Agency Securities, Available for Sale - Summary of Weighted Average Lives of Agency Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value
Less than one year	$ 16	$ 19
Greater than or equal to one year and less than three years	67,434	30,189
Greater than or equal to three years and less than five years	7,452,538	6,037,851
Greater than or equal to five years	2,954,022	6,393,497
Total Agency Securities	10,474,010	12,461,556
Amortized Cost
Less than one year	16	19
Greater than or equal to one year and less than three years	67,080	30,375
Greater than or equal to three years and less than five years	7,396,947	6,039,218
Greater than or equal to five years	2,887,248	6,459,246
Amortized Cost	$ 10,351,291	$ 12,528,858