Mortgage-Backed Securities - Continuous Unrealized Loss Position (Detail) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) item	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) item
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months Number of Securities | item		28		10
Less Than 12 Months Fair Value		$ 24,427		$ 270,737
Less Than 12 Months Unrealized Losses		$ (118)		$ (419)
12 Months or More Number of Securities | item		16		38
12 Months or More Fair Value		$ 8,387		$ 168,095
12 Months or More Unrealized Losses		$ (96)		$ (1,227)
Total Number of Securities | item		44		48
Total Fair Value		$ 32,814		$ 438,832
Total Unrealized Losses		(214)		$ (1,646)
Unrealized gain (loss) on trading investments	$ 989	$ (1,141)	$ 15,895
Non-Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months Number of Securities | item				18
Less Than 12 Months Fair Value				$ 49,281
Less Than 12 Months Unrealized Losses				$ (1,507)
12 Months or More Number of Securities | item				12
12 Months or More Fair Value				$ 75,926
12 Months or More Unrealized Losses				$ (2,647)
Total Number of Securities | item				30
Total Fair Value				$ 125,207
Total Unrealized Losses				$ (4,154)
Impairment charge	$ 606		$ 606