Mortgage-Backed Securities - Aggregate Other Than Temporary Impairments (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
Investments, Debt and Equity Securities [Abstract]
Total other-than-temporary impairment losses									$ (801)	$ (2,556)	$ (5,169)
Portion of loss recognized in other comprehensive income (loss)									(4,052)	(19,235)	(56,687)
Net other-than-temporary credit impairment losses	$ 0	$ 0	$ 0	$ (4,853)	$ (4,269)	$ (7,233)	$ (9,131)	$ (1,158)	$ (4,853)	$ (21,791)	$ (61,856)