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Short-Term Debt - Additional Information (Detail) (USD $)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Short-term Debt [Line Items]      
Securities pledged as collateral $ 734,000,000us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral   $ 762,000,000us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral
Average balance of short-term debt 1,590,000,000us-gaap_ShorttermDebtAverageOutstandingAmount 1,010,000,000us-gaap_ShorttermDebtAverageOutstandingAmount  
Accrued interest payable on short-term debt 2,000,000us-gaap_InterestPayableCurrent   2,000,000us-gaap_InterestPayableCurrent
Committed line of credit 10,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity    
Committed line of credit with financial institutions, outstanding 0us-gaap_LineOfCredit   0us-gaap_LineOfCredit
Residential Loans      
Short-term Debt [Line Items]      
Loan pledged as collateral 915,000,000us-gaap_LoansPledgedAsCollateral
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= rwt_ResidentialLoansMember
  1,220,000,000us-gaap_LoansPledgedAsCollateral
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= rwt_ResidentialLoansMember
Commercial Loans      
Short-term Debt [Line Items]      
Loan pledged as collateral $ 125,000,000us-gaap_LoansPledgedAsCollateral
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= us-gaap_CommercialLoanMember
  $ 161,000,000us-gaap_LoansPledgedAsCollateral
/ us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAxis
= us-gaap_CommercialLoanMember