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Summary of Significant Accounting Policies (Narrative) (Details) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Sep. 30, 2016 USD ($) | Jun. 30, 2016 USD ($) | Mar. 31, 2016 USD ($) | Dec. 31, 2017 USD ($) segment | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Revenue from External Customer [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cash equivalents, qualification, maximum original maturity of short-term investments |  |  |  |  |  |  |  |  | 3 months |  |
Customer deposit requirements, large custom contracts, maximum percentage |  |  |  |  |  |  |  |  | 50.00% |  |
Allowance for doubtful accounts | $ 569,000 |  |  |  | $ 597,000 |  |  |  | $ 569,000 | $ 597,000 |
Revenues recognized | 16,660,000 | $ 15,913,000 | $ 16,218,000 | $ 18,902,000 | 16,703,000 | $ 16,181,000 | $ 17,194,000 | $ 21,576,000 | 67,693,000 | 71,654,000 | $ 95,919,000
Product warranty reserves | 65,000 |  |  |  | 103,000 |  |  |  | 65,000 | 103,000 |
Uncertain tax positions | $ 0 |  |  |  | $ 0 |  |  |  | $ 0 | 0 |
Number of business segments | segment |  |  |  |  |  |  |  |  | 1 |  |
Bill and Hold Arrangement |  |  |  |  |  |  |  |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues recognized |  |  |  |  |  |  |  |  | $ 4,600,000 | $ 5,600,000 |

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