Investments in Agency MBS - Additional Information About Gains and Losses Recognized with Respect to Investments in MBS classified as trading securities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net gains (losses) recognized in earnings for:
MBS sold during the period	$ (37,878)	$ 13,996	$ (147,113)	$ 25,632
Agency MBS
Net gains (losses) recognized in earnings for:
MBS still held at period end	(29,744)	12,715	(103,536)	22,178
MBS sold during the period	(8,113)	1,296	(43,547)	3,421
Total	$ (37,857)	$ 14,011	$ (147,083)	$ 25,599