Commitments and Contingencies Concentration of Lending Activities (Details)	Dec. 31, 2021	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Percent of commercial real estate loans owner occupied	23.00%	28.00%

Percentage of commercial real estate Loans occupied by owners.

No definition available.

Name:	wal_PercentageOfCommercialRealEstateLoansOccupiedByOwners
Namespace Prefix:	wal_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant