Annual report pursuant to Section 13 and 15(d)

Available-for-sale Securities Classified by Rate Type (Details)

v3.8.0.1
Available-for-sale Securities Classified by Rate Type (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Adjustable Rate $ 2,645,930 $ 1,592,627
Fixed Rate 18,574,889 11,523,544
Available-for-sale securities, at fair value 21,220,819 13,116,171
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Adjustable Rate 23,220 30,463
Fixed Rate 18,215,505 11,196,011
Available-for-sale securities, at fair value 18,238,725 11,226,474
Mortgage-backed Securities, Issued by Private Enterprises [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Adjustable Rate 2,622,710 1,562,164
Fixed Rate 359,384 327,533
Available-for-sale securities, at fair value $ 2,982,094 $ 1,889,697