Counterparty Risk (Cash and Cash Equivalents) (Details)	Dec. 31, 2017	Dec. 31, 2016
BlackRock Liquidity Funds FedFund Portfolio, Institutional Class Shares [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	56.00%	41.00%
Bank of New York Mellon Corporation [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	37.00%	27.00%
Deutsche Bank Securities [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	5.00%	0.00%
Bank of America Securities [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	2.00%	0.00%
Goldman Sachs Financial Square Funds - Government Fund [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	0.00%	16.00%
Morgan Stanley Institutional Liquidity Fund - Government Portfolio [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	0.00%	16.00%