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% |