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v2.4.1.9

Fair Value Measurements - Investments by Contractual Maturity Date (Details) (USD $) | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Mar. 31, 2015 | Sep. 30, 2014
Amortized Cost, Available-for-sale securities |  |  |  |
Amortized Cost | $ 2,710,275,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost |  | $ 2,710,275,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost | $ 3,016,432,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost
Gross Unrealized Gains / Losses, Available-for-sale securities |  |  |  |
Gross unrealized gains | 50,132,000 wfsl_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments |  | 50,132,000 wfsl_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments | 37,273,000 wfsl_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
Gross unrealized loss | (3,501,000)wfsl_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments |  | (3,501,000)wfsl_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments | (4,263,000)wfsl_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
Fair Value, Available-for-sale securities |  |  |  |
Fair Value | 2,756,906,000 us-gaap_AvailableForSaleSecurities |  | 2,756,906,000 us-gaap_AvailableForSaleSecurities | 3,049,442,000 us-gaap_AvailableForSaleSecurities
Yield, Available-for-sale securities |  |  |  |
Yield | 1.99%wfsl_AvailableForSaleSecuritiesYield |  | 1.99%wfsl_AvailableForSaleSecuritiesYield | 1.99%wfsl_AvailableForSaleSecuritiesYield
Yield, Held-to-maturity securities |  |  |  |
Investments | 4,190,056,000 us-gaap_Investments |  | 4,190,056,000 us-gaap_Investments | 4,564,697,000 us-gaap_Investments
Gross unrealized gains on investments | 61,864,000 wfsl_GrossUnrealizedGainsOnInvestments |  | 61,864,000 wfsl_GrossUnrealizedGainsOnInvestments | 42,128,000 wfsl_GrossUnrealizedGainsOnInvestments
Gross unrealized losses on investments | (14,537,000)wfsl_GrossUnrealizedLossesOnInvestments |  | (14,537,000)wfsl_GrossUnrealizedLossesOnInvestments | (58,165,000)wfsl_GrossUnrealizedLossesOnInvestments
Total financial assets | 4,237,383,000 us-gaap_InvestmentsFairValueDisclosure |  | 4,237,383,000 us-gaap_InvestmentsFairValueDisclosure | 4,548,660,000 us-gaap_InvestmentsFairValueDisclosure
Yield on investments | 2.39%wfsl_Yield |  | 2.39%wfsl_Yield | 2.38%wfsl_Yield
Available-for-sale securities sold | 0 us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities | 0 us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities |  |
Term of Contractual Due Dates of Substantially All Mortgage-Backed Securities |  |  | 10 years |
US Treasury and Government [Member] |  |  |  |
Amortized Cost, Available-for-sale securities |  |  |  |
1 to 5 years | 142,283,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 142,283,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 171,154,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
5 to 10 years | 107,717,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 107,717,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 203,317,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Over 10 years | 281,951,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 281,951,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 354,828,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Gross Unrealized Gains / Losses, Available-for-sale securities |  |  |  |
Gross unrealized gains, 1 to 5 years | 2,300,000 wfsl_GrossUnrealizedGainsAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 2,300,000 wfsl_GrossUnrealizedGainsAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 2,585,000 wfsl_GrossUnrealizedGainsAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Gross unrealized gains, 5 to 10 years | 403,000 wfsl_GrossUnrealizedGainsAfterFiveThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 403,000 wfsl_GrossUnrealizedGainsAfterFiveThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 300,000 wfsl_GrossUnrealizedGainsAfterFiveThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Gross unrealized gains, Over 10 years | 436,000 wfsl_GrossUnrealizedGainsAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 436,000 wfsl_GrossUnrealizedGainsAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 1,028,000 wfsl_GrossUnrealizedGainsAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Gross unrealized losses, 1 to 5 years | (119,000)wfsl_GrossUnrealizedLossesAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | (119,000)wfsl_GrossUnrealizedLossesAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | (748,000)wfsl_GrossUnrealizedLossesAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Gross unrealized losses, 5 to 10 years | 0 wfsl_GrossUnrealizedLossesAfterFiveYearsThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 0 wfsl_GrossUnrealizedLossesAfterFiveYearsThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | (102,000)wfsl_GrossUnrealizedLossesAfterFiveYearsThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Gross unrealized losses, Over 10 years | (526,000)wfsl_GrossUnrealizedLossesOverTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | (526,000)wfsl_GrossUnrealizedLossesOverTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | (419,000)wfsl_GrossUnrealizedLossesOverTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Fair Value, Available-for-sale securities |  |  |  |
1 to 5 years | 144,464,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 144,464,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 172,991,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
