Other Assets - Derivative Instruments (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Assets [Abstract]
Repurchase agreements recorded upon adoption of revised accounting standard for repurchase agreement financing	$ 0	$ 0	$ 1,519,593
Cumulative effect adjustment on adoption of revised accounting standard for repurchase agreement financing	$ 0	$ 0	$ 4,537