Mortgage-Backed Securities - Gains and Losses from Sales of Investments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Net realized gain (loss)	$ 17,687	$ 1,596	$ (7,526)	$ 8,603	$ 1,213	$ (6,123)	$ 2,167	$ 0	$ 18,770	$ (2,844)	$ 9,123
Non-Agency RMBS
Investment Holdings [Line Items]
Proceeds from sales:									38,659	100,791	68,164
Gross realized gains:									422	7,424	12,171
Gross realized losses:									(1,445)	(7,234)	(20)
Agency RMBS
Investment Holdings [Line Items]
Proceeds from sales:									2,915,545	7,064	570,195
Gross realized gains:									35,221	0	3,068
Gross realized losses:									(24,531)	(1,919)	(7,740)
Agency CMBS
Investment Holdings [Line Items]
Proceeds from sales:									375,489	9,120	173,646
Gross realized gains:									11,328	122	2,300
Gross realized losses:									$ (2,225)	$ (1,237)	$ (656)