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401(k) savings plan (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
401(k) savings plan [Abstract]      
Matching of qualified deferrals by employer, maximum 50.00%    
Percentage of employee's gross salary subject to employer's matching contribution 6.00%    
Matching contributions made by employer $ 2.7 $ 2.5 $ 2.2