Mortgage-Backed and Credit Risk Transfer Securities - Unrealized Losses and Estimated Fair Value of MBS and GSE CRTs by Length of Time (Detail) $ in Thousands	Dec. 31, 2016 USD ($) security	Dec. 31, 2015 USD ($) security
Schedule of Available-for-sale Securities
Fair value, less than 12 Months	$ 5,858,500	$ 6,095,710
Unrealized losses, less than 12 months	$ (110,569)	$ (78,451)
Number of securities, less than 12 months | security	386	368
Fair value, 12 months or more	$ 1,218,879	$ 1,675,435
Unrealized losses, 12 months or more	$ (28,967)	$ (73,001)
Number of securities, 12 months or more | security	95	91
Fair value, total	$ 7,077,379	$ 7,771,145
Unrealized losses, total	$ (139,536)	$ (151,452)
Number of securities, total | security	481	459
Agency RMBS
Schedule of Available-for-sale Securities
Fair value, less than 12 Months	$ 5,006,723	$ 3,491,280
Unrealized losses, less than 12 months	$ (92,163)	$ (40,491)
Number of securities, less than 12 months | security	280	187
Fair value, 12 months or more	$ 615,070	$ 1,197,897
Unrealized losses, 12 months or more	$ (19,571)	$ (40,979)
Number of securities, 12 months or more | security	47	53
Fair value, total	$ 5,621,793	$ 4,689,177
Unrealized losses, total	$ (111,734)	$ (81,470)
Number of securities, total | security	327	240
Fair value option, amount	$ 149,700
Fair value option, unrealized losses	4,000
Agency RMBS | 15 Year Fixed-Rate
Schedule of Available-for-sale Securities
Fair value, less than 12 Months	2,781,777	$ 600,480
Unrealized losses, less than 12 months	$ (66,506)	$ (8,081)
Number of securities, less than 12 months | security	127	33
Fair value, 12 months or more	$ 65,964	$ 77,506
Unrealized losses, 12 months or more	$ (1,556)	$ (1,482)
Number of securities, 12 months or more | security	17	6
Fair value, total	$ 2,847,741	$ 677,986
Unrealized losses, total	$ (68,062)	$ (9,563)
Number of securities, total | security	144	39
Agency RMBS | 30 Year Fixed-Rate
Schedule of Available-for-sale Securities
Fair value, less than 12 Months	$ 747,719	$ 776,065
Unrealized losses, less than 12 months	$ (15,409)	$ (14,827)
Number of securities, less than 12 months | security	45	32
Fair value, 12 months or more	$ 547,763	$ 1,120,391
Unrealized losses, 12 months or more	$ (18,004)	$ (39,497)
Number of securities, 12 months or more | security	27	47
Fair value, total	$ 1,295,482	$ 1,896,456
Unrealized losses, total	$ (33,413)	$ (54,324)
Number of securities, total | security	72	79
Agency RMBS | ARM
Schedule of Available-for-sale Securities
Fair value, less than 12 Months	$ 120,540	$ 200,863
Unrealized losses, less than 12 months	$ (326)	$ (501)
Number of securities, less than 12 months | security	9	11
Fair value, 12 months or more	$ 1,091
Unrealized losses, 12 months or more	$ (7)	$ 0
Number of securities, 12 months or more | security	1	0
Fair value, total	$ 121,631	$ 200,863
Unrealized losses, total	$ (333)	$ (501)
Number of securities, total | security	10	11
Agency RMBS | Hybrid ARM
Schedule of Available-for-sale Securities
Fair value, less than 12 Months	$ 1,356,687	$ 1,913,872
Unrealized losses, less than 12 months	$ (9,922)	$ (17,082)
Number of securities, less than 12 months | security	99	111
Fair value, 12 months or more	$ 252
Unrealized losses, 12 months or more	$ (4)	$ 0
Number of securities, 12 months or more | security	2	0
Fair value, total	$ 1,356,939	$ 1,913,872
Unrealized losses, total	$ (9,926)	$ (17,082)
Number of securities, total | security	101	111
Agency-CMO
Schedule of Available-for-sale Securities
Fair value, less than 12 Months	$ 163,114	$ 166,754
Unrealized losses, less than 12 months	$ (3,812)	$ (3,296)
Number of securities, less than 12 months | security	28	14
Fair value, 12 months or more	$ 22,792	$ 9,118
Unrealized losses, 12 months or more	$ (952)	$ (6,934)
Number of securities, 12 months or more | security	3	9
Fair value, total	$ 185,906	$ 175,872
Unrealized losses, total	$ (4,764)	$ (10,230)
Number of securities, total | security	31	23
Fair value option, unrealized losses	$ 1,700	$ 1,900
Non-Agency RMBS
Schedule of Available-for-sale Securities
Fair value, less than 12 Months	287,647	872,575
Unrealized losses, less than 12 months	$ (7,861)	$ (7,286)
Number of securities, less than 12 months | security	42	66
Fair value, 12 months or more	$ 497,863	$ 316,010
Unrealized losses, 12 months or more	$ (6,671)	$ (10,699)
Number of securities, 12 months or more | security	36	20
Fair value, total	$ 785,510	$ 1,188,585
Unrealized losses, total	$ (14,532)	$ (17,985)
Number of securities, total | security	78	86
GSE CRT
Schedule of Available-for-sale Securities
Fair value, less than 12 Months		$ 340,116
Unrealized losses, less than 12 months	$ 0	$ (10,050)
Number of securities, less than 12 months | security	0	16
Fair value, 12 months or more	$ 35,935	$ 120,877
Unrealized losses, 12 months or more	$ (969)	$ (13,605)
Number of securities, 12 months or more | security	3	7
Fair value, total	$ 35,935	$ 460,993
Unrealized losses, total	$ (969)	$ (23,655)
Number of securities, total | security	3	23
Fair value option, amount		$ 12,400
Fair value option, unrealized losses		56
CMBS
Schedule of Available-for-sale Securities
Fair value, less than 12 Months	$ 401,016	1,224,985
Unrealized losses, less than 12 months	$ (6,733)	$ (17,328)
Number of securities, less than 12 months | security	36	85
Fair value, 12 months or more	$ 47,219	$ 31,533
Unrealized losses, 12 months or more	$ (804)	$ (784)
Number of securities, 12 months or more | security	6	2
Fair value, total	$ 448,235	$ 1,256,518
Unrealized losses, total	$ (7,537)	$ (18,112)
Number of securities, total | security	42	87
Fair value option, amount	$ 13,900
Fair value option, unrealized losses	613
Agency IO
Schedule of Available-for-sale Securities
Fair value option, unrealized losses	$ 3,000	$ 8,300