Counterparty Risk (% of Due from Brokers) (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Concentration risk threshold	15.00%	15.00%
Morgan Stanley [Member]
Concentration Risk [Line Items]
% of Total Due from Brokers		37.00%
J.P. Morgan Securities LLC [Member]
Concentration Risk [Line Items]
% of Total Due from Brokers		30.00%