Fair Value Hierarchy for Financial Instruments Not Fair Valued but For Which Fair Value Is Required To Be Disclosed (Detail) - USD ($) $ in Thousands	Mar. 31, 2019		Dec. 31, 2018
Fair Value Measurement Inputs Disclosure [Line Items]
Junior subordinated notes	$ 5,500		$ 6,110
Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Junior subordinated notes	$ 5,500	[1]	$ 6,110	[2]
Total investment in mortgage loans, held for investment, net, Valuation Technique	us-gaap:ValuationTechniqueDiscountedCashFlowMember		us-gaap:ValuationTechniqueDiscountedCashFlowMember
Investment in mortgage loans, held for sale, Valuation Technique	us-gaap:ValuationTechniqueDiscountedCashFlowMember		us-gaap:ValuationTechniqueDiscountedCashFlowMember
CMBS securitizations | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness, Valuation Technique	us-gaap:ValuationTechniqueDiscountedCashFlowMember		us-gaap:ValuationTechniqueDiscountedCashFlowMember
Estimated Fair Value
Fair Value Measurement Inputs Disclosure [Line Items]
Total investment in mortgage loans, held for investment, net	$ 380,819		$ 444,962
Investment in mortgage loans, held for sale	5,131		4,873
Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Total investment in mortgage loans, held for investment, net	380,819		444,962
Investment in mortgage loans, held for sale	5,131		4,873
Estimated Fair Value | CMBS securitizations | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Notes payable	282,856		320,711
Carrying Amount
Fair Value Measurement Inputs Disclosure [Line Items]
Total investment in mortgage loans, held for investment, net	417,517		480,061
Investment in mortgage loans, held for sale	5,131		4,873
Carrying Amount | CMBS securitizations
Fair Value Measurement Inputs Disclosure [Line Items]
Notes payable	$ 281,126		$ 318,008
7.625% Senior Notes due 2024 | Significant Other Observable Inputs (Level 2)
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness, Valuation Technique	ras:ValuationTechniqueTradingPricingMember		ras:ValuationTechniqueTradingPricingMember
7.625% Senior Notes due 2024 | Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness	$ 41,117		$ 41,229
7.625% Senior Notes due 2024 | Carrying Amount
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness	$ 55,158		$ 55,100
7.125% Senior Notes due 2019 | Significant Other Observable Inputs (Level 2)
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness, Valuation Technique	ras:ValuationTechniqueTradingPricingMember		ras:ValuationTechniqueTradingPricingMember
7.125% Senior Notes due 2019 | Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness	$ 60,128		$ 58,010
7.125% Senior Notes due 2019 | Carrying Amount
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness	$ 65,131		$ 64,997
Junior subordinated notes, at amortized cost | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness, Valuation Technique	us-gaap:ValuationTechniqueDiscountedCashFlowMember		us-gaap:ValuationTechniqueDiscountedCashFlowMember
Junior subordinated notes, at amortized cost | Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Junior subordinated notes	$ 5,780		$ 6,550
Junior subordinated notes, at amortized cost | Carrying Amount
Fair Value Measurement Inputs Disclosure [Line Items]
Junior subordinated notes	$ 25,100		$ 25,100
CDO notes payable, at amortized cost | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness, Valuation Technique	us-gaap:ValuationTechniqueDiscountedCashFlowMember		us-gaap:ValuationTechniqueDiscountedCashFlowMember
CDO notes payable, at amortized cost | Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Notes payable	$ 83,595		$ 94,513
CDO notes payable, at amortized cost | Carrying Amount
Fair Value Measurement Inputs Disclosure [Line Items]
Notes payable	$ 102,918		$ 116,102
Loan Payable On Real Estate | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness, Valuation Technique	us-gaap:ValuationTechniqueDiscountedCashFlowMember		us-gaap:ValuationTechniqueDiscountedCashFlowMember
Loan Payable On Real Estate | Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Loans payable on real estate	$ 40,698		$ 40,608
Loan Payable On Real Estate | Carrying Amount
Fair Value Measurement Inputs Disclosure [Line Items]
Loans payable on real estate	$ 40,330		$ 40,499

[1] During the three months ended March 31, 2019, there were no transfers between Level 1 and Level 2, and there were no transfers into and/or out of Level 3.

[2] During the year ended December 31, 2018, there were no transfers between Level 1 and Level 2, and there were no transfers into and/or out of Level 3.