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Fair Value Measurement, Adjusted Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value by Investment Category (Details) - USD ($) $ in Thousands | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Cash and available-for-sale securities adjusted cost, gross unrealized gains, gross unrealized losses and fair value by significant investment category [Abstract] |  |  |  |
Adjusted Cost | $ 7,611 | $ 3,135 | $ 6,627 | $ 8,726
Adjusted Cost | 4,366 | 2,617 |  |
Unrealized Gains | 0 | 0 |  |
Unrealized Losses | 0 | (1) |  |
Fair Value | 4,366 | 2,616 |  |
Adjusted Cost | 9,414 | 4,589 |  |
Fair Value | 9,414 | 4,588 |  |
Investments Available for Sale | $ 4,366 | 2,616 |  |
Minimum [Member] |  |  |  |
Cash and available-for-sale securities adjusted cost, gross unrealized gains, gross unrealized losses and fair value by significant investment category [Abstract] |  |  |  |
Maturities of available for sale securities | 1 year |  |  |
Maximum [Member] |  |  |  |
Cash and available-for-sale securities adjusted cost, gross unrealized gains, gross unrealized losses and fair value by significant investment category [Abstract] |  |  |  |
Maturities of available for sale securities | 2 years |  |  |
Cash [Member] |  |  |  |
Cash and available-for-sale securities adjusted cost, gross unrealized gains, gross unrealized losses and fair value by significant investment category [Abstract] |  |  |  |
Adjusted Cost | $ 5,048 | 1,972 |  |
Fair Value | 5,048 | 1,972 |  |
Cash and Cash Equivalents | 5,048 | 1,972 |  |
Mutual Funds [Member] |  |  |  |
Cash and available-for-sale securities adjusted cost, gross unrealized gains, gross unrealized losses and fair value by significant investment category [Abstract] |  |  |  |
Adjusted Cost | 1,107 | 616 |  |
Unrealized Gains | 0 | 0 |  |
Unrealized Losses | 0 | 0 |  |
Fair Value | 1,107 | 616 |  |
Investments Available for Sale | 1,107 | 616 |  |
US Agency Securities [Member] |  |  |  |
Cash and available-for-sale securities adjusted cost, gross unrealized gains, gross unrealized losses and fair value by significant investment category [Abstract] |  |  |  |
Adjusted Cost | 3,259 | 2,001 |  |
Unrealized Gains | 0 | 0 |  |
Unrealized Losses | 0 | (1) |  |
Fair Value | 3,259 | 2,000 |  |
Investments Available for Sale | 3,259 | 2,000 |  |
Recurring [Member] |  |  |  |
Cash and available-for-sale securities adjusted cost, gross unrealized gains, gross unrealized losses and fair value by significant investment category [Abstract] |  |  |  |
Fair Value | 7,611 | 3,135 |  |
Fair Value | 1,803 | 1,453 |  |
Cash and Cash Equivalents | 7,611 | 3,135 |  |
Investments Available for Sale | 1,803 | 1,453 |  |
Recurring [Member] | Cash [Member] |  |  |  |
Cash and available-for-sale securities adjusted cost, gross unrealized gains, gross unrealized losses and fair value by significant investment category [Abstract] |  |  |  |
Fair Value | 5,048 | 1,972 |  |
Cash and Cash Equivalents | 5,048 | 1,972 |  |
Recurring [Member] | Level 1 [Member] |  |  |  |
Cash and available-for-sale securities adjusted cost, gross unrealized gains, gross unrealized losses and fair value by significant investment category [Abstract] |  |  |  |
Fair Value | 2,563 | 1,163 |  |
Fair Value | 1,803 | 1,453 |  |
Cash and Cash Equivalents | 2,563 | 1,163 |  |
Investments Available for Sale | 1,803 | 1,453 |  |
Recurring [Member] | Level 1 [Member] | Mutual Funds [Member] |  |  |  |
Cash and available-for-sale securities adjusted cost, gross unrealized gains, gross unrealized losses and fair value by significant investment category [Abstract] |  |  |  |
Fair Value | 1,107 | 616 |  |
Fair Value | 0 | 0 |  |
Cash and Cash Equivalents | 1,107 | 616 |  |
Investments Available for Sale | 0 | 0 |  |
Recurring [Member] | Level 1 [Member] | US Agency Securities [Member] |  |  |  |
Cash and available-for-sale securities adjusted cost, gross unrealized gains, gross unrealized losses and fair value by significant investment category [Abstract] |  |  |  |
Fair Value | 1,456 | 547 |  |
Fair Value | 1,803 | 1,453 |  |
Cash and Cash Equivalents | 1,456 | 547 |  |
Investments Available for Sale | $ 1,803 | $ 1,453 |  |

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