Fair Value Hierarchy for Financial Instruments Not Fair Valued but For Which Fair Value Is Required To Be Disclosed (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Carrying Amount
Commercial mortgages, mezzanine loans and other loans, Carrying Amount	$ 1,369,782		$ 1,122,377
Carrying Amount	2,613,317		2,086,401
Estimated Fair Value
Junior subordinated notes	13,048		11,911
CDO notes payable, Estimated Fair Value	431,142		377,235
Estimated Fair Value
Estimated Fair Value
Commercial mortgages, mezzanine loans and other loans, Estimated Fair Value	1,283,719		1,083,118
Carrying Amount
Carrying Amount
Commercial mortgages, mezzanine loans and other loans, Carrying Amount	1,354,120		1,122,377
7.0% convertible senior notes | Estimated Fair Value
Estimated Fair Value
Convertible senior notes, Estimated Fair Value	40,062		47,778
7.0% convertible senior notes | Carrying Amount
Carrying Amount
Carrying Amount	33,297		32,938
4.0% convertible senior notes | Estimated Fair Value
Estimated Fair Value
Convertible senior notes, Estimated Fair Value	134,308		124,063
4.0% convertible senior notes | Carrying Amount
Carrying Amount
Carrying Amount	133,752		116,184
Junior subordinated notes, at amortized cost | Estimated Fair Value
Estimated Fair Value
Junior subordinated notes	14,648		14,007
Junior subordinated notes, at amortized cost | Carrying Amount
Carrying Amount
Carrying Amount	25,100		25,100
CDO notes payable, at amortized cost | Estimated Fair Value
Estimated Fair Value
CDO notes payable, Estimated Fair Value	919,924		724,885
CDO notes payable, at amortized cost | Carrying Amount
Carrying Amount
Carrying Amount	1,084,289		1,202,772
CMBS securitizations | Estimated Fair Value
Estimated Fair Value
CDO notes payable, Estimated Fair Value	215,527		100,214
CMBS securitizations | Carrying Amount
Carrying Amount
Carrying Amount	215,518		100,139
Loans payable on real estate | Estimated Fair Value
Estimated Fair Value
Loans payable on real estate, Estimated Fair Value	398,517		176,979
Loans payable on real estate | Carrying Amount
Carrying Amount
Carrying Amount	381,095		171,244
7.625% senior notes due 2024 | Estimated Fair Value
Estimated Fair Value
Senior notes, Estimated Fair Value	57,624
7.625% senior notes due 2024 | Carrying Amount
Carrying Amount
Carrying Amount	60,000
7.125% Senior Notes due 2019 | Estimated Fair Value
Estimated Fair Value
Senior notes, Estimated Fair Value	71,013
7.125% Senior Notes due 2019 | Carrying Amount
Carrying Amount
Carrying Amount	71,905
Quoted Prices in Active Markets for Identical Assets (Level 1)
Estimated Fair Value
Commercial mortgages, mezzanine loans and other loans, Estimated Fair Value	0		0
Junior subordinated notes	0	[1]	0	[2]
CDO notes payable, Estimated Fair Value	0	[1]	0	[2]
Quoted Prices in Active Markets for Identical Assets (Level 1) | 7.0% convertible senior notes
Estimated Fair Value
Convertible senior notes, Estimated Fair Value	40,062		47,778
Quoted Prices in Active Markets for Identical Assets (Level 1) | 4.0% convertible senior notes
Estimated Fair Value
Convertible senior notes, Estimated Fair Value	134,308		124,063
Quoted Prices in Active Markets for Identical Assets (Level 1) | Junior subordinated notes, at amortized cost
Estimated Fair Value
Junior subordinated notes	0		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | CDO notes payable, at amortized cost
Estimated Fair Value
CDO notes payable, Estimated Fair Value	0		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | CMBS securitizations
Estimated Fair Value
CDO notes payable, Estimated Fair Value	0		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Loans payable on real estate
Estimated Fair Value
Loans payable on real estate, Estimated Fair Value	0		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | 7.625% senior notes due 2024
Estimated Fair Value
Senior notes, Estimated Fair Value	57,624
Quoted Prices in Active Markets for Identical Assets (Level 1) | 7.125% Senior Notes due 2019
Estimated Fair Value
Senior notes, Estimated Fair Value	71,013
Significant Other Observable Inputs (Level 2)
Estimated Fair Value
Commercial mortgages, mezzanine loans and other loans, Estimated Fair Value	0		0
Junior subordinated notes	0	[1]	0	[2]
CDO notes payable, Estimated Fair Value	0	[1]	0	[2]
Significant Other Observable Inputs (Level 2) | 7.0% convertible senior notes
Estimated Fair Value
Convertible senior notes, Estimated Fair Value	0		0
Significant Other Observable Inputs (Level 2) | 4.0% convertible senior notes
Estimated Fair Value
Convertible senior notes, Estimated Fair Value	0		0
Significant Other Observable Inputs (Level 2) | Junior subordinated notes, at amortized cost
Estimated Fair Value
Junior subordinated notes	0		0
Significant Other Observable Inputs (Level 2) | CDO notes payable, at amortized cost
Estimated Fair Value
CDO notes payable, Estimated Fair Value	0		0
Significant Other Observable Inputs (Level 2) | CMBS securitizations
Estimated Fair Value
CDO notes payable, Estimated Fair Value	0		0
Significant Other Observable Inputs (Level 2) | Loans payable on real estate
Estimated Fair Value
Loans payable on real estate, Estimated Fair Value	0		0
Significant Other Observable Inputs (Level 2) | 7.625% senior notes due 2024
Estimated Fair Value
Senior notes, Estimated Fair Value	0
Significant Other Observable Inputs (Level 2) | 7.125% Senior Notes due 2019
Estimated Fair Value
Senior notes, Estimated Fair Value	0
Significant Unobservable Inputs (Level 3)
Estimated Fair Value
Commercial mortgages, mezzanine loans and other loans, Estimated Fair Value	1,283,719		1,083,118
Junior subordinated notes	13,048	[1]	11,911	[2]
CDO notes payable, Estimated Fair Value	431,142	[1]	377,235	[2]
Significant Unobservable Inputs (Level 3) | 7.0% convertible senior notes
Estimated Fair Value
Convertible senior notes, Estimated Fair Value	0		0
Significant Unobservable Inputs (Level 3) | 4.0% convertible senior notes
Estimated Fair Value
Convertible senior notes, Estimated Fair Value	0		0
Significant Unobservable Inputs (Level 3) | Junior subordinated notes, at amortized cost
Estimated Fair Value
Junior subordinated notes	14,648		14,007
Significant Unobservable Inputs (Level 3) | CDO notes payable, at amortized cost
Estimated Fair Value
CDO notes payable, Estimated Fair Value	919,924		724,885
Significant Unobservable Inputs (Level 3) | CMBS securitizations
Estimated Fair Value
CDO notes payable, Estimated Fair Value	215,527		100,214
Significant Unobservable Inputs (Level 3) | Loans payable on real estate
Estimated Fair Value
Loans payable on real estate, Estimated Fair Value	398,517		176,979
Significant Unobservable Inputs (Level 3) | 7.625% senior notes due 2024
Estimated Fair Value
Senior notes, Estimated Fair Value	0
Significant Unobservable Inputs (Level 3) | 7.125% Senior Notes due 2019
Estimated Fair Value
Senior notes, Estimated Fair Value	$ 0

[1] During the nine-month period ended September 30, 2014, there were no transfers between Level 1 and Level 2, as well as, there were no transfers into and out of Level 3.

[2] During the year ended December 31, 2013, there were no transfers between Level 1 and Level 2, as well as, there were no transfers into and out of Level 3.