Residential Mortgage-Backed Securities - Collateral Characteristics (Detail) (Residential Mortgage Backed Securities [Member], USD $)	Sep. 30, 2012		Dec. 31, 2011
Investment Holdings [Line Items]
Principal or Notional Value at Period-End	$ 11,189,281,000		$ 13,675,493,000
Non-Agency RMBS - Senior [Member]
Investment Holdings [Line Items]
Principal or Notional Value at Period-End	135,000		1,115,000
Weighted Average Amortized Cost Basis	58.37		95.13
Weighted Average Fair Value	60.88		91.55
Weighted Average Coupon	0.00%		1.02%
Weighted Average Yield at Period-End	11.26%	[1]	2.95%	[1]
Non-Agency RMBS - Senior interest-only [Member]
Investment Holdings [Line Items]
Principal or Notional Value at Period-End	3,120,432,000		3,734,452,000
Weighted Average Amortized Cost Basis	4.48		5.34
Weighted Average Fair Value	4.13		5.05
Weighted Average Coupon	1.74%		1.96%
Weighted Average Yield at Period-End	8.81%	[1]	13.28%	[1]
Non-Agency RMBS - Subordinated [Member]
Investment Holdings [Line Items]
Principal or Notional Value at Period-End	1,074,560,000		1,378,891,000
Weighted Average Amortized Cost Basis	44.41		47.44
Weighted Average Fair Value	47.37		44.01
Weighted Average Coupon	3.17%		3.44%
Weighted Average Yield at Period-End	10.83%	[1]	9.57%	[1]
Non-Agency RMBS - Subordinated interest-only [Member]
Investment Holdings [Line Items]
Principal or Notional Value at Period-End	264,052,000		277,560,000
Weighted Average Amortized Cost Basis	6.59		7.89
Weighted Average Fair Value	6.39		7.93
Weighted Average Coupon	2.45%		2.94%
Weighted Average Yield at Period-End	5.96%	[1]	9.93%	[1]
Non-Agency RMBS Transferred To Consolidated Variable Interest Entities ("VIEs") [Member]
Investment Holdings [Line Items]
Principal or Notional Value at Period-End	4,795,819,000		5,265,128,000
Weighted Average Amortized Cost Basis	53.48		55.14
Weighted Average Fair Value	68.87		62.11
Weighted Average Coupon	4.96%		5.32%
Weighted Average Yield at Period-End	15.44%	[1]	14.56%	[1]
Agency RMBS [Member]
Investment Holdings [Line Items]
Principal or Notional Value at Period-End	1,934,283,000		3,018,347,000
Weighted Average Amortized Cost Basis	103.14		103.07
Weighted Average Fair Value	$ 108.78		$ 107.06
Weighted Average Coupon	4.65%		4.66%
Weighted Average Yield at Period-End	3.61%	[1]	3.83%	[1]

[1] Bond Equivalent Yield at period end.