Mortgage-Backed Securities - Continuous Unrealized Loss Position (Detail) $ in Thousands	3 Months Ended						12 Months Ended
	Dec. 31, 2019 USD ($) item	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($) item	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Impairment charge	$ 357	$ 1,145	$ 606	$ 971	$ 141	$ 1,757	$ 2,108	$ 2,869	$ 2,399
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months Number of Securities | item	10			47			10	47
Less Than 12 Months Fair Value	$ 270,737			$ 859,060			$ 270,737	$ 859,060
Less Than 12 Months Unrealized Losses	$ (419)			$ (6,484)			$ (419)	$ (6,484)
12 Months or More Number of Securities | item	38			166			38	166
12 Months or More Fair Value	$ 168,095			$ 1,301,348			$ 168,095	$ 1,301,348
12 Months or More Unrealized Losses	$ (1,227)			$ (36,937)			$ (1,227)	$ (36,937)
Total Number of Securities | item	48			213			48	213
Total Fair Value	$ 438,832			$ 2,160,408			$ 438,832	$ 2,160,408
Total Unrealized Losses	$ (1,646)			$ (43,421)			(1,646)	(43,421)
Unrealized gain (loss) on trading investments							$ 17,036	$ (4,911)	2,793
Non-Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months Number of Securities | item	18			56			18	56
Less Than 12 Months Fair Value	$ 49,281			$ 329,108			$ 49,281	$ 329,108
Less Than 12 Months Unrealized Losses	$ (1,507)			$ (5,886)			$ (1,507)	$ (5,886)
12 Months or More Number of Securities | item	12			12			12	12
12 Months or More Fair Value	$ 75,926			$ 72,514			$ 75,926	$ 72,514
12 Months or More Unrealized Losses	$ (2,647)			$ (5,304)			$ (2,647)	$ (5,304)
Total Number of Securities | item	30			68			30	68
Total Fair Value	$ 125,207			$ 401,622			$ 125,207	$ 401,622
Total Unrealized Losses	$ (4,154)			$ (11,190)			(4,154)	(11,190)
Impairment charge							$ 2,108	$ 2,869	$ 2,399