Agency Securities, Available for Sale - Gross Unrealized Losses and Estimated Fair Value of Agency Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value
Less than 12 Months	$ 123,793	$ 58,363
12 Months or More	25,987	27,640
Total	149,780	86,003
Unrealized Losses
Less than 12 Months	(1,416)	(26)
Unrealized Losses	(476)	(652)
Unrealized Losses	$ (1,892)	$ (678)