Residential Mortgage-Backed Securities - Maturities (Detail) (Residential Mortgage Backed Securities [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Less than one year, at fair value	$ 16,343	$ 50,729
Greater than one year and less than five years, at fair value	2,449,060	2,261,339
Greater than five years and less than ten years, at fair value	2,502,122	3,274,368
Greater than ten years, at fair value	920,136	1,647,040
Fair Value	5,887,661	7,233,476
Less than one year, at amortized cost	15,585	56,923
Greater than one year and less than five years, at amortized cost	2,282,884	2,154,692
Greater than five years and less than ten years, at amortized cost	1,997,460	2,998,188
Greater than ten years, at amortized cost	745,026	1,595,893
Total maturities, at amortized cost	5,040,955	6,805,696
Non-Agency RMBS - Senior [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year, at fair value		892
Greater than five years and less than ten years, at fair value	82	128
Fair Value	82	1,020
Less than one year, at amortized cost		891
Greater than five years and less than ten years, at amortized cost	79	168
Total maturities, at amortized cost	79	1,059
Non-Agency RMBS - Senior interest-only [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year, at fair value	570
Greater than one year and less than five years, at fair value	63,876	85,633
Greater than five years and less than ten years, at fair value	53,324	69,204
Greater than ten years, at fair value	11,098	33,842
Fair Value	128,868	188,679
Less than one year, at amortized cost	798
Greater than one year and less than five years, at amortized cost	77,297	95,974
Greater than five years and less than ten years, at amortized cost	53,085	69,953
Greater than ten years, at amortized cost	8,647	33,361
Total maturities, at amortized cost	139,827	199,288
Non-Agency RMBS - Subordinated [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year, at fair value	521	6,530
Greater than one year and less than five years, at fair value	67,397	101,984
Greater than five years and less than ten years, at fair value	305,436	259,549
Greater than ten years, at fair value	135,707	238,832
Fair Value	509,061	606,895
Less than one year, at amortized cost	357	5,616
Greater than one year and less than five years, at amortized cost	64,832	98,657
Greater than five years and less than ten years, at amortized cost	293,471	300,489
Greater than ten years, at amortized cost	118,594	249,390
Total maturities, at amortized cost	477,254	654,152
Non-Agency RMBS - Subordinated interest-only [Member]
Schedule of Available-for-sale Securities [Line Items]
Greater than five years and less than ten years, at fair value		1,812
Greater than ten years, at fair value	16,867	20,207
Fair Value	16,867	22,019
Greater than five years and less than ten years, at amortized cost		1,946
Greater than ten years, at amortized cost	17,397	19,964
Total maturities, at amortized cost	17,397	21,910
Non-Agency RMBS Transferred To Consolidated Variable Interest Entities ("VIEs") [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year, at fair value	15,028	25,375
Greater than one year and less than five years, at fair value	320,800	338,616
Greater than five years and less than ten years, at fair value	2,143,280	2,119,030
Greater than ten years, at fair value	756,464	787,311
Fair Value	3,235,572	3,270,332
Less than one year, at amortized cost	14,228	32,806
Greater than one year and less than five years, at amortized cost	247,324	296,144
Greater than five years and less than ten years, at amortized cost	1,650,825	1,827,000
Greater than ten years, at amortized cost	600,388	746,052
Total maturities, at amortized cost	2,512,765	2,902,002
Agency RMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than one year, at fair value	224	17,932
Greater than one year and less than five years, at fair value	1,996,987	1,735,106
Greater than five years and less than ten years, at fair value		824,645
Greater than ten years, at fair value		566,848
Fair Value	1,997,211	3,144,531
Less than one year, at amortized cost	202	17,610
Greater than one year and less than five years, at amortized cost	1,893,431	1,663,917
Greater than five years and less than ten years, at amortized cost		798,632
Greater than ten years, at amortized cost		547,126
Total maturities, at amortized cost	$ 1,893,633	$ 3,027,285