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Business and Summary of Significant Accounting Policies - Segment reporting (Details)	12 Months Ended
	Dec. 31, 2017 segment
Business Description And Accounting Policies [Abstract]
Number of reportable segment	1

X

- 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