Mortgage-Backed Securities - Gains and Losses from Sales of Investments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Proceeds from sales									$ 812,005	$ 2,719,329	$ 6,244,613
Gross realized gains									17,538	28,572	94,618
Gross realized losses									(8,415)	(10,417)	(17,544)
Net realized gain (loss)	$ (586)	$ 1	$ 4,541	$ 5,167	$ 11,121	$ 3,079	$ 6,631	$ (2,674)	$ 9,123	$ 18,155	$ 77,074