SECURITIES - Component of Agency Mortgage-Backed Securities Portfolio by Issuing Agency Concentration (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Mortgage-Backed Securities Portfolio [Line Items]
Estimated Fair Value	$ 91,338,611	$ 92,563,572
Agency Mortgage-Backed Securities
Mortgage-Backed Securities Portfolio [Line Items]
Estimated Fair Value	89,290,128	90,551,763
Agency Mortgage-Backed Securities \| Fannie Mae
Mortgage-Backed Securities Portfolio [Line Items]
Estimated Fair Value	59,568,293	63,361,415
Agency Mortgage-Backed Securities \| Freddie Mac
Mortgage-Backed Securities Portfolio [Line Items]
Estimated Fair Value	29,636,443	27,091,978
Agency Mortgage-Backed Securities \| Ginnie Mae
Mortgage-Backed Securities Portfolio [Line Items]
Estimated Fair Value	$ 85,392	$ 98,370

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://www.xbrl.org/2003/role/presentationRef
-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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