Real Estate Securities - Additional Information (Detail) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2013 Investment	Mar. 31, 2013	Sep. 30, 2012	Sep. 30, 2013 Investment	Sep. 30, 2012	Dec. 31, 2012 Investment	Sep. 30, 2013 Senior Securities	Dec. 31, 2012 Senior Securities	Sep. 30, 2013 Trading Securities	Dec. 31, 2012 Trading Securities	Sep. 30, 2013 Interest Only Securities	Sep. 30, 2013 Residential Subordinate Securities	Sep. 30, 2013 Re-REMIC	Dec. 31, 2012 Re-REMIC	Sep. 30, 2013 Subordinate Securities	Dec. 31, 2012 Subordinate Securities	Sep. 30, 2013 Residential Prime Senior Securities	Dec. 31, 2012 Residential Prime Senior Securities	Sep. 30, 2013 Residential Prime Re-REMIC	Sep. 30, 2013 Residential Prime Subordinate Securities	Dec. 31, 2012 Residential Prime Subordinate Securities	Sep. 30, 2013 Residential non-prime Senior Securities	Dec. 31, 2012 Residential non-prime Senior Securities	Sep. 30, 2013 Residential non-prime Re-REMIC	Sep. 30, 2013 Residential non-prime Subordinate Securities	Dec. 31, 2012 Residential non-prime Subordinate Securities	Sep. 30, 2013 Residential	Jun. 30, 2013 Residential	Dec. 31, 2012 Residential	Sep. 30, 2013 Residential Senior Securities	Sep. 30, 2013 Residential Re-REMIC	Sep. 30, 2013 Residential Subordinate Securities
Investment Holdings [Line Items]
Unpaid principal balance									$ 15,000,000	$ 12,000,000
Trading securities	130,854,000			130,854,000		33,172,000	126,307,000	32,543,000			127,000,000	4,000,000			4,547,000	629,000	117,443,000	10,409,000		4,547,000	468,000	8,864,000	22,134,000			161,000
Securities financed through non-recourse resecuritization entity																	141,000,000	172,000,000				135,000,000	152,000,000
Marketable securities, due from five to ten years																											3,000,000
Marketable securities, less than five years																											1,000
Realized gain on sale of available for sale securities	10,532,000	12,000,000	13,920,000	22,762,000	28,693,000
Credit reserve																											136,292,000	190,410,000	187,032,000	26,000,000	33,000,000	77,000,000
Loans underlying																	9,000,000,000		4,000,000,000	16,000,000,000		4,000,000,000		101,000,000	300,000,000						4,000,000,000	17,000,000,000
Available-for-sale securities	$ 1,193,824,000			$ 1,193,824,000		$ 1,075,581,000	$ 558,976,000	$ 711,789,000					$ 156,346,000	$ 163,035,000	$ 478,502,000	$ 200,757,000	$ 363,822,000	$ 466,523,000		$ 478,369,000	$ 184,528,000	$ 195,154,000	$ 245,266,000		$ 133,000	$ 2,160,000	$ 1,193,824,000		$ 1,061,512,000
Percentage of outstanding loan serious delinquencies																														9.51%	9.50%	3.29%
Number of AFS securities	297			297		284
Number of securities in unrealized loss position	63			63		22
Number of securities in a continuous unrealized loss position for twelve consecutive months or longer	7			7		14