Financial Derivatives Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
U.S. Treasury securities sold short, at fair value	$ 374	$ 81,289
US Treasury Securities [Member]
Derivative [Line Items]
U.S. Treasury securities sold short, at fair value	374	81,289
Investment Sold, Not yet Purchased, Balance, Principal Amount	$ 400	$ 82,500