Counterparty Risk (Cash and Cash Equivalents) (Details)	Dec. 31, 2016	Dec. 31, 2015
BlackRock Liquidity Funds FedFund Portfolio, Institutional Class Shares [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	41.00%	0.00%
Bank of New York Mellon Corporation [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	27.00%	13.00%
Goldman Sachs Financial Square Funds - Government Fund [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	16.00%	0.00%
BlackRock Liquidity Funds TempFund Portfolio [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	0.00%	54.00%
JPMorgan Prime Money Market Fund [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	0.00%	27.00%
Morgan Stanley Institutional Liquidity Fund - Government Portfolio [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	16.00%	0.00%
J.P. Morgan US Treasury Plus Money Market Fund [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	0.00%	6.00%