Commitments and Contingencies (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Movement in Standard Product Warranty Accrual [Roll Forward]
Warranty liability balance, beginning of the period	$ 1,124	$ 1,821
Product warranty accruals	32	445
Utilization of accrual	(272)	(672)
Warranty liability balance, end of the period	884	1,594
Product Warranty Accrual, Noncurrent	200		$ 200
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	9,599	$ 46,248		$ 33,087
Unallocated shelf registration limit	$ 14,000
Minimum
Product Warranty Liability [Line Items]
Warranty, Term	12 months
Maximum
Product Warranty Liability [Line Items]
Warranty, Term	24 months
Other Accrued Liabilities [Member]
Movement in Standard Product Warranty Accrual [Roll Forward]
Product Warranty Accrual, Current	$ 700		$ 900