Mortgage-Backed Securities - Non-Agency Securities Credit Losses (Detail) - Non-agency MBS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Cumulative credit loss beginning balance	$ 556,485	$ 529,112	$ 507,548
Additions:
Other-than-temporary impairments not previously recognized	12,669	25,959	41,233
Reductions for securities sold or deconsolidated during the period	(12,405)	(9,429)	(19,746)
Increases related to other-than-temporary impairments on securities with previously recognized other-than-temporary impairments	47,211	32,027	26,138
Reductions for increases in cash flows expected to be collected over the remaining life of the securities	(12,439)	(21,184)	(26,061)
Cumulative credit impairment loss ending balance	$ 591,521	$ 556,485	$ 529,112