Mortgage-Backed and Credit Risk Transfer Securities - Fair Value of MBS and GSE CRTs According to Weighted Average Life Classification (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Less than one year	$ 22,112	$ 268,536
Greater than one year and less than five years	5,303,917	7,836,620
Greater than or equal to five years	2,846,153	13,666,630
Total	$ 8,172,182	$ 21,771,786

The fair value as of the balance sheet date of firm holdings in mortgages, mortgage backed securities (investment instruments that represent ownership of an undivided interest in a group of mortgages, where principal and interest from the individual mortgages are used to pay investors' principal and interest on the mortgage backed security) and asset-backed securities that are primarily paid from the cash flows of a discrete pool of receivables or other financial assets (such as credit-card receivables), either fixed or revolving, that by their terms convert into cash within a finite time period. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=35711157&loc=d3e42546-110969

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Period Type:	instant