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Summary of business and significant accounting policies (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Nov. 24, 2020
Jan. 01, 2018
Apr. 12, 2017
Property, Plant and Equipment [Line Items]            
Restricted Cash $ 2,000 $ 0        
Allowance for Doubtful Other Receivables, Current 500 800        
Operating Lease, Impairment Loss 12,460 0 $ 0      
Deferred Revenue 29,300 16,600        
Deferred Revenue, Revenue Recognized 15,400 15,000        
Advertising Expense 34,100 67,300 $ 73,000      
Accumulated deficit $ (650,516) $ (583,733)        
Product Warranty Liability [Line Items]            
Warranty Period 12 months          
Minimum [Member]            
Property, Plant and Equipment [Line Items]            
Long-term Assets, Amortization Period 24 months          
Maximum [Member]            
Property, Plant and Equipment [Line Items]            
Long-term Assets, Amortization Period 36 months          
Property, Plant and Equipment, Useful Life 9 years          
Cumulative effect of adoption of new accounting standard [Member]            
Property, Plant and Equipment [Line Items]            
Accumulated deficit         $ 15,000  
Accounting Standards Update 2014-09 [Member]            
Property, Plant and Equipment [Line Items]            
Accrued Liabilities         4,900  
Deferred Revenue         2,000  
Accumulated deficit         $ 2,900  
Convertible Senior Notes due 2022 [Member]            
Property, Plant and Equipment [Line Items]            
Interest rate           3.50%
Debt Instrument           $ 175,000
Convertible Senior Notes due 2025 [Member]            
Property, Plant and Equipment [Line Items]            
Interest rate       1.25%    
Debt Instrument       $ 143,800    
Europe [Member]            
Product Warranty Liability [Line Items]            
Warranty Period 2 years