Name:	us-gaap_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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Number of financing receivables that have been modified by troubled debt restructurings.

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

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

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

Amount of financing receivable modified as troubled debt restructuring.

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=123577603&loc=SL6953676-111524

Reference 2: 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

Name:	us-gaap_FinancingReceivableModificationsRecordedInvestment
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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Number of financing receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953682-111524

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

Amount of financing receivable modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953682-111524

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

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

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

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Data Type:	xbrli:monetaryItemType
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Amount of unfunded portion of contractual agreement to lend funds to borrower for financing receivable modified as troubled debt restructuring.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18794-111554

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

Financing Receivable, CARES Act Modification

No definition available.

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Balance Type:	debit
Period Type:	instant

Financing Receivable, Troubled Debt Restructured Loans, Number of Outstanding Contracts

No definition available.

Name:	wal_FinancingReceivableTroubledDebtRestructuredLoansNumberOfOutstandingContracts
Namespace Prefix:	wal_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Loans in ForbearanceResidential HFI mortgage loans in forbearance, COVID-19-related modification.

No definition available.

Name:	wal_LoansInForbearance
Namespace Prefix:	wal_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

This element represents reserves allocated to customers whose loan terms modified in troubled debt restructurings.

No definition available.

Name:	wal_ReservesAllocatedToCustomersWhoseLoanTermsModifiedInTroubledDebtRestructurings
Namespace Prefix:	wal_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=wal_TechInnovationMember
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Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=wal_OthercommercialandindustrialMember
Namespace Prefix:
Data Type:	na
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Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=wal_CommercialRealEstateOwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=wal_HotelfranchisefinanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
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