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Financial Derivatives Narrative (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Derivative [Line Items]    
U.S. Treasury securities sold short, at fair value $ 44,377 $ 81,289
US Treasury Securities [Member]    
Derivative [Line Items]    
U.S. Treasury securities sold short, at fair value 44,377 81,289
Investment Sold, Not yet Purchased, Balance, Principal Amount $ 44,400 $ 82,500