Quarterly report pursuant to Section 13 or 15(d)

Available-for-Sale Securities, at Fair Value (Details)

v3.10.0.1
Available-for-Sale Securities, at Fair Value (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, at fair value $ 25,938,916 $ 21,220,819
Available-for-sale securities, at fair value, pledged as collateral for borrowings 24,600,000 21,000,000
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, at fair value 16,905,970 13,920,721
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, at fair value 4,138,060 3,616,967
Government National Mortgage Association Certificates and Obligations (GNMA) [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, at fair value 1,074,916 701,037
Mortgage-backed Securities, Issued by Private Enterprises [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, at fair value $ 3,819,970 $ 2,982,094