Mortgage-Backed Securities - Continuous Unrealized Loss Position (Detail) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) item	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) item
Schedule Of Available For Sale Securities [Line Items]
Unrealized gain (loss) on trading investments	$ (14,900)	$ 8,900
Impairment charge		18
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months Number of Securities | item	15		47
Less Than 12 Months Fair Value	$ 190,147		$ 859,060
Less Than 12 Months Unrealized Losses	$ (526)		$ (6,484)
12 Months or More Number of Securities | item	148		166
12 Months or More Fair Value	$ 1,156,763		$ 1,301,348
12 Months or More Unrealized Losses	$ (19,843)		$ (36,937)
Total Number of Securities | item	163		213
Total Fair Value	$ 1,346,910		$ 2,160,408
Total Unrealized Losses	(20,369)		$ (43,421)
Unrealized gain (loss) on trading investments	$ 14,906	$ (8,890)
Non-Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months Number of Securities | item	36		56
Less Than 12 Months Fair Value	$ 204,872		$ 329,108
Less Than 12 Months Unrealized Losses	$ (3,523)		$ (5,886)
12 Months or More Number of Securities | item	13		12
12 Months or More Fair Value	$ 73,560		$ 72,514
12 Months or More Unrealized Losses	$ (5,161)		$ (5,304)
Total Number of Securities | item	49		68
Total Fair Value	$ 278,432		$ 401,622
Total Unrealized Losses	(8,684)		(11,190)
Trading Securities | Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Unrealized gain (loss) on trading investments	$ (1,000)		$ (15,900)