Fair Value Hierarchy for Financial Instruments Not Fair Valued but For Which Fair Value Is Required To Be Disclosed (Detail) - USD ($) $ in Thousands	Jun. 30, 2018		Dec. 31, 2017
Fair Value Measurement Inputs Disclosure [Line Items]
Junior subordinated notes	$ 5,111		$ 8,121
Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Junior subordinated notes	$ 5,111	[1]	$ 8,121	[2]
Total investment in mortgage loans, held for investment, net, Valuation Technique	us-gaap:ValuationTechniqueDiscountedCashFlowMember
Investment in mortgage loans, held for sale, Valuation Technique	us-gaap:ValuationTechniqueDiscountedCashFlowMember		us-gaap:ValuationTechniqueDiscountedCashFlowMember
Significant Unobservable Inputs (Level 3) | Other indebtedness
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness, Valuation Technique			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
Carrying Amount
Fair Value Measurement Inputs Disclosure [Line Items]
Total investment in mortgage loans, held for investment, net	$ 714,406		$ 1,255,723
Investment in mortgage loans, held for sale	6,198
Carrying Amount | Other indebtedness
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness			40,955
Carrying Amount | CMBS securitizations
Fair Value Measurement Inputs Disclosure [Line Items]
Notes payable	454,440		736,586
Estimated Fair Value
Fair Value Measurement Inputs Disclosure [Line Items]
Total investment in mortgage loans, held for investment, net	664,235		1,252,780
Investment in mortgage loans, held for sale	6,198
Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Total investment in mortgage loans, held for investment, net	664,235		1,252,780
Investment in mortgage loans, held for sale	6,198
Estimated Fair Value | Significant Unobservable Inputs (Level 3) | Other indebtedness
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness			40,830
Estimated Fair Value | CMBS securitizations | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Notes payable	$ 459,029		$ 744,359
7.0% convertible senior notes | Significant Other Observable Inputs (Level 2)
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness, Valuation Technique	ras:ValuationTechniqueTradingPricingMember		ras:ValuationTechniqueTradingPricingMember
7.0% convertible senior notes | Carrying Amount
Fair Value Measurement Inputs Disclosure [Line Items]
Convertible senior notes	$ 835		$ 833
7.0% convertible senior notes | Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value Measurement Inputs Disclosure [Line Items]
Convertible senior notes	$ 560		$ 533
4.0% convertible senior notes | Significant Other Observable Inputs (Level 2)
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness, Valuation Technique	ras:ValuationTechniqueTradingPricingMember		ras:ValuationTechniqueTradingPricingMember
4.0% convertible senior notes | Carrying Amount
Fair Value Measurement Inputs Disclosure [Line Items]
Convertible senior notes	$ 66,364		$ 106,800
4.0% convertible senior notes | Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value Measurement Inputs Disclosure [Line Items]
Convertible senior notes	$ 67,242		$ 103,457
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 | Carrying Amount
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness	$ 54,984		$ 54,867
7.625% senior notes due 2024 | Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness	$ 44,721		$ 43,009
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 | Carrying Amount
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness	$ 67,753		$ 67,474
7.125% Senior Notes due 2019 | Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness	$ 62,525		$ 61,567
Senior Secured Notes | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness, Valuation Technique	us-gaap:ValuationTechniqueDiscountedCashFlowMember		us-gaap:ValuationTechniqueDiscountedCashFlowMember
Senior Secured Notes | Carrying Amount
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness	$ 9,266		$ 11,063
Senior Secured Notes | Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Indebtedness	$ 9,061		$ 11,197
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 | Carrying Amount
Fair Value Measurement Inputs Disclosure [Line Items]
Junior subordinated notes	$ 25,100		$ 25,100
Junior subordinated notes, at amortized cost | Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Junior subordinated notes	$ 5,652		$ 8,849
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 | Carrying Amount
Fair Value Measurement Inputs Disclosure [Line Items]
Notes payable	$ 209,752		$ 254,724
CDO notes payable, at amortized cost | Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs Disclosure [Line Items]
Notes payable	$ 176,875		$ 196,212
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 | Carrying Amount
Fair Value Measurement Inputs Disclosure [Line Items]
Loans payable on real estate	$ 40,818		$ 61,922
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	$ 40,674		$ 64,377

[1] During the six months ended June 30, 2018, 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, 2017, there were no transfers between Level 1 and Level 2, and there were no transfers into and/or out of Level 3.