Derivatives and Hedging Activities - Net Gain (Loss) on Derivatives Included in Income (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	$ 23.2	$ (37.0)
Net gain on loan origination and sale activities
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	$ 23.2	$ (37.1)
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Net gain on mortgage loan origination and sale activities	Net gain on mortgage loan origination and sale activities
Loan servicing fees, net
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	$ (23.0)	$ 27.9
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Net loan servicing (loss) revenue	Net loan servicing (loss) revenue
Fair value gain adjustments, net
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	$ 3.8	$ 2.6
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Assets, Fair Value Adjustment	Assets, Fair Value Adjustment
Income (loss) from equity investments
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	$ (0.8)	$ (0.5)
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Income (loss) from equity investments	Income (loss) from equity investments
Other non-interest income
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	$ 4.4	$ 2.0
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other income	Other income
Interest rate lock commitments
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	$ (22.5)	$ 15.0
Forward contracts \| Net gain on loan origination and sale activities
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	45.0	(55.6)
Forward contracts \| Loan servicing fees, net
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	(27.8)	7.4
Forward contracts \| Fair value gain adjustments, net
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	3.4	2.4
Futures contracts \| Loan servicing fees, net
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	20.1	(5.8)
Futures contracts \| Fair value gain adjustments, net
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	0.3	0.0
Interest rate contracts \| Net gain on loan origination and sale activities
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	3.7	0.0
Interest rate contracts \| Loan servicing fees, net
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	(15.3)	26.3
Interest rate contracts \| Fair value gain adjustments, net
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	0.1	0.2
Equity warrants
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	(0.8)	(0.5)
Foreign currency contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	4.4	2.0
Other Contract
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	$ (3.0)	$ 3.5

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4A

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Name Accounting Standards Codification
-Section 55
-Paragraph 10
-SubTopic 10
-Topic 825
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482881/825-10-55-10

+ Details

Name:	us-gaap_DerivativeGainLossOnDerivativeNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Indicates line item in statement of income or comprehensive income that includes gain (loss) from derivative.

+ References

+ Details

Name:	us-gaap_DerivativeGainLossStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration
Namespace Prefix:	us-gaap_
Data Type:	enum2:enumerationSetItemType
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/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4EE
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4EE

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4EE
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4EE

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 181
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480401/815-10-55-181

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4EEE
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4EEE

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4EEE
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4EEE

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4EEE
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4EEE

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4EE
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4EE

+ Details

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

- Details

Name:	us-gaap_IncomeStatementLocationAxis=wal_NetGainOnLoanOriginationAndSaleActivitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=wal_LoanServicingFeesNetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=wal_FairValueGainAdjustmentsNetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=wal_IncomeLossFromEquityInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=wal_OtherNonInterestIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateLockCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForwardContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_FutureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_WarrantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_OtherContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: