Counterparty Risk (Cash and Cash Equivalents) (Details)	Dec. 31, 2018
Bank of New York Mellon Corporation [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	64.00%
Deutsche Bank Securities [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	5.00%
Bank of America Securities [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	2.00%
Morgan Stanley Institutional Liquidity Fund - Government Portfolio [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	10.00%
BlackRock Liquidity Funds FedFund Portfolio, Institutional Class Shares [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	9.00%
Goldman Sachs Financial Square Funds - Government Fund [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	9.00%
US Bank N.A. [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	1.00%