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Basis of Presentation and Summary of Significant Accounting Policies - Narrative (Details) - USD ($) shares in Thousands, $ in Thousands |  | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Apr. 01, 2016 | Jun. 30, 2019 | Mar. 31, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Dec. 31, 2018 | Jan. 01, 2019
Concentration Risk [Line Items] |  |  |  |  |  |  |  |
Operating lease liabilities |  | $ 1,741 |  |  | $ 1,741 |  |  |
Operating lease right-of-use assets |  | $ 1,541 |  |  | $ 1,541 |  |  |
Operating lease, impairment loss |  |  | $ 186 |  |  |  |  |
Securities excluded from net loss per share calculation (in shares) |  | 28 |  | 87 | 66 | 83 |  |
Warranty service periods | 10 years |  |  |  |  |  |  |
Minimum |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |
Warranty service periods |  |  |  |  | 1 year |  |  |
Maximum |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |
Warranty service periods |  |  |  |  | 5 years |  |  |
Geographic concentration risk | United States |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |
Concentration risk (less than for one percent) |  |  |  |  | 97.00% |  |  |
Revenue | Geographic concentration risk | Countries outside of the United States |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |
Concentration risk (less than for one percent) |  | 0.00% |  | 5.00% | 1.00% | 3.00% |  |
Distributor To The U.S. Navy | Revenue | Customer concentration risk |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |
Concentration risk (less than for one percent) |  | 13.00% |  | 29.00% | 17.00% | 37.00% |  |
Distributor To The U.S. Navy | Accounts receivable | Customer concentration risk |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |
Concentration risk (less than for one percent) |  |  |  |  | 13.00% |  | 40.00% |
Global Healthcare Systems, Northeast Ohio [Member] | Revenue | Customer concentration risk |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |
Concentration risk (less than for one percent) |  |  |  | 15.00% |  | 12.00% |  |
Commercial Lighting Retrofit Company | Revenue | Customer concentration risk |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |
Concentration risk (less than for one percent) |  | 26.00% |  | 11.00% | 28.00% |  |  |
Commercial Lighting Retrofit Company | Accounts receivable | Customer concentration risk |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |
Concentration risk (less than for one percent) |  |  |  |  | 26.00% |  |  |
U.S. Navy | Revenue | Customer concentration risk |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |
Concentration risk (less than for one percent) |  | 20.00% |  | 33.00% | 26.00% | 41.00% |  |
Accounting Standards Update 2016-02 |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |
Operating lease liabilities |  |  |  |  |  |  |  | $ 2,900
Operating lease right-of-use assets |  |  |  |  |  |  |  | $ 2,900
Facilities | 2017 Restructuring Plan |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |
Reclassification upon adoption of Topic 842 |  |  | $ 273 |  |  |  |  |

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