Revenue - Additional Information (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Disaggregation Of Revenue [Line Items]
Deferred revenue	$ 8,704	$ 8,799	$ 7,206
Systems Revenue [Member]
Disaggregation Of Revenue [Line Items]
Number of days for payment due from customer	30 days
Software Revenue [Member]
Disaggregation Of Revenue [Line Items]
Number of days for payment due from customer	30 days
Parts Revenue [Member]
Disaggregation Of Revenue [Line Items]
Number of days for payment due from customer	30 days
Service Revenue [Member]
Disaggregation Of Revenue [Line Items]
Number of days for payment due from customer	30 days
Minimum [Member] | Systems Revenue [Member]
Disaggregation Of Revenue [Line Items]
Percentage of invoice amount due	80.00%
Assurance warranty period against defects	12 months
Maximum [Member] | Systems Revenue [Member]
Disaggregation Of Revenue [Line Items]
Percentage of invoice amount due	90.00%
Assurance warranty period against defects	15 months