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Counterparty Risk (Cash and Cash Equivalents) (Details)
Jun. 30, 2015
Dec. 31, 2014
BlackRock Liquidity Temp Fund [Member]    
Concentration Risk [Line Items]    
Concentration risk percentage, cash and cash equivalents 40.00% 88.00%
Bank of New York Mellon Corporation [Member]    
Concentration Risk [Line Items]    
Concentration risk percentage, cash and cash equivalents 59.00% 11.00%
US Bank [Member]    
Concentration Risk [Line Items]    
Concentration risk percentage, cash and cash equivalents 1.00% 1.00%