Financial Derivatives Narrative (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
U.S. Treasury securities sold short, at fair value	$ 72,762	$ 74,194
US Treasury Securities [Member]
Derivative [Line Items]
U.S. Treasury securities sold short, at fair value	72,762	74,194
Investment Sold, Not yet Purchased, Balance, Principal Amount	$ 74,800	$ 78,600