Residential Mortgage-Backed Securities - Gross Unrealized Gains (Losses) (Detail) (Residential Mortgage Backed Securities [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Other Comprehensive Income	$ 871,124	$ 563,923
Gross Unrealized Gain Included in Accumulated Deficit	16,287	18,153
Total Gross Unrealized Gain	887,411	582,076
Gross Unrealized Loss Included in Accumulated Other Comprehensive Income	(18,766)	(130,470)
Gross Unrealized Loss Included in Accumulated Deficit	(21,939)	(23,826)
Total Gross Unrealized Loss	(40,705)	(154,296)
Non-Agency RMBS - Senior [Member]
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Other Comprehensive Income	3	2
Total Gross Unrealized Gain	3	2
Gross Unrealized Loss Included in Accumulated Other Comprehensive Income		(41)
Total Gross Unrealized Loss		(41)
Non-Agency RMBS - Senior interest-only [Member]
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Deficit	8,353	11,308
Total Gross Unrealized Gain	8,353	11,308
Gross Unrealized Loss Included in Accumulated Deficit	(19,312)	(21,917)
Total Gross Unrealized Loss	(19,312)	(21,917)
Non-Agency RMBS - Subordinated [Member]
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Other Comprehensive Income	50,307	30,997
Total Gross Unrealized Gain	50,307	30,997
Gross Unrealized Loss Included in Accumulated Other Comprehensive Income	(18,500)	(78,254)
Total Gross Unrealized Loss	(18,500)	(78,254)
Non-Agency RMBS - Subordinated interest-only [Member]
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Deficit	1,714	1,663
Total Gross Unrealized Gain	1,714	1,663
Gross Unrealized Loss Included in Accumulated Other Comprehensive Income	0
Gross Unrealized Loss Included in Accumulated Deficit	(2,244)	(1,554)
Total Gross Unrealized Loss	(2,244)	(1,554)
Non-Agency RMBS Transferred To Consolidated Variable Interest Entities ("VIEs") [Member]
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Other Comprehensive Income	716,876	415,688
Gross Unrealized Gain Included in Accumulated Deficit	6,197	4,817
Total Gross Unrealized Gain	723,073	420,505
Gross Unrealized Loss Included in Accumulated Other Comprehensive Income	(266)	(52,175)
Total Gross Unrealized Loss	(266)	(52,175)
Agency RMBS [Member]
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Other Comprehensive Income	103,938	117,236
Gross Unrealized Gain Included in Accumulated Deficit	23	365
Total Gross Unrealized Gain	103,961	117,601
Gross Unrealized Loss Included in Accumulated Other Comprehensive Income	0
Gross Unrealized Loss Included in Accumulated Deficit	(383)	(355)
Total Gross Unrealized Loss	$ (383)	$ (355)