v3.10.0.1

Counterparty Risk (Cash and Cash Equivalents) (Details) | Jun. 30, 2018 | Dec. 31, 2017
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Bank of New York Mellon Corporation [Member] |  |
Concentration Risk [Line Items] |  |
Concentration risk percentage, cash and cash equivalents | 32.00% | 37.00%
Bank of America Securities [Member] |  |
Concentration Risk [Line Items] |  |
Concentration risk percentage, cash and cash equivalents | 4.00% | 2.00%
BlackRock Liquidity Funds FedFund Portfolio, Institutional Class Shares [Member] |  |
Concentration Risk [Line Items] |  |
Concentration risk percentage, cash and cash equivalents | 10.00% | 56.00%
Goldman Sachs Financial Square Funds - Government Fund [Member] |  |
Concentration Risk [Line Items] |  |
Concentration risk percentage, cash and cash equivalents | 10.00% | 0.00%
Morgan Stanley Institutional Liquidity Fund - Government Portfolio [Member] |  |
Concentration Risk [Line Items] |  |
Concentration risk percentage, cash and cash equivalents | 11.00% | 0.00%
US Bank N.A. [Member] |  |
Concentration Risk [Line Items] |  |
Concentration risk percentage, cash and cash equivalents | 1.00% | 0.00%
Deutsche Bank Securities [Member] |  |
Concentration Risk [Line Items] |  |
Concentration risk percentage, cash and cash equivalents | 32.00% | 5.00%

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