Basis of Presentation and Summary of Significant Accounting Policies - Narrative (Details) - shares shares in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
	Apr. 01, 2016	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Concentration Risk [Line Items]
Securities excluded from net loss per share calculation (in shares)		87	64	83	99
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)				98.00%	98.00%
Revenue | Geographic concentration risk | Countries outside of the United States
Concentration Risk [Line Items]
Concentration risk (less than for one percent)		5.00%	1.00%	3.00%	1.00%
Commercial Lighting Retrofit Company | Revenue | Customer concentration risk
Concentration Risk [Line Items]
Concentration risk (less than for one percent)		11.00%
Commercial Lighting Retrofit Company | Accounts receivable | Customer concentration risk
Concentration Risk [Line Items]
Concentration risk (less than for one percent)				12.00%		17.00%
Global Healthcare Systems, Northeast Ohio | Revenue | Customer concentration risk
Concentration Risk [Line Items]
Concentration risk (less than for one percent)		15.00%		12.00%	15.00%
Global Healthcare Systems, Northeast Ohio | Accounts receivable | Customer concentration risk
Concentration Risk [Line Items]
Concentration risk (less than for one percent)				17.00%		21.00%
U.S. Navy | Revenue | Customer concentration risk
Concentration Risk [Line Items]
Concentration risk (less than for one percent)		33.00%	12.00%	41.00%	17.00%
Distributor To The U.S. Navy [Member] | Revenue | Customer concentration risk
Concentration Risk [Line Items]
Concentration risk (less than for one percent)		29.00%	11.00%	37.00%	12.00%
Distributor To The U.S. Navy [Member] | Accounts receivable | Customer concentration risk
Concentration Risk [Line Items]
Concentration risk (less than for one percent)				30.00%		39.00%