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Available for Sale Securities by Collateral Type (Detail) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Investment Holdings [Line Items] |  |  |  |
Available-for-sale securities | $ 1,267,624 us-gaap_AvailableForSaleSecurities |  | $ 1,558,306 us-gaap_AvailableForSaleSecurities |
Senior Securities |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale securities | 487,557 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = rwt_SeniorSecuritiesMember |  | 855,692 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = rwt_SeniorSecuritiesMember |
Senior Securities | Prime |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale securities | 307,813 us-gaap_AvailableForSaleSecurities / invest_InvestmentSecuritiesClassAxis = us-gaap_ResidentialPrimeFinancingReceivableMember / us-gaap_InvestmentTypeAxis = rwt_SeniorSecuritiesMember |  | 662,306 us-gaap_AvailableForSaleSecurities / invest_InvestmentSecuritiesClassAxis = us-gaap_ResidentialPrimeFinancingReceivableMember / us-gaap_InvestmentTypeAxis = rwt_SeniorSecuritiesMember |
Senior Securities | Non-prime |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale securities | 179,744 us-gaap_AvailableForSaleSecurities / invest_InvestmentSecuritiesClassAxis = us-gaap_ResidentialSubprimeFinancingReceivableMember / us-gaap_InvestmentTypeAxis = rwt_SeniorSecuritiesMember |  | 193,386 us-gaap_AvailableForSaleSecurities / invest_InvestmentSecuritiesClassAxis = us-gaap_ResidentialSubprimeFinancingReceivableMember / us-gaap_InvestmentTypeAxis = rwt_SeniorSecuritiesMember |
Re-REMIC |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale securities | 168,347 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = rwt_ResecuritizationOfRealEstateMortgageInvestmentSecuritiesMember |  | 176,376 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = rwt_ResecuritizationOfRealEstateMortgageInvestmentSecuritiesMember |
Subordinate Securities |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale securities | 611,720 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = rwt_SubordinateSecuritiesMember |  | 526,238 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = rwt_SubordinateSecuritiesMember |
Subordinate Securities | Non-prime | Maximum |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale securities | 1,000 us-gaap_AvailableForSaleSecurities / invest_InvestmentSecuritiesClassAxis = us-gaap_ResidentialSubprimeFinancingReceivableMember / us-gaap_InvestmentTypeAxis = rwt_SubordinateSecuritiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  | 1,000 us-gaap_AvailableForSaleSecurities / invest_InvestmentSecuritiesClassAxis = us-gaap_ResidentialSubprimeFinancingReceivableMember / us-gaap_InvestmentTypeAxis = rwt_SubordinateSecuritiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember |
Subordinate Securities | Prime Mezzanine |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale securities | 448,838 us-gaap_AvailableForSaleSecurities / invest_InvestmentSecuritiesClassAxis = rwt_PrimeMezzanineMember / us-gaap_InvestmentTypeAxis = rwt_SubordinateSecuritiesMember | [1] | 384,849 us-gaap_AvailableForSaleSecurities / invest_InvestmentSecuritiesClassAxis = rwt_PrimeMezzanineMember / us-gaap_InvestmentTypeAxis = rwt_SubordinateSecuritiesMember | [1]
Subordinate Securities | Subordinate |  |  |  |
Investment Holdings [Line Items] |  |  |  |
Available-for-sale securities | $ 162,882 us-gaap_AvailableForSaleSecurities / invest_InvestmentSecuritiesClassAxis = rwt_SubordinateMember / us-gaap_InvestmentTypeAxis = rwt_SubordinateSecuritiesMember | [2] | $ 141,389 us-gaap_AvailableForSaleSecurities / invest_InvestmentSecuritiesClassAxis = rwt_SubordinateMember / us-gaap_InvestmentTypeAxis = rwt_SubordinateSecuritiesMember | [2]
[1] | Mezzanine includes securities initially rated AA, A, and BBB- and issued in 2012 or later.
[2] | Subordinate securities includes less than $1 million of non-prime securities at both December 31, 2014 and 2013.

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