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Summary of significant accounting policies (Details) - USD ($)
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Property, Plant and Equipment [Line Items]      
Allowance for Doubtful Accounts Receivable, Current $ 1,400,000 $ 1,250,000  
Amortization of Long-term Assets 16,800,000 18,000,000 $ 13,500,000
Impairment of Long-Lived Assets Held-for-use $ 0    
Warranty Period 12 months    
Advertising Expense $ 64,700,000 $ 47,200,000 $ 55,500,000
Minimum [Member]      
Property, Plant and Equipment [Line Items]      
Long-term Assets, Amortization Period 24 months    
Useful life (in years) 1 year    
Maximum [Member]      
Property, Plant and Equipment [Line Items]      
Long-term Assets, Amortization Period 36 months    
Useful life (in years) 10 years    
European Union [Member]      
Property, Plant and Equipment [Line Items]      
Warranty Period 2 years    
Deferred Income Tax Charge [Member]      
Property, Plant and Equipment [Line Items]      
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets $ 22,200,000