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Commitments and Contingencies - Contractual Commitments for Lines and Letters of Credit by Maturity (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Loss Contingencies [Line Items]    
Total Amount Committed $ 2,256,117us-gaap_FairValueDisclosureOffbalanceSheetRisksFaceAmountLiability $ 1,943,243us-gaap_FairValueDisclosureOffbalanceSheetRisksFaceAmountLiability
Amount of Commitment Expiration Per Period, Less than one year 886,278us-gaap_OtherCommitmentDueInNextTwelveMonths  
Amount of Commitment Expiration Per Period, 1-3 Years 807,649us-gaap_OtherCommitmentDueInSecondAndThirdYear  
Amount of Commitment Expiration Per Period, 3-5 Years 270,893us-gaap_OtherCommitmentDueInFourthAndFifthYear  
Amount of Commitment Expiration Per Period, After 5 Years 291,297us-gaap_OtherCommitmentDueAfterFifthYear  
Commitments to extend credit [Member]    
Loss Contingencies [Line Items]    
Total Amount Committed 2,164,523us-gaap_FairValueDisclosureOffbalanceSheetRisksFaceAmountLiability
/ us-gaap_OtherCommitmentsAxis
= us-gaap_CommitmentsToExtendCreditMember		 1,878,340us-gaap_FairValueDisclosureOffbalanceSheetRisksFaceAmountLiability
/ us-gaap_OtherCommitmentsAxis
= us-gaap_CommitmentsToExtendCreditMember
Amount of Commitment Expiration Per Period, Less than one year 811,800us-gaap_OtherCommitmentDueInNextTwelveMonths
/ us-gaap_OtherCommitmentsAxis
= us-gaap_CommitmentsToExtendCreditMember		  
Amount of Commitment Expiration Per Period, 1-3 Years 794,270us-gaap_OtherCommitmentDueInSecondAndThirdYear
/ us-gaap_OtherCommitmentsAxis
= us-gaap_CommitmentsToExtendCreditMember		  
Amount of Commitment Expiration Per Period, 3-5 Years 267,156us-gaap_OtherCommitmentDueInFourthAndFifthYear
/ us-gaap_OtherCommitmentsAxis
= us-gaap_CommitmentsToExtendCreditMember		  
Amount of Commitment Expiration Per Period, After 5 Years 291,297us-gaap_OtherCommitmentDueAfterFifthYear
/ us-gaap_OtherCommitmentsAxis
= us-gaap_CommitmentsToExtendCreditMember		  
Credit card guarantees [Member]    
Loss Contingencies [Line Items]    
Total Amount Committed 42,038us-gaap_FairValueDisclosureOffbalanceSheetRisksFaceAmountLiability
/ us-gaap_OtherCommitmentsAxis
= wal_CreditCardCommitmentsAndFinancialGuaranteesMember		 33,632us-gaap_FairValueDisclosureOffbalanceSheetRisksFaceAmountLiability
/ us-gaap_OtherCommitmentsAxis
= wal_CreditCardCommitmentsAndFinancialGuaranteesMember
Amount of Commitment Expiration Per Period, Less than one year 42,038us-gaap_OtherCommitmentDueInNextTwelveMonths
/ us-gaap_OtherCommitmentsAxis
= wal_CreditCardCommitmentsAndFinancialGuaranteesMember		  
Amount of Commitment Expiration Per Period, 1-3 Years 0us-gaap_OtherCommitmentDueInSecondAndThirdYear
/ us-gaap_OtherCommitmentsAxis
= wal_CreditCardCommitmentsAndFinancialGuaranteesMember		  
Amount of Commitment Expiration Per Period, 3-5 Years 0us-gaap_OtherCommitmentDueInFourthAndFifthYear
/ us-gaap_OtherCommitmentsAxis
= wal_CreditCardCommitmentsAndFinancialGuaranteesMember		  
Amount of Commitment Expiration Per Period, After 5 Years 0us-gaap_OtherCommitmentDueAfterFifthYear
/ us-gaap_OtherCommitmentsAxis
= wal_CreditCardCommitmentsAndFinancialGuaranteesMember		  
Standby letters of credit [Member]    
Loss Contingencies [Line Items]    
Total Amount Committed 49,556us-gaap_FairValueDisclosureOffbalanceSheetRisksFaceAmountLiability
/ us-gaap_OtherCommitmentsAxis
= us-gaap_StandbyLettersOfCreditMember		 31,271us-gaap_FairValueDisclosureOffbalanceSheetRisksFaceAmountLiability
/ us-gaap_OtherCommitmentsAxis
= us-gaap_StandbyLettersOfCreditMember
Amount of Commitment Expiration Per Period, Less than one year 32,440us-gaap_OtherCommitmentDueInNextTwelveMonths
/ us-gaap_OtherCommitmentsAxis
= us-gaap_StandbyLettersOfCreditMember		  
Amount of Commitment Expiration Per Period, 1-3 Years 13,379us-gaap_OtherCommitmentDueInSecondAndThirdYear
/ us-gaap_OtherCommitmentsAxis
= us-gaap_StandbyLettersOfCreditMember		  
Amount of Commitment Expiration Per Period, 3-5 Years 3,737us-gaap_OtherCommitmentDueInFourthAndFifthYear
/ us-gaap_OtherCommitmentsAxis
= us-gaap_StandbyLettersOfCreditMember		  
Amount of Commitment Expiration Per Period, After 5 Years $ 0us-gaap_OtherCommitmentDueAfterFifthYear
/ us-gaap_OtherCommitmentsAxis
= us-gaap_StandbyLettersOfCreditMember