Counterparty Risk (Cash and Cash Equivalents) (Details)	Dec. 31, 2015	Dec. 31, 2014
BlackRock Liquidity Temp Fund [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	54.00%	88.00%
J.P. Morgan US Dollar Liquidity Fund [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	27.00%	0.00%
Bank of New York Mellon Corporation [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	13.00%	11.00%
J.P. Morgan US Treasury Plus Premier Fund [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	6.00%	0.00%
US Bank [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	0.00%	1.00%