Quarterly report pursuant to Section 13 or 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
Sep. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Adjustable Rate $ 2,325,207 $ 1,605,313
Fixed Rate 17,873,887 11,523,544
Available-for-sale securities, at fair value 20,199,094 13,128,857
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Adjustable Rate 24,960 30,463
Fixed Rate 17,529,411 11,196,011
Available-for-sale securities, at fair value 17,554,371 11,226,474
Mortgage-backed Securities, Issued by Private Enterprises [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Adjustable Rate 2,300,247 1,574,850
Fixed Rate 344,476 327,533
Available-for-sale securities, at fair value $ 2,644,723 $ 1,902,383