Revenue - Disaggregated by category (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Total revenues	$ 154,139	$ 158,875
Revenue as a percentage	100.00%	100.00%
Product revenue
Disaggregation of Revenue [Line Items]
Total revenues	$ 87,031	$ 86,640
Revenue as a percentage	57.00%	55.00%
Rental revenue
Disaggregation of Revenue [Line Items]
Total revenues	$ 36,163	$ 38,497
Revenue as a percentage	23.00%	24.00%
Field service and other revenue
Disaggregation of Revenue [Line Items]
Total revenues	$ 30,945	$ 33,738
Revenue as a percentage	20.00%	21.00%