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Income taxes, Unrecognized tax benefits (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Contingency [Line Items]      
Unrecognized tax benefits, current $ 800,000 $ 500,000  
Unrecognized tax benefits, noncurrent 1,200,000 1,300,000  
Gross unrecognized tax benefits activity [Roll Forward]      
Gross unrecognized tax benefits, beginning balance 1,761,000 1,457,000 $ 1,248,000
Increases for tax positions for prior years 0 5,000 150,000
Decreases for tax positions for prior years 0 0 0
Increases for tax positions for current year 531,000 299,000 59,000
Settlements (318,000) 0 0
Lapse of statute of limitations 0 0 0
Gross unrecognized tax benefits, ending balance $ 1,974,000 $ 1,761,000 $ 1,457,000
U.S. Federal Tax Authority [Member] | Minimum [Member]      
Gross unrecognized tax benefits activity [Roll Forward]      
Open tax year 2013    
Federal income tax year selected for audit 2011    
U.S. Federal Tax Authority [Member] | Maximum [Member]      
Gross unrecognized tax benefits activity [Roll Forward]      
Open tax year 2016    
Federal income tax year selected for audit 2012    
United Kingdom Tax Authority [Member] | Minimum [Member]      
Gross unrecognized tax benefits activity [Roll Forward]      
Open tax year 2008    
United Kingdom Tax Authority [Member] | Maximum [Member]      
Gross unrecognized tax benefits activity [Roll Forward]      
Open tax year 2016    
Canada Revenue Agency [Member] | Minimum [Member]      
Gross unrecognized tax benefits activity [Roll Forward]      
Open tax year 2010    
Canada Revenue Agency [Member] | Maximum [Member]      
Gross unrecognized tax benefits activity [Roll Forward]      
Open tax year 2016