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Revenue - Disaggregated by category (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disaggregation of Revenue [Line Items]                      
Total revenues $ 140,238 $ 160,808 $ 168,493 $ 158,875 $ 139,824 $ 150,658 $ 138,543 $ 115,110 $ 628,414 $ 544,135 $ 341,191
Product revenue                      
Disaggregation of Revenue [Line Items]                      
Total revenues                 357,087 290,496 189,091
Rental revenue                      
Disaggregation of Revenue [Line Items]                      
Total revenues                 141,816 133,418 77,469
Field service and other revenue                      
Disaggregation of Revenue [Line Items]                      
Total revenues                 $ 129,511 $ 120,221 $ 74,631