Counterparty Risk (Cash and Cash Equivalents) (Details)	Jun. 30, 2018	Dec. 31, 2017
Bank of New York Mellon Corporation [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	32.00%	37.00%
Bank of America Securities [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	4.00%	2.00%
BlackRock Liquidity Funds FedFund Portfolio, Institutional Class Shares [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	10.00%	56.00%
Goldman Sachs Financial Square Funds - Government Fund [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	10.00%	0.00%
Morgan Stanley Institutional Liquidity Fund - Government Portfolio [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	11.00%	0.00%
US Bank N.A. [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	1.00%	0.00%
Deutsche Bank Securities [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	32.00%	5.00%