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Cash and Due from Banks (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Cash and Cash Equivalents [Abstract]    
Average reserve deposited with the Board of Governors of the Federal Reserve Bank $ 8.0wash_CashReserveDepositedAtFederalReserveBank $ 6.7wash_CashReserveDepositedAtFederalReserveBank
Interest-bearing balances due from banks $ 42.7us-gaap_InterestBearingDepositsInBanks $ 51.8us-gaap_InterestBearingDepositsInBanks