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Investments (Tables) | 6 Months Ended
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Jun. 30, 2017
Investments, Debt and Equity Securities [Abstract] |
Schedule of amortized cost and estimated fair values of available-for-sale securities by balance sheet classification | The amortized cost and estimated fair values of available-for-sale securities by balance sheet classification are as follows (dollar amounts in thousands):
As of December 31, 2016 | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value
U.S. government securities funds | $ | 1,799 |  |  | $ | — |  |  | $ | (23 | ) |  | $ | 1,776 |
Total short-term investment securities | $ | 1,799 |  |  | $ | — |  |  | $ | (23 | ) |  | $ | 1,776 |

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- Definition Tabular disclosure of the reconciliation of available-for-sale securities from cost basis to fair value. + References No definition available. + Details
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