Counterparty Risk (% Of Due from Brokers) (Details)	Dec. 31, 2017
Morgan Stanley [Member]
Concentration Risk [Line Items]
% of Total Due from Brokers	35.00%
BNP Paribas Securities Corp. [Member]
Concentration Risk [Line Items]
% of Total Due from Brokers	24.00%
J.P. Morgan Securities LLC [Member]
Concentration Risk [Line Items]
% of Total Due from Brokers	17.00%

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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