Allowance for Loan and Lease Losses and Reserve for Unfunded Commitments - Summary of TDR's by Loan Category (Details) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2021 USD ($) Loan	Dec. 31, 2020 USD ($) Loan
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	16	30
Pre-Modification Outstanding Recorded Investment		$ 8,524
Post-Modification Outstanding Recorded Investment	$ 4,213	8,145
Charge-offs	739	379
Specific Reserves	$ 1,091	$ 3,441
Accruing
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	10	12
Pre-Modification Outstanding Recorded Investment	$ 2,366	$ 2,495
Post-Modification Outstanding Recorded Investment	2,366	2,495
Specific Reserves	$ 272	$ 182
Non-accruing
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	6	18
Pre-Modification Outstanding Recorded Investment	$ 2,586	$ 6,029
Post-Modification Outstanding Recorded Investment	1,847	5,650
Charge-offs	739	379
Specific Reserves	819	$ 3,259
Commercial Real Estate
Financing Receivable Modifications [Line Items]
Pre-Modification Outstanding Recorded Investment	$ 4,952
Commercial Real Estate \| Accruing
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	7	8
Pre-Modification Outstanding Recorded Investment	$ 2,130	$ 2,187
Post-Modification Outstanding Recorded Investment	2,130	2,187
Specific Reserves	$ 209	$ 104
Commercial Real Estate \| Non-accruing
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	3	4
Pre-Modification Outstanding Recorded Investment	$ 856	$ 1,609
Post-Modification Outstanding Recorded Investment	740	1,362
Charge-offs	116	247
Specific Reserves	$ 311	$ 102
Commercial and Industrial \| Accruing
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	1	1
Pre-Modification Outstanding Recorded Investment	$ 63	$ 78
Post-Modification Outstanding Recorded Investment	63	78
Specific Reserves	$ 63	$ 78
Commercial and Industrial \| Non-accruing
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	3	14
Pre-Modification Outstanding Recorded Investment	$ 1,730	$ 4,420
Post-Modification Outstanding Recorded Investment	1,107	4,288
Charge-offs	623	132
Specific Reserves	$ 508	$ 3,157
Residential Real Estate \| Accruing
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	2	3
Pre-Modification Outstanding Recorded Investment	$ 173	$ 230
Post-Modification Outstanding Recorded Investment	173	230
Specific Reserves	$ 0	$ 0

Financing receivable modifications specific reserves.

No definition available.

Name:	by_FinancingReceivableModificationsSpecificReserves
Namespace Prefix:	by_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Reflects the amount of the write-down related to a troubled debt restructuring.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18845-111554

+ Details

Name:	us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=by_AccruingLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=by_NonAccruingLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=by_CommercialAndIndustrialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: