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Revenue recognition - Performance Obligations (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Jun. 30, 2018
Dec. 31, 2017
Nov. 30, 2011
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]          
Contract with Customer, Liability $ 15,802 $ 12,938 $ 10,540 $ 11,891  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | Minimum          
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]          
Revenue, performance obligation, period 1 year        
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | Maximum          
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]          
Revenue, performance obligation, period 3 years        
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01 | Licensing And Collaboration Agreement          
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]          
Deferred consideration payable         $ 30,000
Revenue, performance obligation, period         2 years