v3.8.0.1
Subsequent events (Details) - shares
1 Months Ended 12 Months Ended
Feb. 28, 2018
Dec. 31, 2017
Performance-based restricted stock unit awards    
Subsequent events    
Vesting period   3 years
Executive members | Time-based restricted stock unit awards    
Subsequent events    
Granted (in shares) 44,000  
Vesting period 3 years  
Executive members | Performance-based restricted stock unit awards    
Subsequent events    
Granted (in shares) 44,000  
Executive members | Performance-based restricted stock unit awards | Vesting not to exceed March 15, 2020    
Subsequent events    
Vesting percentage 66.67%  
Executive members | Performance-based restricted stock unit awards | Vesting on May 1, 2020    
Subsequent events    
Vesting percentage 8.33%  
Executive members | Performance-based restricted stock unit awards | Vesting quarterly after May 1, 2020    
Subsequent events    
Vesting percentage upon completion of continuous service 8.33%  
Non-executive personnel | Time-based restricted stock unit awards    
Subsequent events    
Granted (in shares) 168,000  
Vesting period 3 years