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Fair Value Measurement - Additional Information (Details) - USD ($) | Sep. 30, 2020 | Sep. 30, 2019
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Fair value, assets, to or from level 3 transfers, amount | $ 0 | $ 0
Fair value, liabilities, to or from level 3 transfers, amount | 0 | 0
Fair value, equity, to or from level 3 transfers, amount | $ 0 | $ 0

X
- Definition Fair value assets to or from level three transfers amount. + References No definition available. + Details
Name: | by_FairValueAssetsToOrFromLevelThreeTransfersAmount
Namespace Prefix: | by_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Fair value equity to or from level three transfers amount. + References No definition available. + Details
Name: | by_FairValueEquityToOrFromLevelThreeTransfersAmount
Namespace Prefix: | by_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fair value liabilities to or from level three transfers amount. + References No definition available. + Details
Name: | by_FairValueLiabilitiesToOrFromLevelThreeTransfersAmount
Namespace Prefix: | by_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration