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, 2021 USD ($) security	Dec. 31, 2020 USD ($) security
Fair Value
Less than 12 Months	$ 6,861,576	$ 1,526,029
12 Months or More	1,042	1,612
Total	6,862,618	1,527,641
Unrealized Losses
Less than 12 Months	(63,262)	(4,965)
12 Months or More	(1,073)	(203)
Total	$ (64,335)	$ (5,168)
Number of Securities
Less than 12 Months (in securities) | security	64	27
12 Months or More (in securities) | security	9	7
Total (in securities) | security	73	34
30 year fixed-rate
Fair Value
Less than 12 Months	$ 6,838,999	$ 1,496,279
12 Months or More	0	0
Total	6,838,999	1,496,279
Unrealized Losses
Less than 12 Months	(60,741)	(4,108)
12 Months or More	0	0
Total	$ (60,741)	$ (4,108)
Number of Securities
Less than 12 Months (in securities) | security	54	20
12 Months or More (in securities) | security	0	0
Total (in securities) | security	54	20
Total Agency RMBS pass-through
Fair Value
Less than 12 Months	$ 6,838,999	$ 1,496,279
12 Months or More	0	0
Total	6,838,999	1,496,279
Unrealized Losses
Less than 12 Months	(60,741)	(4,108)
12 Months or More	0	0
Total	$ (60,741)	$ (4,108)
Number of Securities
Less than 12 Months (in securities) | security	54	20
12 Months or More (in securities) | security	0	0
Total (in securities) | security	54	20
Agency-CMO
Fair Value
Less than 12 Months	$ 21,810
12 Months or More	0
Total	21,810
Unrealized Losses
Less than 12 Months	(1,389)
12 Months or More	0
Total	$ (1,389)
Number of Securities
Less than 12 Months (in securities) | security	5
12 Months or More (in securities) | security	0
Total (in securities) | security	5
Non-Agency CMBS
Fair Value
Less than 12 Months		$ 27,069
12 Months or More		0
Total		27,069
Unrealized Losses
Less than 12 Months		(419)
12 Months or More		0
Total		$ (419)
Number of Securities
Less than 12 Months (in securities) | security		1
12 Months or More (in securities) | security		0
Total (in securities) | security		1
Non-Agency RMBS
Fair Value
Less than 12 Months	$ 767	$ 2,681
12 Months or More	1,042	1,612
Total	1,809	4,293
Unrealized Losses
Less than 12 Months	(1,132)	(438)
12 Months or More	(1,073)	(203)
Total	$ (2,205)	$ (641)
Number of Securities
Less than 12 Months (in securities) | security	5	6
12 Months or More (in securities) | security	9	7
Total (in securities) | security	14	13
Non-Agency IO
Fair Value
Total	$ 1,700
Unrealized Losses
Total	(2,100)
Non-Agency Residential Mortgage Backed Securities, Excluding Non-Agency IO
Unrealized Losses
Total	$ (136)