Counterparty Risk (% Of Total Receivable For Securities Sold) (Details)	Jun. 30, 2018
Bank of America Securities [Member]
Concentration Risk [Line Items]
% of Total Receivable for Securities Sold	33.00%
Credit Suisse Securities LLC [Member]
Concentration Risk [Line Items]
% of Total Receivable for Securities Sold	34.00%
Wells Fargo Securities [Member]
Concentration Risk [Line Items]
% of Total Receivable for Securities Sold	16.00%