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Commitments and Contingencies Contractual commitments (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Affordable Housing Project Investment [Member]    
Long-term Purchase Commitment [Line Items]    
Contractual obligation $ 4,556us-gaap_ContractualObligation
/ us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis
= wtba_AffordableHousingProjectInvestmentMember
$ 2,727us-gaap_ContractualObligation
/ us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis
= wtba_AffordableHousingProjectInvestmentMember
Coralville, Iowa [Member] | Building Construction [Member]    
Long-term Purchase Commitment [Line Items]    
Payments for construction in process 2,870us-gaap_PaymentsForConstructionInProcess
/ wtba_LocationsOfBankBranchesAxis
= wtba_CoralvilleIowaMember
/ us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis
= wtba_BuildingConstructionMember
 
Contractual obligation $ 3,287us-gaap_ContractualObligation
/ wtba_LocationsOfBankBranchesAxis
= wtba_CoralvilleIowaMember
/ us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis
= wtba_BuildingConstructionMember