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Cash and Due from Banks (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Cash and Cash Equivalents [Abstract]    
Average reserve deposited with the Board of Governors of the Federal Reserve Bank $ 10.1wash_CashReserveDepositedAtFederalReserveBank $ 8.0wash_CashReserveDepositedAtFederalReserveBank
Interest-bearing deposits in other banks $ 47.0us-gaap_InterestBearingDepositsInBanks $ 42.7us-gaap_InterestBearingDepositsInBanks