5 to 10 years | 108,120,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 108,120,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 203,515,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Over 10 years | 281,861,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 281,861,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 355,437,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Yield, Available-for-sale securities |  |  |  |
1 to 5 years | 1.41%wfsl_AvailableforsaleSecuritiesYieldAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 1.41%wfsl_AvailableforsaleSecuritiesYieldAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 1.26%wfsl_AvailableforsaleSecuritiesYieldAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
5 to 10 years | 1.19%wfsl_AvailableforsaleSecuritiesYieldAfterFiveThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 1.19%wfsl_AvailableforsaleSecuritiesYieldAfterFiveThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 1.45%wfsl_AvailableforsaleSecuritiesYieldAfterFiveThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Over 10 years | 1.14%wfsl_AvailableforsaleSecuritiesYieldAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember |  | 1.14%wfsl_AvailableforsaleSecuritiesYieldAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 1.25%wfsl_AvailableforsaleSecuritiesYieldAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Equity Securities [Member] |  |  |  |
Amortized Cost, Available-for-sale securities |  |  |  |
Within 1 year | 500,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 500,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
1 to 5 years | 100,000,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 100,000,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 100,500,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
5 to 10 years | 0 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 0 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
Gross Unrealized Gains / Losses, Available-for-sale securities |  |  |  |
Gross unrealized gains, Within 1 year | 17,000 wfsl_GrossUnrealizedGainsWithinOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 17,000 wfsl_GrossUnrealizedGainsWithinOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
Gross unrealized gains, 1 to 5 years | 1,760,000 wfsl_GrossUnrealizedGainsAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 1,760,000 wfsl_GrossUnrealizedGainsAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 887,000 wfsl_GrossUnrealizedGainsAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Gross unrealized gains, 5 to 10 years | 0 wfsl_GrossUnrealizedGainsAfterFiveThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 0 wfsl_GrossUnrealizedGainsAfterFiveThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
Gross unrealized losses, Within 1 year | 0 wfsl_GrossUnrealizedLossesWithinOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 0 wfsl_GrossUnrealizedLossesWithinOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
Gross unrealized losses, 1 to 5 years | 0 wfsl_GrossUnrealizedLossesAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 0 wfsl_GrossUnrealizedLossesAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 0 wfsl_GrossUnrealizedLossesAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Gross unrealized losses, 5 to 10 years | 0 wfsl_GrossUnrealizedLossesAfterFiveYearsThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 0 wfsl_GrossUnrealizedLossesAfterFiveYearsThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
Fair Value, Available-for-sale securities |  |  |  |
Within 1 year | 517,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 517,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
1 to 5 years | 101,760,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 101,760,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 101,387,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
5 to 10 years | 0 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 0 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
Yield, Available-for-sale securities |  |  |  |
Within 1 year | 1.80%wfsl_AvailableforsaleSecuritiesYieldWithinOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 1.80%wfsl_AvailableforsaleSecuritiesYieldWithinOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
1 to 5 years | 1.90%wfsl_AvailableforsaleSecuritiesYieldAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 1.90%wfsl_AvailableforsaleSecuritiesYieldAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 1.90%wfsl_AvailableforsaleSecuritiesYieldAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
5 to 10 years | 0.00%wfsl_AvailableforsaleSecuritiesYieldAfterFiveThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 0.00%wfsl_AvailableforsaleSecuritiesYieldAfterFiveThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
Corporate Debt Securities [Member] |  |  |  |
Amortized Cost, Available-for-sale securities |  |  |  |
Within 1 year | 15,000,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 15,000,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 15,000,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
1 to 5 years | 327,715,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 327,715,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 302,540,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
5 to 10 years | 133,271,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 133,271,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 138,201,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Over 10 years | 50,000,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 50,000,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 50,000,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Gross Unrealized Gains / Losses, Available-for-sale securities |  |  |  |
