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Servicing Assets - Unpaid Principal Balances of Loans Serviced for Others (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Loan portfolios serviced for:
Unpaid principal balances of loans serviced	$ 1,112,901	$ 1,017,928
SBA guaranteed loans
Loan portfolios serviced for:
Unpaid principal balances of loans serviced	1,021,143	911,803
USDA guaranteed loans
Loan portfolios serviced for:
Unpaid principal balances of loans serviced	$ 91,758	$ 106,125

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- Definition

This is the result of the principal amount outstanding of total loans managed or securitized less securitized loans and loans that are in the process of being securitized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=d3e107314-111719

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_FinancialInstrumentAxis=by_SBAGuaranteedLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancialInstrumentAxis=by_USDAGuaranteedLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: