Mortgage-Backed Securities - Continuous Unrealized Loss Position (Detail) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($) item	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) item	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) item
Schedule Of Available For Sale Securities [Line Items]
Unrealized gain (loss) on trading investments	$ 1,000	$ 2,700	$ 15,900	$ (11,600)
Impairment charge	$ 606	1,757	$ 606	1,757
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months Number of Securities | item	10		10		47
Less Than 12 Months Fair Value	$ 45,613		$ 45,613		$ 859,060
Less Than 12 Months Unrealized Losses	$ (91)		$ (91)		$ (6,484)
12 Months or More Number of Securities | item	81		81		166
12 Months or More Fair Value	$ 893,331		$ 893,331		$ 1,301,348
12 Months or More Unrealized Losses	$ (6,153)		$ (6,153)		$ (36,937)
Total Number of Securities | item	91		91		213
Total Fair Value	$ 938,944		$ 938,944		$ 2,160,408
Total Unrealized Losses	(6,244)		(6,244)		$ (43,421)
Unrealized gain (loss) on trading investments	$ 989	(2,677)	$ 15,895	(11,567)
Non-Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months Number of Securities | item	19		19		56
Less Than 12 Months Fair Value	$ 91,521		$ 91,521		$ 329,108
Less Than 12 Months Unrealized Losses	$ (1,666)		$ (1,666)		$ (5,886)
12 Months or More Number of Securities | item	15		15		12
12 Months or More Fair Value	$ 86,213		$ 86,213		$ 72,514
12 Months or More Unrealized Losses	$ (4,247)		$ (4,247)		$ (5,304)
Total Number of Securities | item	34		34		68
Total Fair Value	$ 177,734		$ 177,734		$ 401,622
Total Unrealized Losses	(5,913)		(5,913)		(11,190)
Impairment charge	$ 606	$ 1,757	$ 606	$ 1,757
Trading Securities | Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Unrealized gain (loss) on trading investments					$ (15,900)