Gross unrealized gains, Within 1 year | 0 wfsl_GrossUnrealizedGainsWithinOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 0 wfsl_GrossUnrealizedGainsWithinOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 75,000 wfsl_GrossUnrealizedGainsWithinOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Gross unrealized gains, 1 to 5 years | 1,860,000 wfsl_GrossUnrealizedGainsAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 1,860,000 wfsl_GrossUnrealizedGainsAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 2,372,000 wfsl_GrossUnrealizedGainsAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Gross unrealized gains, 5 to 10 years | 1,432,000 wfsl_GrossUnrealizedGainsAfterFiveThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 1,432,000 wfsl_GrossUnrealizedGainsAfterFiveThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 1,789,000 wfsl_GrossUnrealizedGainsAfterFiveThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Gross unrealized gains, Over 10 years | 250,000 wfsl_GrossUnrealizedGainsAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 250,000 wfsl_GrossUnrealizedGainsAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 0 wfsl_GrossUnrealizedGainsAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Gross unrealized losses, Within 1 year | (15,000)wfsl_GrossUnrealizedLossesWithinOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | (15,000)wfsl_GrossUnrealizedLossesWithinOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 0 wfsl_GrossUnrealizedLossesWithinOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Gross unrealized losses, 1 to 5 years | 0 wfsl_GrossUnrealizedLossesAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 0 wfsl_GrossUnrealizedLossesAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 0 wfsl_GrossUnrealizedLossesAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Gross unrealized losses, 5 to 10 years | (949,000)wfsl_GrossUnrealizedLossesAfterFiveYearsThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | (949,000)wfsl_GrossUnrealizedLossesAfterFiveYearsThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | (970,000)wfsl_GrossUnrealizedLossesAfterFiveYearsThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Gross unrealized losses, Over 10 years | 0 wfsl_GrossUnrealizedLossesOverTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 0 wfsl_GrossUnrealizedLossesOverTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 0 wfsl_GrossUnrealizedLossesOverTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Fair Value, Available-for-sale securities |  |  |  |
Within 1 year | 14,985,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 14,985,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 15,075,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
1 to 5 years | 329,575,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 329,575,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 304,912,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
5 to 10 years | 133,754,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 133,754,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 139,020,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Over 10 years | 50,250,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 50,250,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 50,000,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Yield, Available-for-sale securities |  |  |  |
Within 1 year | 1.00%wfsl_AvailableforsaleSecuritiesYieldWithinOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 1.00%wfsl_AvailableforsaleSecuritiesYieldWithinOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 1.00%wfsl_AvailableforsaleSecuritiesYieldWithinOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
1 to 5 years | 0.74%wfsl_AvailableforsaleSecuritiesYieldAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 0.74%wfsl_AvailableforsaleSecuritiesYieldAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 0.71%wfsl_AvailableforsaleSecuritiesYieldAfterOneThroughFiveYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
5 to 10 years | 1.48%wfsl_AvailableforsaleSecuritiesYieldAfterFiveThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 1.48%wfsl_AvailableforsaleSecuritiesYieldAfterFiveThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 1.43%wfsl_AvailableforsaleSecuritiesYieldAfterFiveThroughTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Over 10 years | 3.00%wfsl_AvailableforsaleSecuritiesYieldAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 3.00%wfsl_AvailableforsaleSecuritiesYieldAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 3.00%wfsl_AvailableforsaleSecuritiesYieldAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Municipal Bonds [Member] |  |  |  |
Amortized Cost, Available-for-sale securities |  |  |  |
Over 10 years | 20,392,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  | 20,392,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 20,402,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Gross Unrealized Gains / Losses, Available-for-sale securities |  |  |  |
Gross unrealized gains, Over 10 years | 3,621,000 wfsl_GrossUnrealizedGainsAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  | 3,621,000 wfsl_GrossUnrealizedGainsAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 3,279,000 wfsl_GrossUnrealizedGainsAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Gross unrealized losses, Over 10 years | 0 wfsl_GrossUnrealizedLossesOverTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  | 0 wfsl_GrossUnrealizedLossesOverTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 0 wfsl_GrossUnrealizedLossesOverTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Fair Value, Available-for-sale securities |  |  |  |
Over 10 years | 24,013,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  | 24,013,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 23,681,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Yield, Available-for-sale securities |  |  |  |
