Mortgage-Backed and Credit Risk Transfer Securities - Unrealized Losses and Estimated Fair Value of MBS and GSE CRT by Length of Time (Detail) $ in Thousands	Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Fair Value
Less than 12 Months	$ 2,375,171	$ 5,527,120
12 Months or More	155,270	4,216,435
Total	2,530,441	9,743,555
Unrealized Losses
Less than 12 Months	(58,717)	(75,521)
12 Months or More	(13,879)	(135,699)
Total	$ (72,596)	$ (211,220)
Number of Securities
Less than 12 Months (in securities) | security	116	359
12 Months or More (in securities) | security	34	285
Total (in securities) | security	150	644
15 year fixed-rate
Fair Value
Less than 12 Months	$ 957	$ 86,241
12 Months or More	362	16,660
Total	1,319	102,901
Unrealized Losses
Less than 12 Months	(1)	(814)
12 Months or More	(3)	(189)
Total	$ (4)	$ (1,003)
Number of Securities
Less than 12 Months (in securities) | security	2	50
12 Months or More (in securities) | security	4	22
Total (in securities) | security	6	72
30 year fixed-rate
Fair Value
Less than 12 Months	$ 255,649	$ 3,966,347
12 Months or More	34,009	2,846,090
Total	289,658	6,812,437
Unrealized Losses
Less than 12 Months	(207)	(49,182)
12 Months or More	(256)	(94,716)
Total	$ (463)	$ (143,898)
Number of Securities
Less than 12 Months (in securities) | security	3	158
12 Months or More (in securities) | security	5	95
Total (in securities) | security	8	253
Hybrid ARM
Fair Value
Less than 12 Months	$ 434	$ 9,390
12 Months or More	1,524	503,417
Total	1,958	512,807
Unrealized Losses
Less than 12 Months	(2)	(87)
12 Months or More	(46)	(9,175)
Total	$ (48)	$ (9,262)
Number of Securities
Less than 12 Months (in securities) | security	1	3
12 Months or More (in securities) | security	3	81
Total (in securities) | security	4	84
Agency RMBS
Fair Value
Less than 12 Months	$ 257,040	$ 4,061,978
12 Months or More	35,895	3,366,167
Total	292,935	7,428,145
Unrealized Losses
Less than 12 Months	(210)	(50,083)
12 Months or More	(305)	(104,080)
Total	$ (515)	$ (154,163)
Number of Securities
Less than 12 Months (in securities) | security	6	211
12 Months or More (in securities) | security	12	198
Total (in securities) | security	18	409
Fair value option, fair value	$ 271,300	$ 6,100,000
Fair value option, unrealized losses	268	130,200
Agency-CMO
Fair Value
Less than 12 Months	67,875	152,962
12 Months or More	6,155	101,705
Total	74,030	254,667
Unrealized Losses
Less than 12 Months	(1,194)	(6,315)
12 Months or More	(1,513)	(5,100)
Total	$ (2,707)	$ (11,415)
Number of Securities
Less than 12 Months (in securities) | security	15	34
12 Months or More (in securities) | security	13	19
Total (in securities) | security	28	53
Agency IO
Number of Securities
Fair value option, fair value	$ 11,100	$ 21,800
Fair value option, unrealized losses	2,300	6,300
CMO
Number of Securities
Fair value option, fair value		66,000
Fair value option, unrealized losses		845
Agency CMBS
Fair Value
Less than 12 Months	1,743,800
12 Months or More	0
Total	1,743,800
Unrealized Losses
Less than 12 Months	(50,521)
12 Months or More	0
Total	$ (50,521)
Number of Securities
Less than 12 Months (in securities) | security	58
12 Months or More (in securities) | security	0
Total (in securities) | security	58
Non-Agency CMBS
Fair Value
Less than 12 Months	$ 203,129	1,214,691
12 Months or More	101,021	659,298
Total	304,150	1,873,989
Unrealized Losses
Less than 12 Months	(2,783)	(17,778)
12 Months or More	(11,425)	(25,381)
Total	$ (14,208)	$ (43,159)
Number of Securities
Less than 12 Months (in securities) | security	19	94
12 Months or More (in securities) | security	7	52
Total (in securities) | security	26	146
Fair value option, fair value	$ 181,500	$ 831,300
Fair value option, unrealized losses	2,800	26,300
Non-Agency RMBS
Fair Value
Less than 12 Months	26,283	87,850
12 Months or More	12,199	89,265
Total	38,482	177,115
Unrealized Losses
Less than 12 Months	(3,935)	(1,152)
12 Months or More	(636)	(1,138)
Total	$ (4,571)	$ (2,290)
Number of Securities
Less than 12 Months (in securities) | security	14	19
12 Months or More (in securities) | security	2	16
Total (in securities) | security	16	35
Fair value option, fair value	$ 17,600	$ 6,200
Fair value option, unrealized losses	261	79
Non-Agency IO
Number of Securities
Fair value option, fair value	8,500	3,700
Fair value option, unrealized losses	3,700	269
GSE CRT
Fair Value
Less than 12 Months	77,044	9,639
12 Months or More	0	0
Total	77,044	9,639
Unrealized Losses
Less than 12 Months	(74)	(193)
12 Months or More	0	0
Total	$ (74)	$ (193)
Number of Securities
Less than 12 Months (in securities) | security	4	1
12 Months or More (in securities) | security	0	0
Total (in securities) | security	4	1