Residential Mortgage-Backed Securities - Unrealized Loss Positions (Detail) (Residential Mortgage Backed Securities [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2012 Item	Dec. 31, 2011 Item
Investment Holdings [Line Items]
Estimated fair value of RMBS in continuous loss position for less than 12 consecutive months	$ 32,646	$ 440,955
Unrealized losses on RMBS in continuous loss position for less than 12 consecutive months	(5,456)	(73,489)
Number of Securities of RMBS in continuous loss position for less than 12 months	15	63
Estimated fair value of RMBS in continuous loss position for 12 or more consecutive months	280,586	723,310
Unrealized losses on RMBS in continuous loss position for 12 or more consecutive months	(35,249)	(80,807)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months	40	48
Estimated fair value of RMBS in continuous loss position	313,232	1,164,265
Unrealized losses on RMBS in continuous loss position	(40,705)	(154,296)
Number of Securities in continuous loss position	55	111
Non-Agency RMBS - Senior [Member]
Investment Holdings [Line Items]
Estimated fair value of RMBS in continuous loss position for 12 or more consecutive months		127
Unrealized losses on RMBS in continuous loss position for 12 or more consecutive months		(41)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months		1
Estimated fair value of RMBS in continuous loss position		127
Unrealized losses on RMBS in continuous loss position		(41)
Number of Securities in continuous loss position		1
Non-Agency RMBS - Senior interest-only [Member]
Investment Holdings [Line Items]
Estimated fair value of RMBS in continuous loss position for less than 12 consecutive months	18,928	99,351
Unrealized losses on RMBS in continuous loss position for less than 12 consecutive months	(3,048)	(18,756)
Number of Securities of RMBS in continuous loss position for less than 12 months	10	26
Estimated fair value of RMBS in continuous loss position for 12 or more consecutive months	50,449	17,647
Unrealized losses on RMBS in continuous loss position for 12 or more consecutive months	(16,264)	(3,161)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months	20	12
Estimated fair value of RMBS in continuous loss position	69,377	116,998
Unrealized losses on RMBS in continuous loss position	(19,312)	(21,917)
Number of Securities in continuous loss position	30	38
Non-Agency RMBS - Subordinated [Member]
Investment Holdings [Line Items]
Estimated fair value of RMBS in continuous loss position for less than 12 consecutive months		321,416
Unrealized losses on RMBS in continuous loss position for less than 12 consecutive months		(52,824)
Number of Securities of RMBS in continuous loss position for less than 12 months	1	33
Estimated fair value of RMBS in continuous loss position for 12 or more consecutive months	192,675	111,167
Unrealized losses on RMBS in continuous loss position for 12 or more consecutive months	(18,500)	(25,430)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months	17	17
Estimated fair value of RMBS in continuous loss position	192,675	432,583
Unrealized losses on RMBS in continuous loss position	(18,500)	(78,254)
Number of Securities in continuous loss position	18	50
Non-Agency RMBS - Subordinated interest-only [Member]
Investment Holdings [Line Items]
Estimated fair value of RMBS in continuous loss position for less than 12 consecutive months	9,870	16,300
Unrealized losses on RMBS in continuous loss position for less than 12 consecutive months	(2,244)	(1,554)
Number of Securities of RMBS in continuous loss position for less than 12 months	1	2
Estimated fair value of RMBS in continuous loss position	9,870	16,300
Unrealized losses on RMBS in continuous loss position	(2,244)	(1,554)
Number of Securities in continuous loss position	1	2
Non-Agency RMBS Transferred To Consolidated Variable Interest Entities ("VIEs") [Member]
Investment Holdings [Line Items]
Estimated fair value of RMBS in continuous loss position for 12 or more consecutive months	36,527	594,369
Unrealized losses on RMBS in continuous loss position for 12 or more consecutive months	(266)	(52,175)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months	2	18
Estimated fair value of RMBS in continuous loss position	36,527	594,369
Unrealized losses on RMBS in continuous loss position	(266)	(52,175)
Number of Securities in continuous loss position	2	18
Agency RMBS [Member]
Investment Holdings [Line Items]
Estimated fair value of RMBS in continuous loss position for less than 12 consecutive months	3,848	3,888
Unrealized losses on RMBS in continuous loss position for less than 12 consecutive months	(164)	(355)
Number of Securities of RMBS in continuous loss position for less than 12 months	3	2
Estimated fair value of RMBS in continuous loss position for 12 or more consecutive months	935
Unrealized losses on RMBS in continuous loss position for 12 or more consecutive months	(219)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months	1
Estimated fair value of RMBS in continuous loss position	4,783	3,888
Unrealized losses on RMBS in continuous loss position	$ (383)	$ (355)
Number of Securities in continuous loss position	4	2