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Loans, Leases and Allowance for Credit Losses - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Leases [Line Items]      
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease) $ (0.2)us-gaap_FinancingReceivableAllowanceForCreditLossesPeriodIncreaseDecrease    
Reduction in interest income due to nonaccrual loans 3.8us-gaap_LoansAndLeasesReceivableImpairedInterestLostOnNonaccrualLoans 5.4us-gaap_LoansAndLeasesReceivableImpairedInterestLostOnNonaccrualLoans 5.5us-gaap_LoansAndLeasesReceivableImpairedInterestLostOnNonaccrualLoans
Loans And Loans Receivable Purchases 166.4wal_LoansAndLoansReceivablePurchases 241.8wal_LoansAndLoansReceivablePurchases  
TDR loans [Member]      
Leases [Line Items]      
Loan commitments outstanding 1.2wal_LoanCommitmentsOutstanding
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= wal_TroubledDebtRestructuredLoansMember
0wal_LoanCommitmentsOutstanding
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= wal_TroubledDebtRestructuredLoansMember
 
Related parties [Member]      
Leases [Line Items]      
Loan commitments outstanding 47.0wal_LoanCommitmentsOutstanding
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= wal_RelatedPartiesMember
51.6wal_LoanCommitmentsOutstanding
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= wal_RelatedPartiesMember
 
Commercial real estate [Member]      
Leases [Line Items]      
Loans And Loans Receivable Purchases   4.4wal_LoansAndLoansReceivablePurchases
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
 
Commercial and industrial [Member]      
Leases [Line Items]      
Loans And Loans Receivable Purchases $ 164.7wal_LoansAndLoansReceivablePurchases
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= wal_CommercialAndIndustrialMember
$ 232.8wal_LoansAndLoansReceivablePurchases
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= wal_CommercialAndIndustrialMember