Counterparty Risk (% of Total Receivable for Securities Sold) (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Concentration risk threshold	15.00%	15.00%
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%