Counterparty Risk (% of Total Receivable for Securities Sold) (Details)	Dec. 31, 2018
J.P. Morgan Securities LLC [Member]
Concentration Risk [Line Items]
% of Total Receivable for Securities Sold	25.00%
Bank of America Securities [Member]
Concentration Risk [Line Items]
% of Total Receivable for Securities Sold	26.00%
CS First Boston Limited [Member]
Concentration Risk [Line Items]
% of Total Receivable for Securities Sold	34.00%