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Stock-based compensation - Restricted stock units (Details) - Restricted stock units
$ / shares in Units, $ in Millions
6 Months Ended 12 Months Ended
Jan. 05, 2016
shares
Mar. 29, 2015
$ / shares
Jun. 30, 2016
USD ($)
Dec. 31, 2014
tranche
Stock-based compensation        
Total unrecognized compensation cost, net of estimated forfeitures, related to unvested stock options and restricted stock units | $     $ 1.9  
Weighted average period for recognition of compensation cost related to unvested stock awards     3 years 170 days  
CEO        
Stock-based compensation        
Vesting period 4 years      
Grant date stock price (in dollars per share) | $ / shares   $ 43.47    
Number of shares issued (in shares) | shares 250,000      
Service-based | CEO        
Stock-based compensation        
Vesting period       24 months
Performance based restricted stock units | CEO        
Stock-based compensation        
Number of tranches | tranche       2
Vesting period       1 year
First year | CEO        
Stock-based compensation        
Percentage of shares vesting 25.00%      
Next 12 quarters | CEO        
Stock-based compensation        
Vesting period 3 years