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Equity - Performance shares (Details)
$ / shares in Units, shares in Thousands, $ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Mar. 31, 2019
Sep. 30, 2019
USD ($)
item
$ / shares
shares
Dec. 31, 2018
Performance Shares      
Share-based compensation      
Number of tranches | item   2  
Vesting terms     3 years
Granted (in shares)   70  
Weighted-average fair value per share, Granted (in dollars per share) | $ / shares   $ 92.57  
Unrecognized compensation cost | $   $ 5.0  
Remaining requisite service period (in years)   2 years  
Cancelled (in shares)   3  
Market-based performance shares      
Share-based compensation      
Percentage of share-based compensation award   50.00%  
Performance shares calculation period (in years)   3 years  
Vesting terms   3 years  
Market-based performance shares | Minimum      
Share-based compensation      
Performance shares available for vesting (as a percent)   0.00%  
Market-based performance shares | Maximum      
Share-based compensation      
Performance shares available for vesting (as a percent)   200.00%  
Internal-based performance based      
Share-based compensation      
Percentage of share-based compensation award   50.00%  
Performance shares calculation period (in years)   3 years  
Vesting terms   3 years  
Internal-based performance based | Minimum      
Share-based compensation      
Performance shares available for vesting (as a percent)   0.00%  
Internal-based performance based | Maximum      
Share-based compensation      
Performance shares available for vesting (as a percent)   200.00%  
Performance Shares Award Granted in 2016      
Share-based compensation      
Award pay out achieved (as a percent) 0.00%