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Basis of Presentation and Summary of Significant Accounting Policies - Narrative (Details) - USD ($) shares in Thousands, $ in Thousands | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Oct. 31, 2019 | Sep. 30, 2019 | Mar. 31, 2019 | Sep. 30, 2018 | Mar. 31, 2016 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018 | Jan. 01, 2019
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |
Operating lease liabilities |  | $ 1,612 |  |  |  | $ 1,612 |  |  |
Operating lease right-of-use assets |  | $ 1,415 |  |  |  | $ 1,415 |  |  |
Operating lease, impairment loss |  |  | $ 200 |  |  |  |  |  |
Securities excluded from net loss per share calculation (in shares) |  | 0 |  | 62 |  | 31 | 90 |  |
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% |  | 1.00% |  |  | 2.00% |  |
Distributor To The U.S. Navy | Revenue | Customer concentration risk |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |
Concentration risk (less than for one percent) |  | 27.00% |  | 48.00% |  | 20.00% | 41.00% |  |
Distributor To The U.S. Navy | Accounts receivable | Customer concentration risk |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |
Concentration risk (less than for one percent) |  |  |  |  |  | 37.00% |  | 40.00% |
Global Healthcare Systems, Northeast Ohio | Revenue | Customer concentration risk |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |
Concentration risk (less than for one percent) |  | 15.00% |  |  |  | 13.00% |  |  |
Commercial Lighting Retrofit Company | Revenue | Customer concentration risk |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |
Concentration risk (less than for one percent) |  | 14.00% |  | 15.00% |  | 23.00% | 10.00% |  |
Commercial Lighting Retrofit Company | Accounts receivable | Customer concentration risk |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |
Concentration risk (less than for one percent) |  |  |  |  |  | 11.00% |  |  |
U.S. Navy | Revenue | Customer concentration risk |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |
Concentration risk (less than for one percent) |  | 30.00% |  | 53.00% |  | 27.00% | 45.00% |  |
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 |  |  |  |  |  |
Commercial LEDFL Tubular LED Lamps (Excluding Battery Backup TLED), Troffer Luminaires, And Certain Globe Lights |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |
Warranty service periods |  |  |  |  |  | 10 years |  |  |
Other LED Products |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |
Warranty service periods |  |  |  |  |  | 5 years |  |  |
Subsequent Event | LEDFL Tubular LED Lamps (Excluding Red Caps) |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |
Warranty service periods | 10 years |  |  |  |  |  |  |  |
Subsequent Event | All Other Products |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |
Warranty service periods | 5 years |  |  |  |  |  |  |  |

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