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Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets. fair value | $ 903,709 | $ 633,110
Book value | 28,043 | 19,009
Held-to-maturity Securities | Short-term investments |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets. fair value | 474,205 | 206,161
Unrealized gain (loss) | (217) | $ (96)
Held-to-maturity Securities | Long-term investments |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets. fair value | 14,738 |
Unrealized gain (loss) | 118 |
Book value | $ 14,620 |

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