Derivatives and Hedging Activities - Net Gain (Loss) on Derivatives Included in Income (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	$ 2.6	$ (4.5)	$ 25.8	$ (41.5)
Income Statement Location [Axis]: us-gaap:AssetsFairValueAdjustment
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	$ 10.3	$ 2.1	$ 14.1	$ 4.7
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Assets, Fair Value Adjustment	Assets, Fair Value Adjustment	Assets, Fair Value Adjustment	Assets, Fair Value Adjustment
Income Statement Location [Axis]: us-gaap:NoninterestIncomeOtherOperatingIncome
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	$ 5.5	$ 0.3	$ 9.9	$ 2.3
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other income	Other income	Other income	Other income
Income Statement Location [Axis]: wal:IncomefromEquityInvestments
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	$ 3.8	$ 0.2	$ 3.0	$ (0.3)
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Income (loss) from equity investments	Income (loss) from equity investments	Income (loss) from equity investments	Income (loss) from equity investments
Income Statement Location [Axis]: wal:LoanServicingFeesNet
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	$ (10.8)	$ 6.6	$ (33.8)	$ 34.5
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Net loan servicing revenue	Net loan servicing revenue	Net loan servicing revenue	Net loan servicing revenue
Income Statement Location [Axis]: wal:NetGainOnMortgageLoanOriginationAndSaleActivities
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	$ 2.7	$ (4.5)	$ 25.8	$ (41.5)
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	Net gain on mortgage loan origination and sale activities	Net gain on mortgage loan origination and sale activities
Interest rate lock commitments
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	$ 23.2	$ 10.8	$ 0.7	$ 25.8
Forward contracts \| Income Statement Location [Axis]: wal:LoanServicingFeesNet
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	3.4	(9.3)	(24.4)	(1.9)
Forward contracts \| Income Statement Location [Axis]: wal:NetGainOnMortgageLoanOriginationAndSaleActivities
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	(28.6)	(17.5)	16.3	(73.0)
Futures contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	4.1	(0.6)	24.2	(6.4)
Interest rate contracts \| Income Statement Location [Axis]: us-gaap:AssetsFairValueAdjustment
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	4.2	2.0	7.6	4.5
Interest rate contracts \| Income Statement Location [Axis]: wal:LoanServicingFeesNet
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	(18.3)	16.5	(33.6)	42.8
Interest rate contracts \| Income Statement Location [Axis]: wal:NetGainOnMortgageLoanOriginationAndSaleActivities
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	3.0	2.3	6.7	2.3
Other Contract
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	5.1	(0.1)	2.1	3.4
Covered call options
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	5.8	0.0	6.1	0.0
Risk participation agreements
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	0.3	0.1	0.4	0.2
Equity warrants
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	3.8	0.2	3.0	(0.3)
Foreign currency contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total change in fair value of derivatives	$ 5.5	$ 0.3	$ 9.9	$ 2.3

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

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