Schedule of Components of Purchase Discount on its Non-Agency MBS (Detail) - Non-Agency MBS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Market Yield Adjustment
Schedule Of Available For Sale Securities [Line Items]
Balance, beginning of period	$ 137,237	$ 147,198
Accretion of discount	(5,948)	(6,633)
Purchases		(777)
Sales	(319)	1,413
Transfer		(3,964)
Balance, end of period	130,970	137,237
Non-Accretable Discount
Schedule Of Available For Sale Securities [Line Items]
Balance, beginning of period	(332,886)	(332,480)
Accretion of discount		(100)
Purchases		(23,419)
Realized credit losses, net of recoveries	16,401	22,018
Sales	408
Impairment charge	(2,108)	(2,869)
Transfer		3,964
Balance, end of period	$ (318,185)	$ (332,886)