Securities - Summary of Amortized Cost and Fair Values of Securities Available-for-sale (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	$ 1,281,305	$ 1,183,404
Available-for-sale Securities, Gross Unrealized Gains	26,372	7,593
Available-for-sale Securities, Gross Unrealized Losses	(8,194)	(4,705)
Available-for-sale Securities, Fair Value	1,299,483	1,186,292
U.S. Treasury Notes
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	37,424	41,403
Available-for-sale Securities, Gross Unrealized Gains	877	427
Available-for-sale Securities, Fair Value	38,301	41,830
U.S. Government Agencies
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	132,687	165,162
Available-for-sale Securities, Gross Unrealized Gains	1,024	542
Available-for-sale Securities, Gross Unrealized Losses		(754)
Available-for-sale Securities, Fair Value	133,711	164,950
Obligations of States, Municipalities, and Political Subdivisions
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	98,321	92,806
Available-for-sale Securities, Gross Unrealized Gains	3,078	2,075
Available-for-sale Securities, Gross Unrealized Losses	(107)	(49)
Available-for-sale Securities, Fair Value	101,292	94,832
Agency, Residential Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	596,664	490,427
Available-for-sale Securities, Gross Unrealized Gains	16,569	2,163
Available-for-sale Securities, Gross Unrealized Losses	(12)	(2,354)
Available-for-sale Securities, Fair Value	613,221	490,236
Non-Agency, Residential Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	87,683	109,501
Available-for-sale Securities, Gross Unrealized Gains	385	593
Available-for-sale Securities, Gross Unrealized Losses	(959)	(272)
Available-for-sale Securities, Fair Value	87,109	109,822
Agency, Commercial Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	195,255	159,650
Available-for-sale Securities, Gross Unrealized Gains	3,784	1,092
Available-for-sale Securities, Gross Unrealized Losses	(579)	(1,041)
Available-for-sale Securities, Fair Value	198,460	159,701
Non-Agency, Commercial Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	31,064	31,144
Available-for-sale Securities, Gross Unrealized Gains		130
Available-for-sale Securities, Gross Unrealized Losses	(521)
Available-for-sale Securities, Fair Value	30,543	31,274
Corporate Securities
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	51,691	48,796
Available-for-sale Securities, Gross Unrealized Gains	655	571
Available-for-sale Securities, Gross Unrealized Losses	(1,257)	(37)
Available-for-sale Securities, Fair Value	51,089	49,330
Asset-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	50,516	44,515
Available-for-sale Securities, Gross Unrealized Losses	(4,759)	(198)
Available-for-sale Securities, Fair Value	$ 45,757	$ 44,317

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

+ Details

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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