Annual report pursuant to Section 13 and 15(d)

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

v3.8.0.1
Available-for-Sale Securities, at Fair Value (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, at fair value $ 21,220,819 $ 13,116,171
Available-for-sale securities, at fair value, pledged as collateral for borrowings 20,990,890 13,117,329
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, at fair value 13,920,721 8,274,507
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, at fair value 3,616,967 2,742,630
Government National Mortgage Association Certificates and Obligations (GNMA) [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, at fair value 701,037 209,337
Mortgage-backed Securities, Issued by Private Enterprises [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, at fair value $ 2,982,094 $ 1,889,697