CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025
Statement of Cash Flows [Abstract]
Loans transferred with original designation of HFS, operating cash flows	$ 16.4	$ 129.9

Loan and Lease, Transfer from Held-in-Portfolio to Held-for-Sale, Operating Activity

No definition available.

Name:	wal_LoanAndLeaseTransferFromHeldInPortfolioToHeldForSaleOperatingActivity
Namespace Prefix:	wal_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration