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Business and Summary of Significant Accounting Policies - Segment reporting (Details) | 12 Months Ended
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Dec. 31, 2017 segment
Business Description And Accounting Policies [Abstract] |
Number of reportable segment | 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

X
- Definition Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. + References No definition available. + Details
Name: | us-gaap_NumberOfReportableSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration