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The percentage points (also referred to as 'margin') added to the reference rate as stated in the loan agreement and used to compute the variable rate on the loans receivable. For instance, the loan agreement might state that the interest rate the borrower is obligated to pay is the London Interbank Offered Rate (LIBOR) plus three quarters (3/4) of a percentage point adjusted quarterly (each three months). LIBOR in this example is the index or reference rate, 3/4 percentage point is the margin, the reference rate plus the margin is the fully indexed rate. This element would be used to report separately the margin.

No definition available.

Name:	us-gaap_LoansReceivableBasisSpreadOnVariableRate
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Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

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Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

No definition available.

Name:	us-gaap_SubsequentEventLineItems
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Debt Instrument, Exit Percent Fee

No definition available.

Name:	wmc_DebtInstrumentExitPercentFee
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Financing Receivable, Percentage Of Excess Proceeds

No definition available.

Name:	wmc_FinancingReceivablePercentageOfExcessProceeds
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Mortgage Loans On Real Estate, Percentage Of Projected Cash Flow

No definition available.

Name:	wmc_MortgageLoansOnRealEstatePercentageOfProjectedCashFlow
Namespace Prefix:	wmc_
Data Type:	num:percentItemType
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Mortgage Loans on Real Estate, Number of Loans, Nonperforming, Percentage

No definition available.

Name:	wmc_MortgageLoansonRealEstateNumberofLoansNonperformingPercentage
Namespace Prefix:	wmc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Represents the number of counterparties to repurchase agreement from whom the entity borrowed the funds.

No definition available.

Name:	wmc_NumberOfCounterpartiesFromWhomEntityBorrowedUnderRepurchaseAgreement
Namespace Prefix:	wmc_
Data Type:	xbrli:integerItemType
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Period Type:	instant

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Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=wmc_CommercialAndResidentialPortfolioSegmentMember
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Namespace Prefix:
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Name:	us-gaap_FinancialInstrumentAxis=wmc_OtherSecuritiesMember
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Name:	us-gaap_FinancialInstrumentAxis=wmc_WholeLoansMember
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