Mortgage-Backed and Credit Risk Transfer Securities - Components of Carrying Value of MBS and GSE CRT Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Principal/notional balance	$ 23,354,075	$ 20,114,683
Unamortized premium	440,503	395,907
Unamortized discount	(2,762,302)	(3,148,755)
Gross unrealized gains	812,106	246,027
Gross unrealized losses	(72,596)	(211,220)
Fair Value	21,771,786	17,396,642
Mortgage-backed and credit risk transfer securities
Debt Securities, Available-for-sale [Line Items]
Principal/notional balance	20,957,410	17,442,367
Unamortized premium	440,503	395,907
Unamortized discount	(419,983)	(549,988)
Gross unrealized gains	807,324	238,579
Gross unrealized losses	(66,064)	(204,664)
Fair Value	21,719,190	17,322,201
Interest-Only Securities
Debt Securities, Available-for-sale [Line Items]
Principal/notional balance	2,396,665	2,672,316
Unamortized premium	0	0
Unamortized discount	(2,342,319)	(2,598,767)
Gross unrealized gains	4,782	7,448
Gross unrealized losses	(6,532)	(6,556)
Fair Value	$ 52,596	$ 74,441