Mortgage-Backed Securities - Gains and Losses from Sales of Investments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Proceeds from sales	$ 32,551	$ 153,681	$ 32,551	$ 173,743
Gross realized gains	4,383	5,380	4,383	10,566
Gross realized losses	(2,216)	(839)	(2,216)	(858)
Net realized gain (loss)	$ 2,167	$ 4,541	$ 2,167	$ 9,708