Deposits - Summary of Total Earnings Credit and Referral Costs (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2026 |
Jun. 30, 2025 |
Jun. 30, 2026 |
Jun. 30, 2025 |
|
| Deposit Liability [Line Items] | ||||
| Deposits, Earnings Credit And Referral Costs | $ 234.2 | $ 208.5 | $ 448.4 | $ 400.7 |
| Income Statement Location [Axis]: us-gaap:InterestAndDividendIncomeOperating | ||||
| Deposit Liability [Line Items] | ||||
| Deposits, Earnings Credit And Referral Costs | 52.5 | 61.3 | 101.2 | 119.4 |
| Income Statement Location [Axis]: wal:NonInterestExpenseDepositCosts | ||||
| Deposit Liability [Line Items] | ||||
| Deposits, Earnings Credit And Referral Costs | 173.3 | 142.8 | 330.5 | 272.7 |
| Other deposit related costs | 5.9 | 4.6 | 12.0 | 11.5 |
| Income Statement Location [Axis]: wal:ServiceChargesAndLoanFees | ||||
| Deposit Liability [Line Items] | ||||
| Deposits, Earnings Credit And Referral Costs | $ 8.4 | $ 4.4 | $ 16.7 | $ 8.6 |