Mortgage-Backed Securities - Summary of Investment Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Principal balance	$ 18,052,407	$ 14,514,868
Unamortized Premium (Discount)	(313,291)	(299,501)
Amortized Cost	17,739,116	14,215,367
Unrealized Gain/(Loss), net	585,092	(1,218)
Fair value	18,324,208	14,214,149
Net Weighted Average Coupon	4.23%	4.52%
Weighted Average Yield	3.51%	3.83%
Weighted Average Yield	3.31%	3.93%
Agency RMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Principal balance	12,253,579	9,779,292
Unamortized Premium (Discount)	749,458	566,781
Amortized Cost	13,003,037	10,346,073
Unrealized Gain/(Loss), net	426,918	177,458
Fair value	13,429,955	10,523,531
Net Weighted Average Coupon	4.25%	4.53%
Weighted Average Yield	3.01%	3.33%
Weighted Average Yield	2.81%	3.19%
Agency-CMO [Member]
Schedule of Available-for-sale Securities [Line Items]
Principal balance	1,346,339	765,172
Unamortized Premium (Discount)	(828,433)	(592,342)
Amortized Cost	517,906	172,830
Unrealized Gain/(Loss), net	2,681	(4,368)
Fair value	520,587	168,462
Net Weighted Average Coupon	2.88%	2.86%
Weighted Average Yield	3.58%	3.52%
Weighted Average Yield	2.22%	1.32%
Non-Agency RMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Principal balance	2,829,094	2,719,797
Unamortized Premium (Discount)	(235,582)	(252,135)
Amortized Cost	2,593,512	2,467,662
Unrealized Gain/(Loss), net	32,281	(108,434)
Fair value	2,625,793	2,359,228
Net Weighted Average Coupon	4.12%	4.57%
Weighted Average Yield	4.87%	5.07%
Weighted Average Yield	4.91%	6.05%
CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Principal balance	1,623,395	1,250,607
Unamortized Premium (Discount)	1,266	(21,805)
Amortized Cost	1,624,661	1,228,802
Unrealized Gain/(Loss), net	123,212	(65,874)
Fair value	1,747,873	1,162,928
Net Weighted Average Coupon	5.44%	5.38%
Weighted Average Yield	5.31%	5.60%
Weighted Average Yield	5.24%	5.69%
15 Year Fixed-Rate [Member]
Schedule of Available-for-sale Securities [Line Items]
Principal balance	2,099,509	2,289,495
Unamortized Premium (Discount)	110,129	123,610
Amortized Cost	2,209,638	2,413,105
Unrealized Gain/(Loss), net	74,540	36,454
Fair value	2,284,178	2,449,559
Net Weighted Average Coupon	4.10%	4.18%
Weighted Average Yield	2.65%	2.85%
Weighted Average Yield	2.40%	2.75%
30 Year Fixed-Rate [Member]
Schedule of Available-for-sale Securities [Line Items]
Principal balance	9,419,335	6,055,045
Unamortized Premium (Discount)	620,748	410,257
Amortized Cost	10,040,083	6,465,302
Unrealized Gain/(Loss), net	328,440	116,309
Fair value	10,368,523	6,581,611
Net Weighted Average Coupon	4.37%	4.95%
Weighted Average Yield	3.12%	3.66%
Weighted Average Yield	2.92%	3.52%
ARM [Member]
Schedule of Available-for-sale Securities [Line Items]
Principal balance	119,157	113,413
Unamortized Premium (Discount)	3,768	2,398
Amortized Cost	122,925	115,811
Unrealized Gain/(Loss), net	3,046	2,065
Fair value	125,971	117,876
Net Weighted Average Coupon	3.17%	3.40%
Weighted Average Yield	2.68%	3.07%
Weighted Average Yield	2.75%	2.90%
Hybrid ARM [Member]
Schedule of Available-for-sale Securities [Line Items]
Principal balance	615,578	1,321,339
Unamortized Premium (Discount)	14,813	30,516
Amortized Cost	630,391	1,351,855
Unrealized Gain/(Loss), net	20,892	22,630
Fair value	$ 651,283	$ 1,374,485
Net Weighted Average Coupon	3.20%	3.29%
Weighted Average Yield	2.64%	2.59%
Weighted Average Yield	2.61%	2.49%