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Employee benefit plans - Fair Value Assumptions for Restricted Stock Units and ESPP (Details) - $ / shares
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Employee Stock Purchase Plan Shares [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate   45.50%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term 6 months 7 months 6 days
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate   0.10%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate 0.00% 0.00%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate, Minimum 0.10%  
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate, Maximum 0.20%  
Employee Stock Purchase Plan Shares [Member] | Minimum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate 39.00%  
Employee Stock Purchase Plan Shares [Member] | Maximum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate 45.00%  
Restricted Stock Units (RSUs) [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate   50.90%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term   10 years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate   2.69%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Weighted-average Fair Value at Purchase Date   $ 18.40