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Equity - Performance shares (Details)
$ / shares in Units, shares in Thousands, $ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2020
USD ($)
item
$ / shares
shares
Dec. 31, 2018
USD ($)
Dec. 31, 2019
USD ($)
Mar. 31, 2019
USD ($)
Share-based compensation        
Share-based payments subject to redemption | $ $ 23.0 $ 37.0 $ 31.0 $ 21.0
Performance Shares        
Share-based compensation        
Number of tranches | item 2      
Vesting terms   3 years    
Granted (in shares) | shares 81      
Weighted-average fair value per share, Granted (in dollars per share) | $ / shares $ 94.48      
Unrecognized compensation cost | $ $ 9.0      
Remaining requisite service period (in years) 2 years 4 months 24 days      
Cancelled (in shares) | shares 0      
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 2017        
Share-based compensation        
Award pay out achieved (as a percent) 0.00%