Indebtedness - Additional Information (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	6 Months Ended								6 Months Ended						3 Months Ended								6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 CMBS Facilities	Jun. 30, 2013 Recourse Indebtedness		Dec. 31, 2012 Recourse Indebtedness		Jun. 30, 2013 Recourse Indebtedness 7.0% convertible senior notes		Dec. 31, 2012 Recourse Indebtedness 7.0% convertible senior notes		Jun. 30, 2013 Recourse Indebtedness Secured credit facility	Dec. 31, 2012 Recourse Indebtedness Secured credit facility	Jun. 30, 2013 Recourse Indebtedness Junior subordinated notes, at fair value		Dec. 31, 2012 Recourse Indebtedness Junior subordinated notes, at fair value		Jun. 30, 2013 Recourse Indebtedness CMBS Facilities	Jun. 30, 2013 Recourse Indebtedness Commercial mortgage facility	Jun. 30, 2013 Non-recourse indebtedness	Dec. 31, 2012 Non-recourse indebtedness	Jun. 30, 2013 Non-recourse indebtedness CDO notes payable, at fair value		Dec. 31, 2012 Non-recourse indebtedness CDO notes payable, at fair value
Debt Disclosure [Line Items]
Convertible note conversion rate									142.6266
Principal amount of convertible note, conversion rate									$ 1
Initial conversion price									$ 7.01
Carrying Amount	1,919,620		1,799,595	1,760,167	181,985	[1]	172,476	[2]	110,437	[3]	109,631	[3]	7,121	8,090	11,933	[4]	29,655	[4]	12,569	14,825	1,737,635	1,627,119	304,687	[4],[5],[6]	187,048	[4],[5],[7]
Contractual Maturity, month and year									2031-04	[3]	2031-04	[3]	2016-12	2016-12	2035-03	[4]				2013-12
Payment of notes	8,368	1,605													19,381								66,325
Unpaid Principal Balance	2,545,805		2,612,362		193,286	[1]	186,242	[2]	115,000	[3]	115,000	[3]	7,121	8,090	18,671	[4]	38,052	[4]	12,569	14,825	2,352,519	2,426,120	918,055	[4],[5],[6]	984,380	[4],[5],[7]
Line of credit facility, borrowing capacity																			250,000	150,000
Outstanding borrowings																			12,569	14,825
Line of credit facility, remaining borrowing capacity																			$ 237,431	$ 135,175

[1] Excludes senior secured notes issued by us with an aggregate principal amount equal to $90,000 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.

[3] Our 7.0% convertible senior notes are redeemable at par, at the option of the holder, in April 2016, April 2021, and April 2026.

[4] Relates to liabilities which we elected to record at fair value under FASB ASC Topic 825.

[5] Excludes CDO notes payable purchased by us which are eliminated in consolidation.

[6] Collateralized by $1,035,821 principal amount of investments in securities and security-related receivables and loans, before fair value adjustments. The fair value of these investments as of June 30, 2013 was $791,642. These obligations were issued by separate legal entities and consequently the assets of the special purpose entities that collateralize these obligations are not available to our creditors.

[7] Collateralized by $1,118,346 principal amount of investments in securities and security-related receivables and loans, before fair value adjustments. The fair value of these investments as of December 31, 2012 was $849,919. These obligations were issued by separate legal entities and consequently the assets of the special purpose entities that collateralize these obligations are not available to our creditors.