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Business and Summary of Significant Accounting Policies - Cash and cash equivalents (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Business Description And Accounting Policies [Abstract] |  |
Reserve requirement | $ 15.8 | $ 14.1

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- Definition Business description and accounting policies. + References No definition available. + Details
Name: | by_BusinessDescriptionAndAccountingPoliciesAbstract
Namespace Prefix: | by_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Cash on hand or on deposit with the Federal Reserve Bank required to meet regulatory reserve and clearing requirements. + References No definition available. + Details
Name: | by_ReserveRequirementCashAndCashEquivalentsBank
Namespace Prefix: | by_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant