Carrying Amount and Fair Value of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Mar. 31, 2012	Dec. 31, 2011
Assets
Commercial mortgages, mezzanine loans and other loans, Carrying Amount	$ 1,124,044		$ 1,075,129
Investments in securities and security-related receivables, Carrying Amount	670,245		655,509
Cash and cash equivalents, Carrying Amount	63,854		100,041		53,841	29,720
Restricted cash, Carrying Amount	88,397		90,641
Derivative assets	1,158		1,132
Liabilities
Junior subordinated notes	11,268		29,655
Carrying Amount	1,888,850		1,799,595
Notes payable, Estimated Fair Value	282,355		187,048
Derivative liabilities	141,518		151,438
Recourse Indebtedness
Liabilities
Carrying Amount	169,936	[1]	172,476	[2]
Non-recourse indebtedness
Liabilities
Carrying Amount	1,718,914		1,627,119
Carrying Amount
Assets
Commercial mortgages, mezzanine loans and other loans, Carrying Amount	1,124,044		1,075,129
Investments in securities and security-related receivables, Carrying Amount	670,245		655,509
Cash and cash equivalents, Carrying Amount	63,854		100,041
Restricted cash, Carrying Amount	88,397		90,641
Carrying Amount | Derivative assets
Assets
Derivative assets	1,158		1,132
Carrying Amount | 7.0% convertible senior notes
Liabilities
Carrying Amount	110,035		109,631
Carrying Amount | Junior subordinated notes, at amortized cost
Liabilities
Carrying Amount	25,100		25,100
Carrying Amount | CDO notes payable, at amortized cost
Liabilities
Carrying Amount	1,292,202		1,295,400
Carrying Amount | Loans payable on real estate
Liabilities
Carrying Amount	144,357		144,671
Carrying Amount | Recourse Indebtedness | 7.0% convertible senior notes
Liabilities
Carrying Amount	110,035		109,631
Carrying Amount | Recourse Indebtedness | Secured credit facility
Liabilities
Carrying Amount	7,608		8,090
Carrying Amount | Recourse Indebtedness | Junior subordinated notes, at fair value
Liabilities
Carrying Amount	11,268		29,655
Carrying Amount | Recourse Indebtedness | Junior subordinated notes, at amortized cost
Liabilities
Carrying Amount	25,100		25,100
Carrying Amount | Recourse Indebtedness | CMBS Facilities
Liabilities
Carrying Amount	3,250
Carrying Amount | Recourse Indebtedness | Commercial Mortgage Loan
Liabilities
Carrying Amount	12,675
Carrying Amount | Non-recourse indebtedness | CDO notes payable, at amortized cost
Liabilities
Carrying Amount	1,292,202		1,295,400
Carrying Amount | Non-recourse indebtedness | CDO notes payable, at fair value
Liabilities
Carrying Amount	282,355		187,048
Carrying Amount | Non-recourse indebtedness | Loans payable on real estate
Liabilities
Carrying Amount	144,357		144,671
Carrying Amount | Non-recourse indebtedness | Derivative Liabilities
Liabilities
Carrying Amount	141,518		151,438
Estimated Fair Value
Assets
Commercial mortgages, mezzanine loans and other loans, Estimated Fair Value	1,103,791		1,063,716
Investments in securities and security-related receivables, Carrying Amount	670,245		655,509
Cash and cash equivalents, Estimated Fair Value	63,854		100,041
Restricted cash, Carrying Amount	88,397		90,641
Estimated Fair Value | Derivative assets
Assets
Derivative assets	1,158		1,132
Estimated Fair Value | 7.0% convertible senior notes
Liabilities
Convertible senior notes, Estimated Fair Value	148,580		115,230
Estimated Fair Value | Junior subordinated notes, at amortized cost
Liabilities
Junior subordinated notes	13,570		12,700
Estimated Fair Value | CDO notes payable, at amortized cost
Liabilities
Notes payable, Estimated Fair Value	720,138		722,371
Estimated Fair Value | Loans payable on real estate
Liabilities
Loans payable on real estate, Estimated Fair Value	157,431		156,510
Estimated Fair Value | Recourse Indebtedness | 7.0% convertible senior notes
Liabilities
Convertible senior notes, Estimated Fair Value	148,580		115,230
Estimated Fair Value | Recourse Indebtedness | Secured credit facility
Liabilities
Secured credit facilities, Estimated Fair Value	7,608		8,090
Estimated Fair Value | Recourse Indebtedness | Junior subordinated notes, at fair value
Liabilities
Junior subordinated notes	11,268		29,655
Estimated Fair Value | Recourse Indebtedness | Junior subordinated notes, at amortized cost
Liabilities
Junior subordinated notes	13,570		12,700
Estimated Fair Value | Recourse Indebtedness | CMBS Facilities
Liabilities
Carrying Amount	3,250
Estimated Fair Value | Recourse Indebtedness | Commercial Mortgage Loan
Liabilities
Carrying Amount	12,675
Estimated Fair Value | Non-recourse indebtedness | CDO notes payable, at amortized cost
Liabilities
Notes payable, Estimated Fair Value	720,138		722,371
Estimated Fair Value | Non-recourse indebtedness | CDO notes payable, at fair value
Liabilities
Notes payable, Estimated Fair Value	282,355		187,048
Estimated Fair Value | Non-recourse indebtedness | Loans payable on real estate
Liabilities
Loans payable on real estate, Estimated Fair Value	157,431		156,510
Estimated Fair Value | Non-recourse indebtedness | Derivative Liabilities
Liabilities
Derivative liabilities	$ 141,518		$ 151,438

[1] Excludes senior secured notes issued by us with an aggregate principal amount equal to $92,000 with a weighted average coupon of 7.0%, which are eliminated in consolidation

[2] Excludes senior secured notes issued by us with an aggregate principal amount equal to $94,000 with a weighted average coupon of 7.0%, which are eliminated in consolidation.