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Borrowings under Repurchase Agreements (Details) (USD $)
3 Months Ended 12 Months Ended 3 Months Ended
Mar. 31, 2015
item
Dec. 31, 2014
Mar. 31, 2014
Borrowings Under Repurchase Agreements      
Number of counterparties to master repurchase agreement 25wmc_NumberOfCounterpartiesToMasterRepurchaseAgreement    
Number of counterparties from whom the Company had borrowings 20wmc_NumberOfCounterpartiesFromWhomEntityBorrowedUnderRepurchaseAgreement    
Certain characteristics of the Company's repurchase agreements      
Repurchase Agreement Borrowings $ 3,648,939,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase $ 3,875,721,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase  
Weighted Average Interest Rate on Borrowings Outstanding at end of period (as a percent) 0.68%us-gaap_ShortTermDebtWeightedAverageInterestRate 0.66%us-gaap_ShortTermDebtWeightedAverageInterestRate  
Weighted Average Remaining Maturity (days) 39 days 34 days  
Accrued interest payable 16,326,000us-gaap_InterestPayableCurrentAndNoncurrent 17,573,000us-gaap_InterestPayableCurrentAndNoncurrent  
Repurchase agreements      
Certain characteristics of the Company's repurchase agreements      
Average borrowings under repurchase agreements 3,900,000,000us-gaap_ShorttermDebtAverageOutstandingAmount
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
  2,600,000,000us-gaap_ShorttermDebtAverageOutstandingAmount
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
Maximum balance 4,000,000,000us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
  2,800,000,000us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
Accrued interest payable 3,300,000us-gaap_InterestPayableCurrentAndNoncurrent
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
   
Repurchase agreements | Agency RMBS      
Certain characteristics of the Company's repurchase agreements      
Repurchase Agreement Borrowings 2,793,272,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_ResidentialMortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
2,994,351,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_ResidentialMortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
 
Weighted Average Interest Rate on Borrowings Outstanding at end of period (as a percent) 0.41%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_ResidentialMortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
0.40%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_ResidentialMortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
 
Weighted Average Remaining Maturity (days) 38 days 32 days  
Amount of secured collateral 2,940,283,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseFairValueOfCollateral
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_ResidentialMortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
3,158,544,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseFairValueOfCollateral
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_ResidentialMortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
 
Repurchase agreements | Non-Agency RMBS      
Certain characteristics of the Company's repurchase agreements      
Repurchase Agreement Borrowings 413,517,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
473,942,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
 
Weighted Average Interest Rate on Borrowings Outstanding at end of period (as a percent) 1.60%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
1.54%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
 
Weighted Average Remaining Maturity (days) 39 days 49 days  
Amount of secured collateral 600,712,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseFairValueOfCollateral
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
670,526,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseFairValueOfCollateral
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
 
Repurchase agreements | Agency and Non-Agency CMBS      
Certain characteristics of the Company's repurchase agreements      
Repurchase Agreement Borrowings 345,903,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
325,864,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
 
Weighted Average Interest Rate on Borrowings Outstanding at end of period (as a percent) 1.57%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
1.55%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
 
Weighted Average Remaining Maturity (days) 36 days 29 days  
Amount of secured collateral 473,556,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseFairValueOfCollateral
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
431,938,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseFairValueOfCollateral
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
 
Repurchase agreements | Other securities      
Certain characteristics of the Company's repurchase agreements      
Amount of secured collateral 125,919,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseFairValueOfCollateral
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_OtherSecuritiesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
108,743,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseFairValueOfCollateral
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_OtherSecuritiesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
 
Repurchase agreements | Rehypothecated securities      
Certain characteristics of the Company's repurchase agreements      
Maximum balance 530,000us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_RehypothecatedSecuritiesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
  0us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_RehypothecatedSecuritiesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
Repurchase agreements | Minimum      
Certain characteristics of the Company's repurchase agreements      
Term of repurchase agreements 1 month    
Repurchase agreements | Maximum      
Certain characteristics of the Company's repurchase agreements      
Term of repurchase agreements 3 months    
Repurchase agreements entered at balance sheet date and with subsequent settlement      
Certain characteristics of the Company's repurchase agreements      
Repurchase Agreement Borrowings 20,400,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
/ us-gaap_ShortTermDebtTypeAxis
= wmc_RepurchaseAgreementsEnteredAndSettledOnNextDayMember
   
Weighted Average Interest Rate on Borrowings Outstanding at end of period (as a percent) 1.43%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ us-gaap_ShortTermDebtTypeAxis
= wmc_RepurchaseAgreementsEnteredAndSettledOnNextDayMember
   
Weighted Average Remaining Maturity (days) 82 days    
Amount of secured collateral 26,900,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseFairValueOfCollateral
/ us-gaap_ShortTermDebtTypeAxis
= wmc_RepurchaseAgreementsEnteredAndSettledOnNextDayMember
   
Existing counterparty with intention to exit the repurchase market | Repurchase agreements      
Borrowings Under Repurchase Agreements      
Number of counterparties to master repurchase agreement 2wmc_NumberOfCounterpartiesToMasterRepurchaseAgreement
/ us-gaap_CounterpartyNameAxis
= wmc_ExistingCounterpartyWithIntentionToExitRepurchaseMarketMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
   
Certain characteristics of the Company's repurchase agreements      
Repurchase Agreement Borrowings 0us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
/ us-gaap_CounterpartyNameAxis
= wmc_ExistingCounterpartyWithIntentionToExitRepurchaseMarketMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
   
Residential whole-loans | Repurchase agreements      
Certain characteristics of the Company's repurchase agreements      
Repurchase Agreement Borrowings 13,100,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
4,900,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
 
Residential whole-loans | Repurchase agreements | Other securities      
Certain characteristics of the Company's repurchase agreements      
Repurchase Agreement Borrowings 96,247,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_OtherSecuritiesMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
81,564,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_OtherSecuritiesMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
 
Weighted Average Interest Rate on Borrowings Outstanding at end of period (as a percent) 1.64%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_OtherSecuritiesMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
1.55%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_OtherSecuritiesMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
 
Weighted Average Remaining Maturity (days) 62 days 34 days  
Residential whole-loans | Repurchase agreements | VIE | Other securities      
Certain characteristics of the Company's repurchase agreements      
Amount of secured collateral $ 17,900,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseFairValueOfCollateral
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= wmc_OtherSecuritiesMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember