Mortgage-Backed Securities - Aggregate Other Than Temporary Impairments (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
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Total other-than-temporary impairment losses	$ (923)	$ (784)	$ (749)	$ (2,713)	$ (1,034)	$ (993)	$ (3,129)	$ (4,423)
Portion of loss recognized in other comprehensive income (loss)	$ (17,256)	$ (10,684)	$ (12,760)	$ (15,988)	$ (13,746)	$ (10,581)	$ (17,816)	$ (6,255)
Residential Mortgage-Backed Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Total other-than-temporary impairment losses									$ (5,169)	$ (9,589)	$ (8,700)
Portion of loss recognized in other comprehensive income (loss)									(56,687)	(48,398)	(58,744)
Net other-than-temporary credit impairment losses									$ (61,856)	$ (57,987)	$ (67,444)