Revenue - Additional Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Disaggregation Of Revenue [Line Items]
Beginning Balance, December 31, 2017	$ 7,206
Systems Revenue [Member]
Disaggregation Of Revenue [Line Items]
Number of days for payment due from customer	30 days
Software Revenue
Disaggregation Of Revenue [Line Items]
Number of days for payment due from customer	30 days
Parts Revenue
Disaggregation Of Revenue [Line Items]
Number of days for payment due from customer	30 days
Service Revenue
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