Counterparty Risk (% Of Total Receivable For Securities Sold) (Details)	Mar. 31, 2017
J.P. Morgan Securities LLC [Member]
Concentration Risk [Line Items]
% of Total Receivable for Securities Sold	21.00%
Bank of America Securities [Member]
Concentration Risk [Line Items]
% of Total Receivable for Securities Sold	17.00%
Goldman, Sachs & Co. [Member]
Concentration Risk [Line Items]
% of Total Receivable for Securities Sold	20.00%
Barclays Capital Inc [Member]
Concentration Risk [Line Items]
% of Total Receivable for Securities Sold	18.00%