Mortgage-Backed Securities - Continuous Unrealized Loss Position (Detail) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) item
Schedule Of Available For Sale Securities [Line Items]
Unrealized gain (loss) on trading investments	$ 1,900	$ 3,000	$ 17,800	$ (14,600)
Impairment charge	$ 1,145	141	$ 1,751	1,898
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months Number of Securities | item	4		4		47
Less Than 12 Months Fair Value	$ 27,716		$ 27,716		$ 859,060
Less Than 12 Months Unrealized Losses	$ (50)		$ (50)		$ (6,484)
12 Months or More Number of Securities | item	64		64		166
12 Months or More Fair Value	$ 386,835		$ 386,835		$ 1,301,348
12 Months or More Unrealized Losses	$ (4,510)		$ (4,510)		$ (36,937)
Total Number of Securities | item	68		68		213
Total Fair Value	$ 414,551		$ 414,551		$ 2,160,408
Total Unrealized Losses	(4,560)		(4,560)		$ (43,421)
Unrealized gain (loss) on trading investments	$ 1,939	(3,017)	$ 17,834	(14,584)
Non-Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months Number of Securities | item	10		10		56
Less Than 12 Months Fair Value	$ 41,092		$ 41,092		$ 329,108
Less Than 12 Months Unrealized Losses	$ (607)		$ (607)		$ (5,886)
12 Months or More Number of Securities | item	10		10		12
12 Months or More Fair Value	$ 67,689		$ 67,689		$ 72,514
12 Months or More Unrealized Losses	$ (3,439)		$ (3,439)		$ (5,304)
Total Number of Securities | item	20		20		68
Total Fair Value	$ 108,781		$ 108,781		$ 401,622
Total Unrealized Losses	(4,046)		(4,046)		(11,190)
Impairment charge	$ 1,145	$ 141	1,751	$ 1,898
Trading Securities | Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Unrealized gain (loss) on trading investments			$ 1,900		$ (15,900)