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Significant Accounting Policies Derivative Instruments (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Jul. 26, 2017
Jun. 06, 2017
Derivative      
Derivative, Notional Amount   $ 23,096 $ 20,253
Future [Member]      
Derivative      
Derivative, Notional Amount $ 147,988    
Derivative Financial Instruments, Assets [Member] | Future [Member]      
Derivative      
Derivative, Notional Amount $ 44,464