Over 10 years | 6.45%wfsl_AvailableforsaleSecuritiesYieldAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  | 6.45%wfsl_AvailableforsaleSecuritiesYieldAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 6.45%wfsl_AvailableforsaleSecuritiesYieldAfterTenYears / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Agency Pass-through Certificates [Member] |  |  |  |
Amortized Cost, Available-for-sale securities |  |  |  |
Without single maturity date | 1,420,784,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |  | 1,420,784,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 1,561,639,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Gross Unrealized Gains / Losses, Available-for-sale securities |  |  |  |
Gross unrealized gains, Without single maturity date | 38,053,000 wfsl_GrossUnrealizedGainsWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |  | 38,053,000 wfsl_GrossUnrealizedGainsWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 24,893,000 wfsl_GrossUnrealizedGainsWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Gross unrealized losses, Without single maturity date | (1,892,000)wfsl_GrossUnrealizedLossesWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |  | (1,892,000)wfsl_GrossUnrealizedLossesWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | (2,024,000)wfsl_GrossUnrealizedLossesWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Fair Value, Available-for-sale securities |  |  |  |
Without single maturity date | 1,456,945,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |  | 1,456,945,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 1,584,508,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Yield, Available-for-sale securities |  |  |  |
Without single maturity date | 2.57%wfsl_AvailableforsaleSecuritiesYieldWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |  | 2.57%wfsl_AvailableforsaleSecuritiesYieldWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 2.57%wfsl_AvailableforsaleSecuritiesYieldWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Amortized Cost, Held-to-maturity securities |  |  |  |
Without single maturity date | 1,479,781,000 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateNetCarryingAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |  | 1,479,781,000 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateNetCarryingAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 1,548,265,000 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateNetCarryingAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |  |  |
Gross unrealized gains, Without single maturity date | 11,732,000 wfsl_GrossUnrecognizedGainsWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |  | 11,732,000 wfsl_GrossUnrecognizedGainsWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 4,855,000 wfsl_GrossUnrecognizedGainsWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Gross unrealized losses, Without single maturity date | (11,036,000)wfsl_GrossUnrecognizedLossesWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |  | (11,036,000)wfsl_GrossUnrecognizedLossesWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | (53,902,000)wfsl_GrossUnrecognizedLossesWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Fair Value, Held-to-maturity securities |  |  |  |
Without single maturity date | 1,480,477,000 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |  | 1,480,477,000 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 1,499,218,000 us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Yield, Held-to-maturity securities |  |  |  |
Without a single maturity date | 3.13%wfsl_HeldtomaturitySecuritiesYieldWithoutASingleMaturityDate / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |  | 3.13%wfsl_HeldtomaturitySecuritiesYieldWithoutASingleMaturityDate / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 3.13%wfsl_HeldtomaturitySecuritiesYieldWithoutASingleMaturityDate / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Commercial Mortgage Backed Securities [Member] |  |  |  |
Amortized Cost, Available-for-sale securities |  |  |  |
Without single maturity date | 110,662,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  | 110,662,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 98,851,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Gross Unrealized Gains / Losses, Available-for-sale securities |  |  |  |
Gross unrealized gains, Without single maturity date | 0 wfsl_GrossUnrealizedGainsWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  | 0 wfsl_GrossUnrealizedGainsWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 65,000 wfsl_GrossUnrealizedGainsWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Gross unrealized losses, Without single maturity date | 0 wfsl_GrossUnrealizedLossesWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  | 0 wfsl_GrossUnrealizedLossesWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 0 wfsl_GrossUnrealizedLossesWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Fair Value, Available-for-sale securities |  |  |  |
Without single maturity date | $ 110,662,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  | $ 110,662,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | $ 98,916,000 us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Yield, Available-for-sale securities |  |  |  |
Without single maturity date | 1.45%wfsl_AvailableforsaleSecuritiesYieldWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  | 1.45%wfsl_AvailableforsaleSecuritiesYieldWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 1.49%wfsl_AvailableforsaleSecuritiesYieldWithoutSingleMaturity / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember

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