v3.19.1
Stock-based compensation (Narrative) (Details) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Mar. 31, 2017
Apr. 30, 2019
Apr. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shares authorized (in shares)       2,600,000.0  
Percentage of capital stock       3.00%  
Expected dividend yield       0.00% 0.00%
Unrecognized stock compensation expense to be recognized in future       $ 12.4  
Stock options          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Expiration period       10 years  
Forfeiture period       90 days  
Weighted-average vesting period of non-vested awards expected to vest       1 year 11 months  
Restricted stock units          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Weighted-average vesting period of non-vested awards expected to vest       2 years 10 months  
Award vesting period       4 years  
Performance units awards (in shares)       421,000  
Aggregate intrinsic value       $ 6.9 $ 2.9
Unrecognized stock-based compensation expense related to restricted stock units to be recognized in future       $ 52.0  
Performance restricted stock units          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Performance units awards (in shares) 129,963   146,964    
Performance restricted stock units | Minimum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award vesting rights percentage 0.00%   0.00%    
Performance restricted stock units | Maximum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award vesting rights percentage 200.00%   150.00%    
Performance restricted stock awards          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Performance units awards (in shares)   227,760      
Performance restricted stock awards | Minimum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award vesting rights percentage   0.00%      
Performance restricted stock awards | Maximum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award vesting rights percentage   200.00%      
Restricted stock awards          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Weighted-average vesting period of non-vested awards expected to vest       2 years 1 month  
Award vesting period       4 years  
Performance units awards (in shares)       0  
Aggregate intrinsic value       $ 0.8  
Unrecognized stock-based compensation expense related to restricted stock units to be recognized in future       $ 6.2