Amortized cost excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

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

- Definition

Percentage, excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty to total financing receivables.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesExcludingAccruedInterestPercent
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
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

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-34

Reference 3: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-33

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

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

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ContractualInterestRateReductionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_PaymentDeferralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

Loans, Leases and Allowance for Credit Losses - Amortized Cost Basis of Loans Modified (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 1	$ 70	$ 238	$ 177
% of Total Class of Financing Receivable	0.00%	0.10%	0.40%	0.30%
Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 0	$ 0	$ 40	$ 78
Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0	0	1	0
Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	1	70	197	99
Tech & innovation
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			$ 24	$ 29
% of Total Class of Financing Receivable			0.70%	0.90%
Tech & innovation \| Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			$ 5	$ 0
Tech & innovation \| Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			1	0
Tech & innovation \| Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			18	29
Other commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 1		$ 84	$ 8
% of Total Class of Financing Receivable	0.00%		0.80%	0.10%
Other commercial and industrial \| Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 0		$ 0	$ 8
Other commercial and industrial \| Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	0		0	0
Other commercial and industrial \| Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis	$ 1		84	0
CRE - owner occupied
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				$ 31
% of Total Class of Financing Receivable				1.80%
CRE - owner occupied \| Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				$ 31
CRE - owner occupied \| Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0
CRE - owner occupied \| Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis				0
Other CRE - non-owner occupied
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		$ 70	$ 91	$ 70
% of Total Class of Financing Receivable		1.10%	1.40%	1.10%
Other CRE - non-owner occupied \| Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		$ 0	$ 35	$ 0
Other CRE - non-owner occupied \| Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		0	0	0
Other CRE - non-owner occupied \| Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis		$ 70	56	70
Construction and land development
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			$ 39	$ 39
% of Total Class of Financing Receivable			0.90%	0.80%
Construction and land development \| Term Extension
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			$ 0	$ 39
Construction and land development \| Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			0	0
Construction and land development \| Payment Delay
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amortized cost basis			$ 39	$ 0