by_AvailableFederalFundsLineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |