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Employee benefit plans Employee retirement plan (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Defined Contribution Plan Minimum Annual Contributions Per Employee Percent 1.00%  
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent 86.00%  
Defined Contribution Plan, Employer Discretionary Contribution Amount $ 5,500 $ 2,700
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value $ 108,846 $ 1,425,339