Schedule of Components of Purchase Discount on its Non-Agency MBS (Detail) - Non-Agency MBS - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2017
Market Yield Adjustment
Schedule Of Available For Sale Securities [Line Items]
Balance, beginning of period	$ 120,099	$ 83,577
Accretion of discount	(1,129)	(2,245)
Purchases	10,427	48,065
Balance, end of period	129,397	129,397
Non-Accretable Discount
Schedule Of Available For Sale Securities [Line Items]
Balance, beginning of period	(277,366)	(223,291)
Purchases	(15,908)	(75,475)
Realized credit losses, net of recoveries	5,014	11,238
Impairment charge	(905)	(1,637)
Balance, end of period	$ (289,165)	$ (289,165)