Carrying Amount and Fair Value of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013		Sep. 30, 2013	Dec. 31, 2012
Assets
Commercial mortgages, mezzanine loans, other loans and preferred equity interests, Carrying Amount	$ 1,369,782		$ 1,122,377
Investments in securities and security-related receivables, Carrying Amount	568,279		567,302
Cash and cash equivalents, Carrying Amount	116,767		88,847		116,623	100,041
Restricted cash, Carrying Amount	133,374		121,589
Derivative assets	732		1,305
Liabilities
Junior subordinated notes	13,048		11,911
Carrying Amount	2,613,317		2,086,401
Notes payable, Estimated Fair Value	431,142		377,235
Derivative liabilities	81,998		113,331
Recourse Indebtedness
Liabilities
Carrying Amount	501,273	[1]	235,011	[2]
Non-recourse indebtedness
Liabilities
Carrying Amount	2,112,044		1,851,390
Carrying Amount
Assets
Commercial mortgages, mezzanine loans, other loans and preferred equity interests, Carrying Amount	1,354,120		1,122,377
Investments in securities and security-related receivables, Carrying Amount	568,279		567,302
Cash and cash equivalents, Carrying Amount	116,767		88,847
Restricted cash, Carrying Amount	133,374		121,589
Carrying Amount | 7.0% convertible senior notes
Liabilities
Carrying Amount	33,297		32,938
Carrying Amount | 4.0% convertible senior notes
Liabilities
Carrying Amount	133,752		116,184
Carrying Amount | 7.625% senior notes due 2024
Liabilities
Carrying Amount	60,000
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	1,084,289		1,202,772
Carrying Amount | CMBS securitizations
Liabilities
Carrying Amount	215,518		100,139
Carrying Amount | Loans payable on real estate
Liabilities
Carrying Amount	381,095		171,244
Carrying Amount | 7.125% Senior Notes due 2019
Liabilities
Carrying Amount	71,905
Carrying Amount | Recourse Indebtedness | 7.0% convertible senior notes
Liabilities
Carrying Amount	33,297		32,938
Carrying Amount | Recourse Indebtedness | 4.0% convertible senior notes
Liabilities
Carrying Amount	133,752		116,184
Carrying Amount | Recourse Indebtedness | 7.625% senior notes due 2024
Liabilities
Carrying Amount	60,000
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	159,171		37,749
Carrying Amount | Recourse Indebtedness | 7.125% Senior Notes due 2019
Liabilities
Carrying Amount	71,905
Carrying Amount | Non-recourse indebtedness | Derivative Liabilities
Liabilities
Carrying Amount	81,998		113,331
Carrying Amount | Non-recourse indebtedness | CDO notes payable, at amortized cost
Liabilities
Carrying Amount	1,084,289		1,202,772
Carrying Amount | Non-recourse indebtedness | CDO notes payable, at fair value
Liabilities
Carrying Amount	431,142		377,235
Carrying Amount | Non-recourse indebtedness | CMBS securitizations
Liabilities
Carrying Amount	215,518		100,139
Carrying Amount | Non-recourse indebtedness | Loans payable on real estate
Liabilities
Carrying Amount	381,095		171,244
Carrying Amount | Non-recourse indebtedness | Warrants and investor SARs
Liabilities
Carrying Amount	29,731		31,304
Estimated Fair Value
Assets
Commercial mortgages, mezzanine loans, other loans and preferred equity interests, Estimated Fair Value	1,283,719		1,083,118
Investments in securities and security-related receivables, Carrying Amount	568,279		567,302
Cash and cash equivalents, Estimated Fair Value	116,767		88,847
Restricted cash, Carrying Amount	133,374		121,589
Estimated Fair Value | 7.0% convertible senior notes
Liabilities
Convertible senior notes, Estimated Fair Value	40,062		47,778
Estimated Fair Value | 4.0% convertible senior notes
Liabilities
Convertible senior notes, Estimated Fair Value	134,308		124,063
Estimated Fair Value | 7.625% senior notes due 2024
Liabilities
Senior notes, Estimated Fair Value	57,624
Estimated Fair Value | Junior subordinated notes, at amortized cost
Liabilities
Junior subordinated notes	14,648		14,007
Estimated Fair Value | CDO notes payable, at amortized cost
Liabilities
Notes payable, Estimated Fair Value	919,924		724,885
Estimated Fair Value | CMBS securitizations
Liabilities
Notes payable, Estimated Fair Value	215,527		100,214
Estimated Fair Value | Loans payable on real estate
Liabilities
Notes payable, Estimated Fair Value	398,517		176,979
Estimated Fair Value | 7.125% Senior Notes due 2019
Liabilities
Senior notes, Estimated Fair Value	71,013
Estimated Fair Value | Recourse Indebtedness | 7.0% convertible senior notes
Liabilities
Convertible senior notes, Estimated Fair Value	40,062		47,778
Estimated Fair Value | Recourse Indebtedness | 4.0% convertible senior notes
Liabilities
Convertible senior notes, Estimated Fair Value	134,308		124,063
Estimated Fair Value | Recourse Indebtedness | 7.625% senior notes due 2024
Liabilities
Senior notes, Estimated Fair Value	57,624
Estimated Fair Value | Recourse Indebtedness | Junior subordinated notes, at amortized cost
Liabilities
Junior subordinated notes	14,648		14,007
Estimated Fair Value | Recourse Indebtedness | CMBS Facilities
Liabilities
Carrying Amount	159,171		37,749
Estimated Fair Value | Recourse Indebtedness | 7.125% Senior Notes due 2019
Liabilities
Senior notes, Estimated Fair Value	71,013
Estimated Fair Value | Non-recourse indebtedness | Derivative Liabilities
Liabilities
Derivative liabilities	81,998		113,331
Estimated Fair Value | Non-recourse indebtedness | CDO notes payable, at amortized cost
Liabilities
Notes payable, Estimated Fair Value	919,924		724,885
Estimated Fair Value | Non-recourse indebtedness | CDO notes payable, at fair value
Liabilities
Notes payable, Estimated Fair Value	431,142		377,235
Estimated Fair Value | Non-recourse indebtedness | CMBS securitizations
Liabilities
Notes payable, Estimated Fair Value	215,527		100,214
Estimated Fair Value | Non-recourse indebtedness | Loans payable on real estate
Liabilities
Notes payable, Estimated Fair Value	398,517		176,979
Estimated Fair Value | Non-recourse indebtedness | Warrants and investor SARs
Liabilities
Derivative liabilities	29,731		31,304
Derivative assets | Carrying Amount
Assets
Derivative assets	732		1,305
Derivative assets | Estimated Fair Value
Assets
Derivative assets	732		1,305
Secured credit facilities | Recourse Indebtedness
Liabilities
Carrying Amount	5,000		11,129
Secured credit facilities | Carrying Amount | Recourse Indebtedness
Liabilities
Carrying Amount	5,000		11,129
Secured credit facilities | Estimated Fair Value | Recourse Indebtedness
Liabilities
Secured credit facilities, Estimated Fair Value	5,000		11,129
Junior subordinated notes, at fair value | Carrying Amount | Recourse Indebtedness
Liabilities
Carrying Amount	13,048		11,911
Junior subordinated notes, at fair value | Estimated Fair Value | Recourse Indebtedness
Liabilities
Junior subordinated notes	$ 13,048		$ 11,911

[1] Excludes senior secured notes issued by us with an aggregate principal amount equal to $80,000 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 $86,000 with a weighted average coupon of 7.0%, which are eliminated in consolidation.