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RESIDENTIAL INVESTMENT SECURITIES - Component of Agency Mortgage-Backed Securities Portfolio by Issuing Agency Concentration (Details) - Agency Mortgage-Backed Securities - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Mortgage-Backed Securities Portfolio [Line Items]    
Estimated Fair Value $ 88,579,097 $ 90,551,763 [1]
Fannie Mae    
Mortgage-Backed Securities Portfolio [Line Items]    
Estimated Fair Value 62,734,494 63,361,415
Freddie Mac    
Mortgage-Backed Securities Portfolio [Line Items]    
Estimated Fair Value 25,750,396 27,091,978
Ginnie Mae    
Mortgage-Backed Securities Portfolio [Line Items]    
Estimated Fair Value $ 94,207 $ 98,370
[1] Derived from the audited consolidated financial statements at December 31, 2017.