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Stock plans - Additional Information (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award        
Shares or units awarded 2,548 2,362    
Unrecognized compensation expense $ 92 $ 32    
2008 Plan        
Share-based Compensation Arrangement by Share-based Payment Award        
Share based compensation expense   156 $ 71  
2017 Plan        
Share-based Compensation Arrangement by Share-based Payment Award        
Common stock available for grant as awards 86,994      
Share based compensation expense $ 272 $ 21    
Non-Performance-based restricted stock        
Share-based Compensation Arrangement by Share-based Payment Award        
Shares or units vesting period   3 years    
Non-Performance-based restricted stock | 2008 Plan        
Share-based Compensation Arrangement by Share-based Payment Award        
Shares or units awarded   2,362    
Vested shares 2,493      
Non-Performance-based restricted stock | 2017 Plan        
Share-based Compensation Arrangement by Share-based Payment Award        
Shares or units awarded 2,548      
Vested shares 114      
Performance-based restricted stock units        
Share-based Compensation Arrangement by Share-based Payment Award        
Shares or units vesting period   3 years    
Cumulative diluted earnings per share period used for conditions for vesting of performance-based restricted stock units   3 years    
Average return on equity period used for conditions for vesting of performance-based restricted stock units   3 years    
Performance-based restricted stock units | 2008 Plan        
Share-based Compensation Arrangement by Share-based Payment Award        
Number of nonvested restricted stock awards   8,266    
Performance-based restricted stock units | 2017 Plan        
Share-based Compensation Arrangement by Share-based Payment Award        
Number of nonvested restricted stock awards 8,920      
Restricted Stock        
Share-based Compensation Arrangement by Share-based Payment Award        
Vested shares 2,607 10,542 1,947  
Number of nonvested restricted stock awards 21,309 12,448 12,362 14,309
Shares or units cliff vesting period   5 years    
Unrecognized compensation expense $ 525      
Weighted average vesting period 1 year 8 months 12 days      
Stock appreciation rights granted 11,468 10,628    
Restricted Stock | 2008 Plan        
Share-based Compensation Arrangement by Share-based Payment Award        
Vested shares   10,542