Mortgage-Backed Securities - Gains and Losses from Sales of Investments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Investment Holdings [Line Items]
Net realized gain (loss)	$ 3,079	$ 3,539	$ 7,035	$ 42,789
Residential Mortgage-Backed Securities
Investment Holdings [Line Items]
Proceeds from sales	58,036	196,192	2,697,239	3,433,527
Gross realized gains	3,622	4,612	16,798	46,007
Gross realized losses	(543)	(1,073)	(9,763)	(3,218)
Net realized gain (loss)	$ 3,079	$ 3,539	$ 7,035	$ 42,789