Mortgage-Backed and Credit Risk Transfer Securities - Components of Carrying Value of MBS and GSE CRT Portfolio (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Principal/ notional balance	$ 20,464,863	$ 20,855,353
Unamortized premium	494,730	521,626
Unamortized discount	(3,241,443)	(3,362,618)
Gross unrealized gains	282,242	341,656
Gross unrealized losses	(378,158)	(165,263)
Fair value	17,622,234	18,190,754
MBS and GSE CRT Securities
Schedule of Available-for-sale Securities [Line Items]
Principal/ notional balance	17,696,263	17,974,390
Unamortized premium	494,730	521,626
Unamortized discount	(559,366)	(577,344)
Gross unrealized gains	277,526	336,543
Gross unrealized losses	(368,916)	(155,146)
Fair value	17,540,237	18,100,069
Interest-Only Securities
Schedule of Available-for-sale Securities [Line Items]
Principal/ notional balance	2,768,600	2,880,963
Unamortized premium	0	0
Unamortized discount	(2,682,077)	(2,785,274)
Gross unrealized gains	4,716	5,113
Gross unrealized losses	(9,242)	(10,117)
Fair value	$ 81,997	$ 90,685