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401(k) plan (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
401(k) plan      
Matching contribution up to the first 6% of employee's base salary (as a percent) 50.00%    
Percentage of employee's base salary, matched by employer 6.00%    
Expense recorded $ 0.7 $ 0.3 $ 0.1